Principles of Horticulture 4th ed. by C.R.Adams.pdf

PRINCIPLES OFHORTICULTURE

Fourth Edition

C.R. Adams, BSc (Agric) Hons, FIHort, Dip Applied EdM.P. Early, MSc, BSc Hons, DTA, Cert Ed

AMSTERDAM BOSTON HEIDELBERG LONDON NEW YORK OXFORDPARIS SAN DIEGO SAN FRANCISCO SINGAPORE SYDNEY TOKYO

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Elsevier Butterworth-HeinemannLinacre House, Jordan Hill, Oxford OX2 8DP200 Wheeler Road, Burlington, MA 01803

First published 1984Reprinted 1985, 1987, 1988, 1990, 1991, 1992Second edition 1993Third edition 1998Reprinted 1999Fourth edition 2004

Copyright © 1984, 1993, 1998, C.R. Adams, K.M. Bamford, M.P. Early

Copyright © 2004, C.R. Adams, M.P. Early. All rights reserved

The right of C.R. Adams and M.P. Early to be identified as the authors of this work has been asserted in accordance with the Copyright, Designs and Patents Act 1988

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British Library Cataloguing in Publication DataAdams, C.R. (Charles R.)

Principles of horticulture – 4th ed.1. Horticulture 2. GardeningI. Title II. Early, M.P. (Micheal P.)635

ISBN 0 7506 6088 0

Typeset by Charon Tec Pvt. Ltd, Chennai, IndiaPrinted and bound in Great Britain

For information on all Elsevier publications visit ourwebsite at http://www.books.elsevier.com

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Preface viiiAcknowledgements x

1 Horticulture in context 1The plant 2Plant communities 2

Single species communities 3Competition between species 4Food chains 6Decomposers 6Sustainable development 7Biomass 7Garden considerations 7Companion planting 8

Organisms occurring on plantsurfaces 8

Rhizosphere 8Phyllosphere 9

Conservation 9Organic growing 11

2 Climate and microclimate 13The sun’s energy 13

Effect of latitude 14Movement of heat and

weather systems 14Weather and climate 16

Climate of the British Isles 17The growing season 18World climates 21

Local climate 21Microclimates 22

Measurement 24Temperature 24Precipitation 25Humidity 25Wind 26Light 27Automation 27

3 Classification and naming 28Principles of classification 28Kingdom plantae (plants) 28

Divisions of the plant kingdom 29Naming of cultivated plants 30

The binomial system 30Hybrids 31Identifying plants 31Geographical origins of plants 32Further classifications of plants 34

Kingdom Fungi 35Kingdom Animalia (animals) 35Kingdom Prokaryotae (monera) 36Kingdom Protoctista (protista) 36Viruses 36

4 Plant organization 37Plant form 37

Plant form in design 39Plant size and growth rate 39

The anatomy of the plant 40The cell 40

Tissues of the stem 41Dicotyledonous stem 41

Contents

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Monocotyledonous stem 42Stem growth 43

5 Water and minerals in the plant 46Water 46

Functions of water 46Movement of water 46

Water uptake 46Movement of water up the stem 48

Transpiration 48Minerals 50

Functions and deficiency symptomsof minerals in the plant 50

Mineral uptake 51

6 Plant growth 53Photosynthesis 53

Requirements for photosynthesis 54Carbon dioxide 54Light 55Temperature 56Water 56Minerals 57The leaf 57Movement of sugar 58Leaf adaptations 58Leaf form 58Pollution 59Respiration 59

Storage of plants 60

7 Plant development 61Plant hormones 61The seed 62

Seed structure 62Seed dormancy 63Seed germination 64The Seeds Acts 65

The seedling 65Seedling development 66Conditions for early plant growth 67

The vegetative plant 67Juvenility 67Vegetative propagation 68

Artificial methods of propagation 69Tissue culture 70Grafting 71Apical dominance 72Pruning 72Growth retardation 73

The flowering plant 73Photoperiodism 74Artificial control of lighting

for flowering 74Flower initiation 75

Flower structure 76Extended flower life 76Pollination and fertilization 77Bees in pollination 77Colour in flowers 78Removal of dead flowers 78The fruiting plant 78Fruit set 78The ageing plant 79

8 Genetics and plant breeding 80The cell 80

Cell division 81Inheritance of characteristics 81F1 hybrids 84Other breeding programmes 84Polyploids 85Mutations 85Recombinant DNA technology 85The Plant Varieties and

Seeds Act, 1964 86Gene banks 86

9 Weeds 87Damage 87Identification 88Weed biology 88Annual weeds 90Chickweed (Stellaria media) 90Groundsel (Senecio vulgaris) 91Speedwells (Veronica persica and

V. filiformis) 92Perennial weeds 92Creeping thistle (Cirsium arvense) 92

iv Contents

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Couch grass (Agropyron repens) 93Yarrow (Achillea millifolium) 94Broad-leaved dock

(Rumex obtusifolius) 95Mixed weed populations 95

Mosses and liverworts 95

10 Horticultural pests 97Mammal and bird pests 97

The rabbit (Oryctolagus cuniculus) 97The brown rat(Rattus norvegicus) 97The grey squirrel

(Sciurus carolinensis) 98The mole (Talpa europea) 99Deer 99The wood-pigeon

(Columba palumbus) 99The bullfinch

(Pyrrhula pyrrhula) 100Invertebrate pests 100

Slugs 100Insects 101

Structure and biology 101Aphids and their relatives

(order Hemiptera) 104Thrips (order Thysanoptera) 108Earwigs (Forficula auricularia) 109Moths and butterflies 109Flies 111Beetles 112Sawflies 113Springtails (order Collembola) 114

Mites 114Other arthropods 116Nematodes 117

11 Fungi, bacteria and viruses 121Fungi 121

Structure and biology 121Flower and leaf diseases 122Stem diseases 126Root diseases 127

Bacteria 129

Viruses 131Structure and biology 131General symptoms 131

12 Control measures 135Legislation and control 135Cultural control 136

Cultivation 136Partial soil sterilization 136Chemical sterilization 136Soil fertility 137Crop rotation 137Planting and harvesting times 137Clean seed and planting

material 137Hygienic growing 138Traps and repellants 138Alternate hosts 138Removal of infected plant

material 139Resistance 139Biological control 139

Phytoseiulus persimilis 141Encarsia formosa 141

Chemical control 142Herbicides 142Insecticides and acaricides 144Nematicides 145Fungicides 145Resistance to pesticides 145Formulations 146Application of herbicides

and pesticides 146Integrated control 147Supervised control 148Safety with herbicides and

pesticides 148Phytotoxicity 149

13 Soil as a growing medium 151Root requirements 151Composition of soils 152Soil formation 152

Weathering and erosion 152Igneous rocks 153

Contents v

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Sedimentary rocks 153Metamorphic rocks 154

Soil development 154Sedentary soils 155Transported soils 155

Soil types 156Soil components 157

Particle size classes 157Sand 157Clay 158Silt 160Stones and gravel 160

Soil texture 160Mechanical analysis of soils 161Texturing by feel 162

Soil structure 162Porosity 163Soil structures 164Development of soil structures 164Structural stability 166Tilth 166

Cultivations 166Ploughing and digging 166Rotary cultivators 167Harrowing and raking 167Subsoiling 167

Management of main soil types 167Sandy soils 167Silts and fine sands 168Clay soils 169Peat soils 169

14 Soil water 171Wetting of a dry soil 171

Saturated soils 171Field capacity (FC) 172Watertables 173Capillary rise 173

Drying of a wet soil 173Evaporation 173Evapotranspiration 174Available water 175Workability of soils 175

Drainage 175Symptoms of poor drainage 176Low permeability soils 177

Maintenance of drainagesystems 178

Irrigation 178Irrigation plan 178SMD 180Methods of applying water 181

Water quality 181Water conservation 182

15 Soil organic matter 184Living organisms in the soil 184

Plant roots 184Earthworms 185Bacteria 185Fungi 186The rhizosphere 186

Nutrient cycles 186Carbon cycle 186Nitrogen cycle 187Sulphur cycle 188Carbon to nitrogen ratio 188

Humus 189Organic matter levels 189

Climate 189Soil type 190Cultivation 190Organic soils 190

Benefits of organic matter 191Addition of organic matter 191

Composting 191Straw 192FYM 192Horticultural peats 192Leaves 192Air-dried digested sludge 193Leys 193Mulching 193

Organic production 194

16 Plant nutrition 195Control of soil pH 195

Acidity and alkalinity 196Soil acidity 196Effects of liming 197Plant tolerance 198Lime requirement 198

vi Contents

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Liming materials 198Lime application 199Decreasing soil pH 199

Fertilizers 199Application methods 200Formulations 201

Plant nutrients 202Nitrogen 202Phosphorus 202Potassium 203Magnesium 204Calcium 204Sulphur 204

Trace elements 204Deficiencies 204

Fertilizer programme 205Growing medium analysis 205Fertilizer recommendations 207Sampling 207

Soil conductivity 208

17 Alternatives to growingin the soil 210

Growing in containers 210Composts 212

Loam composts 212Loamless or soilless composts 213Alternatives to peat 213Compost formulations 215Compost mixing 216

Plant containers 217Blocks 217Modules 217Hydroponics 217NFT 218Aggregate culture 219

Sport surfaces 220

Index 222

Contents vii

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By studying the principles of horticulture, one is ableto learn how and why plants grow and develop. Inthis way, horticulturists are better able to understandthe responses of the plant to various conditions, andtherefore to perform their function more efficiently.They are able to manipulate the plant so that theyachieve their own particular requirements of maxi-mum yield and/or quality at the correct time.The texttherefore introduces the plant in its own right, andexplains how a correct naming method is vital fordistinguishing one plant from another. The internalstructure of the plant is studied in relation to thefunctions performed in order that we can understandwhy the plant takes it particular form. The environ-ment of a plant contains many variable factors, all ofwhich have their effects, and some of which can dra-matically modify growth and development. It istherefore important to distinguish the effects ofthese factors in order to have precise control ofgrowth. The environment which surrounds the partsof the plant above the ground includes factors suchas light, day-length, temperature, carbon dioxide andoxygen, and all of these must ideally be provided inthe correct proportions to achieve the type of growthand development required. The growing medium isthe means of providing nutrients, water, air and usu-ally anchorage for the plants.

In the wild, a plant will interact with other plants,often to different species and other organisms to cre-ate a balanced community. Ecology is the study ofthis balance. In growing plants for our own ends wehave created a new type of community which createsproblems – problems of competition for the environ-mental factors between one plant and another of thesame species, between the crop plant and a weed, or

between the plant and a pest or disease organism.These latter two competitive aspects create the needfor crop protection.

It is only by identification of these competitiveorganisms (weeds, pests and diseases) that the horti-culturist may select the correct method of control.With the larger pests there is little problem of recog-nition, but the smaller insects, mites, nematodes,fungi and bacteria are invisible to the naked eye and,in this situation, the grower must rely on the symp-toms produced (type of damage). For this reason, thepests are covered under major headings of the organ-ism, whereas the diseases are described under symp-toms. Symptoms (other than those caused by anorganism) such as frost damage, herbicide damageand mineral deficiencies may be confused with pestor disease damage, and reference is made in the textto this problem. Weeds are broadly identified asperennial or annual problems. References at the endof each chapter encourage students to expand theirknowledge of symptoms. In an understanding of cropprotection, the structure and life cycle of the organ-ism must be emphasized in order that specific meas-ures, e.g. chemical control, may be used at the correcttime and place to avoid complications such as phyto-toxicity, resistant pest production or death of benefi-cial organisms. For this reason, each weed, pest anddisease is described in such a way that control meas-ures follow logically from an understanding of itsbiology. More detailed explanations of specific typesof control, such as biological control, are contained ina separate chapter where concepts such as economicdamage are discussed.

This book is not intended to be a reference sourceof weeds, pests and diseases; its aim is to show the

Preface

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Preface ix

range of these organisms in horticulture. Referencesare given to texts which cover symptoms and lifecycle stages of a wider range of organisms. Latinnames of species are included in order that confusionabout the varied common names may be avoided.

Growing media include soils and soil substitutessuch as composts, aggregate culture and nutrient filmtechnique. Usually the plant’s water and mineralrequirements are taken up from the growing mediumby roots. Active roots need a supply of oxygen, andtherefore the root environment must be managed toinclude aeration as well as to supply water and miner-als. The growing medium must also provide anchor-age and stability, to avoid soils that ‘blow’, trees thatuproot in shallow soils or tall pot plants that topple inlightweight composts.

The components of the soil are described toenable satisfactory root environments to be producedand maintained where practicable. Soil conditions aremodified by cultivations, irrigation, drainage and lim-ing, while fertilizers are used to adjust the nutrientstatus to achieve the type of growth required.

The use of soil substitutes, and the management ofplants grown in pots, troughs, peat bags and othercontainers where there is a restricted rooting zone,are also discussed in the final chapter.

In this, the fourth edition of the book the authorshave continued to make changes to align the textwith the significant syllabus changes to courses forwhich the book has been produced to support.

Revision has also been made in the light of rec-ommendations from key users.This has lead to a sub-stantial increase in the nature of the plant’simmediate environment, its microclimate, its meas-urement and methods of modifying it. This is put incontext by the inclusion of a full discussion of the cli-mate, the underlying factors that drive the weathersystems and the nature of local climates in the BritishIsles. There has been an expansion of the geneticssection to accommodate the need for more details ondominance and dihybrid crosses along with an intro-duction to genetic modification (GM), and the asso-ciated crown gall, to reflect the interest in this topicin the industry. The changes in the classification

system have been accommodated and the plant divisions revised without losing the familiar names ofplant groups, such as monocotyledon, in the text.Theplant science chapters have also been revised toaccommodate requests for more detail on the princi-ples underlying horticultural practices such as graft-ing, photosynthesis and the subject of growth.Concerns about biodiversity and the interest in plantconservation are addressed along with more detailon ecology and companion planting. More examplesof plant adaptions have been provided and moreemphasis has been given to the practical applicationof plant form in the amenity use of plants. The use ofpesticides has been revised in the light of continuedregulations about their use. More details have beenincluded on the use of inert growing media such asrockwool with a substantial expansion on the use ofwater in protected culture.

For the first time there has been the use of colourto illustrate the text and the number of supportingdiagrams has been increased.There has been a deter-mination by the authors to further integrate topicsacross the whole span of the text and to provide fur-ther practical examples related to the growing ofplants in all aspects of horticulture. The indexing andkey word cross-referencing have been improved tohelp the reader integrate the subject areas and topursue related topics without laborious searching.

The text of this new edition has been amended tosupport students studying for the National Certificatein Horticulture, National Diplomas in Horticultureand the RHS General Examination in Horticulture.The knowledge and understanding acquired from thistextbook will be of value in preparation for VocationalQualifications (NVQ and SVQ) assessments up toLevel III. Furthermore, the content of the text willprovide an excellent introductory text for those study-ing the RHS Advanced Certificate and Diploma mod-ules, Higher National Diplomas and those without astrong science background embarking on FoundationDegrees. It is also intended to be a comprehensivesource of information for the keen gardener, espe-cially for those taking City and Guilds Certificate inGardening modules.

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We are indebted to the following people:Marjorie Adams and Ann Taylor have shown

tremendous patience, skill and fortitude ininterpreting our script for typing and RichardAdams for technical support.

Gill Parks has developed and printed manyof the photographs, and we are also grateful toJohn Parkin for the microscope photographs ofplant structure.

Drs J. Bridge, D. Govier and S. Dowbigginread the early drafts and provided valuable sug-gestions. Ms Marion Brown very kindly hadread the proofs.

Thanks are also due to the following individ-uals,firms and organizations that provided photo-graphs and tables:

Agricultural Lime Producers’ Association.Dr C.C. Doncaster, Rothamsted ExperimentalStation.Dr P.R. Ellis, National Vegetable ResearchStation.Mr P. Evans, Rothamsted Experimental Station.Dr D. Govier, Rothamsted ExperimentalStation.Dr M. Hollings, Glasshouse Crops ResearchInstitute.Mr Neale Holmes-SmithDr M.S. Ledieu, Glasshouse Crops ResearchInstitute.

Ministry of Agriculture, Fisheries and Food.Muntons Microplants Ltd, Stowmarket, Suffolk.Shell Chemicals.Soil Survey of England and Wales.Mr Alex TaylorDr E. Thomas, Rothamsted ExperimentalStation.

Two figures illustrating weed biology andchemical weed control are reproduced aftermodification, with permission of Drs H.A.Roberts, R.J. Chancellor and J.M. Thurston andthose illustrating the carbon and nitrogen cyclesare adapted from diagrams devised by Dr E.G. Coker who also provided the photograph of the apple tree with exposed root system.

Thanks for help in revision are due to:

Christine Atkinson; Chris Bird, SparsholtCollege; David Carey, Hadlow College;Tom Cole, Capel Manor; Stephen Dando,Brackenhurst College; Tom Deans, ReaseheathCollege; Josie Hutchinson, Brooksby College;Rita Lait, Harlow Carr; Andy Parrett, YvonneSharp, Oaklands College; John Sales; JohnSalter, Elmwood College; David Steed; HilaryThomas, Capel Manor and John Truscott, deMontford University. Particular thanks aregiven to Bob Evans for his help in the prepar-ation of the fourth edition.

Acknowledgements

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Horticulture may be described as the practice ofgrowing plants in a relatively intensive manner.This contrasts with agriculture, which, in mostwestern European countries, relies on a high levelof machinery use over an extensive area of land,consequently involving few people in the produc-tion process. However, the boundary between thetwo is far from clear, especially when consideringlarge-scale vegetable production. Horticulture ofteninvolves the manipulation of plant material, e.g. bypropagation (see Chapter 7),by changing the above-ground environment (see Chapter 2),or by changingthe root environment (see Chapter 13). There is a fundamental difference between production

horticulture, whether producing plants themselvesor plant products, and service horticulture, i.e. thedevelopment and upkeep of gardens and land-scape for their amenity, cultural and recreationalvalues. Increasingly, horticulture can be seen to be involved with social well-being and welfarethrough the impact of plants for human physicaland mental health. It encompasses environmentalprotection and conservation through large- andsmall-scale landscape design and management.Where the tending of plants for leisure movesfrom being horticulture to countryside manage-ment is another moot point. In contrast, thechange associated with replacing plants with alter-native materials, as in the creation of artificialplaying surfaces, tests what is meant by horticul-ture in a quite different way.

This book concerns itself with the principlesunderlying the growing of plants in the followingsectors of horticulture:

• Turf culture, which includes decorative lawnsand sports surfaces for football, cricket, golf, etc.

• Landscaping, garden construction and main-tenance which involves the skills of constructiontogether with the development of planted areas(soft landscaping). Closely associated with thissector is grounds maintenance, the maintenanceof trees and woodlands (arboriculture and treesurgery), specialist features within the gardensuch as walls and patios (hard landscaping) andthe use of water (aquatic gardening).

1

Horticulture in Context

The many facets of horticulture have much incommon, each being concerned with the grow-ing of plants. Despite the wide range of theindustry, embracing as it does activities fromthe preparation of a cricket square to the pro-duction of uniformly sized cucumbers, thereare common principles which guide the suc-cessful management of the plants involved.This chapter puts the industry, the plant, theplant communities and ecology into perspec-tive, and considers the aspects of conservationand organic growing and looks forward to themore detailed explanations of horticulturalpractice in the following chapters.

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• Interior landscaping is the provision of semi-permanent plant arrangements inside conser-vatories, offices and many public buildings, andinvolves the skills of careful plant selection andmaintenance.

• Protected cropping enables plant material to be supplied outside its normal availability, e.g.chrysanthemums all the year round, tomatoesto a high specification over an extended season,and cucumbers from an area where the climateis not otherwise suitable. Plant propagation,providing seedlings and cuttings, serves outdoorgrowing as well as the greenhouse industry.Protected culture, mainly using low or walk-inpolythene tunnels, is increasingly import-ant inthe production of vegetables, salads, beddingplants and flowers.

• Nursery stock is concerned with the productionof soil-grown or container-grown shrubs andtrees. Young stock of fruit may also be estab-lished by this sector for sale to the fruit growers:soft fruit (strawberries, etc.), cane fruit (rasp-berries, etc.) and top fruit (apples, pears, etc.).

• Professional gardening covers the growing ofplants in gardens including both public and pri-vate gardens and may reflect many aspects ofthe areas of horticulture described. It oftenembraces both the decorative and productiveaspects of horticulture.

• Garden centres provide plants for sale to thepublic, which involves handling plants, maintain-ing them and providing horticultural advice.A few have some production on site, but stockis usually bought in.

THE PLANT

There is a feature common to all the above aspectsof horticulture: the grower or gardener benefitsfrom knowing about the factors that may increaseor decrease the plant’s growth and development.The main aim of this book is to provide an under-standing of how these factors contribute to theideal performance of the plant in particular cir-cumstances. In most cases this will mean optimumgrowth, as in the case of a salad crop such as lettuce

where a fast turnover of the crop with once overharvesting that grades out well. However, the aimmay equally be restricted growth, as in the pro-duction of dwarf chrysanthemum pot plants or inthe case of a lawn that would otherwise requirefrequent cutting. The main factors to be con-sidered are summarized in Figure 1.1, which shows where in this book each is discussed.

It must be stressed that the incorrect function-ing of any one factor may result in undesired plantperformance. It should also be understood thatfactors such as the soil conditions, which affect theunderground parts of the plant, are just as import-ant as those such as light, which affect the aerialparts.The nature of soil is dealt with in Chapter 13.Increasingly, plants are grown in alternatives tosoil such as composts and rockwool and these arereviewed in Chapter 17.

Weather generally plays an important part in horticulture.It is not surprising that those involved ingrowing plants have such a keen interest in weatherforecasting in order to establish whether conditionsare suitable to work in or because of the direct effectof temperature, water and light on the growth ofplants. The climate is dealt with in Chapter 2, whichalso gives particular attention to the microclimate(the environment the plant actually experiences).

A single plant growing in isolation with no com-petition is as unusual in horticulture as it is innature. However, specimen plants such as leeks,marrows and potatoes, lovingly reared by enthusi-asts looking for prizes in local shows, grow toenormous sizes when freed from competition. Inlandscaping, specimen plants are placed awayfrom the influence of others so that they not onlystand out and act as a point of focus, but also canattain perfection of form. A pot plant such as afuchsia is isolated in its container, but the influ-ence of other plants, and the consequent effect onits growth, depend on spacing. Generally, plantsare to be found in groups, or communities.

PLANT COMMUNITIES

Neighbouring plants can have a significant effecton each other since there is competition for factors

2 Principles of Horticulture

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such as root space, nutrient supply and light. As innatural plant communities, some of the effects canbe beneficial whilst others are detrimental to theachievement of horticultural objectives.

Single species communities

When a plant community is made up of onespecies it is referred to as a monoculture. On afootball field there may be only ryegrass (Loliumspecies) with all plants closely spaced just a fewmillimetres apart. Each plant species, whethergrowing in the wild or in the garden, may be con-sidered in terms of its own characteristic spacingdistance (or plant density).

In a decorative border, the bedding plantAlyssum will be spaced at 15 cm intervals, whereasa Pelargonium plant may require 45 cm betweenplants. For decorative effect, the larger plants are

normally placed towards the back of the borderand at a wider spacing.

In a field of potatoes, the plant spacing will becloser within the row (40 cm) than between therows (70 cm) so that suitable soil ridges can beproduced to encourage tuber production, andmachinery can pass unhindered along the row.

In nursery stock production, small trees are oftenplanted in a square formation with a spacing idealfor the plant species, e.g. the conifer Chamaecyparisat 1.5 m. The recent trend in producing commercialtop fruit, e.g. apples, is towards small trees (usingdwarf rootstocks) in order to produce manageableplants with easily harvested fruit. This has resultedin spacing reduced from 6 to 4 m.

A correct plant-spacing distance is that mostlikely to provide the requirements shown inFigure 1.1 at their optimum level. Too much com-petition for soil space by the roots of adjacentplants, or for light by their leaves, would quickly

Horticulture in Context 3

MicroclimateChapter 2

LightChapters 2, 6 and 7

Harmful substancesChapters 6, 12 and 13

PestsChapters 10 and 12

DiseasesChapters 11 and 12

Soil organismsChapters 10,11 and 15

Selected plant materialChapters 3, 4, 7 and 8

TemperatureChapters 2, 6 and 7

WeedsChapters 9 and 12

OxygenChapters 6, 13 and 17

pH and nutrientsChapters 5 and 16

SeedChapters 7 and 8

WaterChapters 2, 5 and 14

Growing mediaChapters 13 and 17

Figure 1.1 The requirements of the plant for the healthy growth and development.

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lead to reduced growth.Three ways of overcomingthis problem may be seen in the horticulturist’sactivities of transplanting seedlings from trays intopots, increasing the spacing of pot plants in green-houses, and hoeing out a proportion of young vege-table seedlings from a densely sown row. Aninteresting horticultural practice, which reducesroot competition, is the deep-bed system, in whicha 1 m depth of well-structured and fertilized soilenables deep root penetration. However, growersoften deliberately grow plants closer to restrictgrowth in order to produce the correct size andthe desired uniformity as in the growing of carrotsfor the processing companies.

Whilst spacing is a vital aspect of plant growth,it should be realized that the grower might need toadjust the physical environment in one of manyother specific ways in order to favour a chosenplant species.This may involve the selection of thecorrect light intensity; a rose, for example, whetherin the garden, greenhouse or conservatory, willrespond best to high-light levels, while a fern willgrow better in low light.

Another factor may be the artificial alteration ofday length, as in the use of ‘black-outs’ and cycliclighting in the commercial production of chrysan-themums to induce flowering. Correct soil acidity(pH) is a vital aspect of good growing: heathers prefer high acidity, whilst saxifrages grow moreactively in non-acid (alkaline) soils. Soil texture, e.g.on golf greens may need to be adjusted to a loamysand type at the time of green preparation in orderto reduce compaction and maintain drainage.

Each crop species has particular requirements,and it requires the skill of the horticulturist tobring all these together. In greenhouse produc-tion, sophisticated control equipment may moni-tor air and root-medium conditions every fewminutes, in order to provide the ideal day andnight requirements.

Competition between species

The subject of ‘ecology’ deals with the interrela-tionship of plant (and animal) species and their

environment. Below are described some of the eco-logical terms and concepts which most commonlyapply to the natural environment, where humaninterference is minimal. It will be seen, however,that such concepts have relevance to horticulture,with its more controlled environment.

Habitat. This term refers to the place where aplant or animal lives. For a water lily, its only habi-tat is a pond or a slow-moving river. In contrast, aspecies such as a blackberry may be found in morethan one habitat, e.g. heathland, woodland and inhedges. The common rat, often associated withhumans, is seen in various habitats (e.g. farms,sewers, hedgerows and food stores).

On a smaller scale, the term microhabitat isused to pinpoint a particular part of a plant or soilwhere a particular plant, or a small organism occurs.The glasshouse whitefly occupies the under-leafmicrohabitat of a Fuchsia plant. The wilt fungusVerticillium albo-atrum lives in the xylem micro-habitat of plants.

Niche. Given the dynamic way that plants andanimals grow in size and numbers, and competeagainst each other, it is not surprising to find thateach species of plant or animal has an ideal loca-tion for its best growth and survival (this location is called its niche). The term ‘niche’ carries with itan idea of the specialization that a species mayexhibit within a community of other plants andanimals.A niche involves, for plants, such factors astemperature, light intensity, humidity, pH, nutrientlevels, etc. For animals such as pests and their pred-ators, there are also factors such as preferred foodand chosen time of activity determining the niche.

The term is rather hard to apply in an exact way,since each species shows a certain tolerance of thefactors mentioned above, but it is useful in empha-sizing specialization within a habitat. The biolo-gist, Gause, showed that no two species can existtogether if they occupy the same niche. Onespecies will, sooner or later, start to dominate.

For the horticulturalist, here is the importantconcept that for each species planted in the ground,there is an ideal combination of factors to be con-sidered if the plant is to grow well. Although thisconcept is an important one, it should not be taken


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to an extreme. Most plants tolerate a range of con-ditions, but the closer the grower gets to the ideal,the more likely they are to establish a healthy plant.

Biome. This term refers to a wider grouping oforganisms than that of a habitat. As with the termhabitat, the term biome is biological in emphasis,concentrating on the species present.This is in con-trast to the wider ecosystem concept describedbelow. Commonly recognized biomes would be‘temperate woodland’, ‘tropical rainforest’, ‘desert’,‘alpine’ and ‘steppe’. About 35 types of biomes arerecognized worldwide, the classification being basedlargely on climate; on whether they are land basedor water based; on geology and soil; and on altitudeabove sea level. Each example of a biome will havewithin it many habitats. Different biomes may becharacterized by markedly different potential forannual growth. For example, a square metre of temperate-forest biome may produce about 10 timesthe growth of an alpine biome.

Ecosystem. This term brings emphasis to boththe community of living organisms and to theirnon-living environment. Examples of ecosystemsare a wood, a meadow, a chalk hillside, a shorelineand a pond. Implicit within this term (unlike theterms habitat, niche and biome) is the idea of awhole integrated system, involving both the living(biotic) plant and animal species, and the non-living(abiotic) units such as soil and climate, all reactingtogether within the ecosystem. Ecosystems can bedescribed in terms of their energy flow, showinghow much light is stored (or lost) within the systemas plant products such as starch (in the plant) or as organic matter (in the soil). Also, the severalother systems such as carbon, nitrogen and sulphurcycles (see pages 186–8), and water conservation(see page 182) may also be presented as features ofthe ecosystem in question.

Succession. Communities of plants and animalschange with time. The species composition willchange as will the number of individuals withineach species. This process of change is known as‘succession’. Two types of succession are recog-nized.The first one, known as primary succession isseen in a situation of uncolonized ground. Sanddunes, disused quarries and landslide locations are

good examples. This process runs in parallel withthe formation of soils (see page 152). It can be seenthat plant and animal species from outside the newhabitat will be the ones involved in colonization.

The second (and more common example inBritain) is secondary succession, where a barehabitat is formed after vegetation has been burnt,or chopped down, or covered over with a flood siltdeposit. In this situation, there will often be plantseeds and animals which survive under the barrensurface to begin colonization again, by bringing topsoil to the surface, or at least some of its associatedbeneficial bacteria and other micro-organisms.

The first species to establish are aptly called the‘pioneer community’. In felled woodland, thesemay well be mosses, lichens, ferns and fungi. In con-trast, a drained pond will probably have Sphagnummoss, reeds and rushes, more at home in this wetterhabitat.

The second succession stage will see plants suchas grasses, foxgloves and willow herb taking overin the ex-woodland area. Grasses and sedges arethe most common examples seen in the drainedpond. Such early colonizing species are sometimesreferred to as opportunistic. They often have simi-lar characteristics to horticultural weeds (see page88), viz. extended seed germination period, rapidplant establishment, short time to maturity andconsiderable seed production. They quickly coverover the previously bare ground.

The third succession stage involves larger plants,which, over a period of about 5 years, graduallyreduce the opportunists’ dominance. Honeysuckle,elder, and bramble are often species that appear inex-woodland, whilst willows and alder occupy asimilar position in the drained pond. The termcompetitive is applied to such species.

The fourth stage introduces tree species thathave the potential to achieve considerable heights.It may well happen that both the ex-woodland andthe drained pond situation end up with the sametree species such as birch, oak and beech.These aredescribed as climax species, and will dominate thehabitat for a long time so long as it remains undis-turbed, by natural or human forces. Within the climax community, there often remain some


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specimens of the preceding succession stages, butthey are now held in check by the ever-larger trees.

This short discussion of succession has empha-sized the plant members of the community.As suc-cession progresses along the four stages described,there is usually an increase in biodiversity, i.e.increase in numbers of plant species. It should alsobe borne in mind that, for every plant speciesthere will be several animal species dependent onit for food, and thus succession brings biodiversityin the plant, animal, fungal and bacterial realms.

Succession to the climax stage is often quite rapid,occurring within 20 years from the occurrence ofthe bare habitat. Once established, a climax com-munity of plants and animals in a natural habitatwill usually remain quite stable for many years.

Food chains

At any one stage along the succession sequence ina habitat, there will be a particular combination ofliving things (organisms) associated with the plantcommunity. In a crop situation, e.g. strawberries,the crop plant itself is the main source of energyfor the other organisms, and is referred to, alongwith any weeds present, as the primary producer inthat habitat. Any pest (e.g. aphid) or disease (e.g.mildew) feeding on the strawberries is termed aprimary consumer, whilst a ladybird eating theaphid is called a secondary consumer. A habitatmay include also tertiary, quaternary consumers,etc. Any combination of species such as the aboveis referred to as a food chain and each stage withina food chain is called a trophic level: e.g.

strawberry → aphids → ladybird

In the pond habitat, a comparable food chainwould be

green algae → Daphnia crustacean →minnow fish

Within any plant community, there will be com-parable food chains to the one described above. Itis normally observed that in a monoculture such asstrawberry, there will be a relatively short period of

time (up to 5 years) for a complex food chain todevelop. However, in a long-term stable habitatsuch as oak woodland or a mature garden-growingperennial, there will be many plant (primary pro-ducer) species, allowing many food chains to occur.Furthermore, primary consumer species, e.g. cater-pillars and pigeons, may be eating from several dif-ferent plant types, whilst secondary consumerssuch as predatory beetles and tits will be devouringa range of primary consumers on several plantspecies. In this way, a more complex, intercon-nected community is developed, called a food web.An interesting feature of succession is that, as timepasses, the habitat acquires a greater diversity ofspecies, and more complex food webs, including theimportant rotting organisms such as fungi whichbreak down ageing and fallen trees. Effectivecountryside management particularly utilizes thesefood webs, and succession principles when strikinga balance between the production of species diver-sity and the maintenance of an acceptably orderlymanaged area.

Decomposers

At this point, the whole group of organismsinvolved in the recycling of dead organic matter(called decomposers or detritovores) should bementioned in relation to the food-web concept.Theorganic matter (see also Chapter 15) derived fromdead plants and animals of all kinds is digested by asuccession of species: large animals by crows, largetrees by bracket fungi, small insects by ants, rootsand fallen leaves by earthworms, mammal and birdfaeces by dung beetles, etc. Subsequently, progres-sively small organic particles are consumed by millipedes, springtails, mites, nematodes, fungi andbacteria to eventually create the organic moleculesof humus that are so vital a source of nutrients, anda means of soil stability in most plant-growth situ-ations. It can thus be seen that although decomposersdo not normally link directly to the food web; theyare often eaten by secondary consumers. They alsoare extremely important in supplying inorganicnutrients to the primary producer plant community.


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Sustainable development

From the content of preceding paragraphs, it canbe seen that the provision of as extensive a systemof varied habitats, each with its complex food web,in as many locations as possible, is increasinglybeing considered desirable in a nation’s environ-ment provision. In this way, a wide variety ofspecies numbers (biodiversity) is maintained,habitats are more attractive and species of poten-tial use to mankind are preserved. In addition,a society that bequeaths its natural habitats and ecosystems to future generations in an accept-ably varied, useful and pleasant condition, is con-tributing to the sustainable development of thatnation.

Biomass

At any one time in a habitat, the amount of livingplant and animal tissue (biomass) can be meas-ured or estimated. In production horticulture, it isclearly desirable to have as close to 100 per cent ofthis biomass in the form of the primary producer(crop), with as little primary consumer (pest ordisease) as possible present. On the other hand, ina natural woodland habitat, the primary producerwould represent approximately 85 per cent of thebiomass, the primary consumer 3 per cent, the sec-ondary consumer 0.1 per cent and the decomposers12 per cent. This weight relationship between dif-ferent trophic levels in a habitat (particularly thefirst three) is often summarized in graphical formas the ‘pyramid of species’.

A further concept relevant to the plant–animalrelationships relates to energy. The process ofphotosynthesis enables the plant to retain, aschemical energy, approximately 1 per cent of thesun’s radiant energy falling on the particular leaf’ssurface. As the plant is consumed by primary consumers, approximately 90 per cent of the leafenergy is lost from the biomass, either by respirationin the primary consumer, by heat radiation fromthe primary consumer’s body or as dead organicmatter excreted by the primary consumer. This

organic matter, when incorporated in the soil,remains usefully within the habitat.

The relative levels in a habitat of its total biomassas against its total organic matter are an importantfeature. This balance can be markedly affected byphysical factors such as soil type, by climatic factorssuch as temperature, rainfall and humidity, and canalso be affected by the management system operat-ing in that habitat (or ecosystem). For example, atemperate woodland on ‘heavy’ soil with 750 mmannual rainfall will maintain a relatively large soilorganic matter content, permitting good nutrientretention, good water retention and resisting soilerosion even under extreme weather conditions.For these reasons, the habitat is seen to be relativelystable. On the other hand, a tropical forest on a‘light’ soil with 3000 mm rainfall will have a muchsmaller soil organic matter reserve, with most of itscarbon compounds being used in the living plantsand animal tissues. As a consequence, nutrient andmoisture retention and resistance to soil erosionare usually low; serious habitat loss can result whenwind damage or human interference occurs. Fortemperate horticulturists, the main lesson to keepin mind is that high levels of soil organic matter areusually highly desirable, especially in sandy soilsthat readily lose organic matter.

Garden considerations

When contemplating the distribution of ourfavourite species in the garden (ranging from tinyannuals to huge trees), a thought may be given totheir position in the succession process back in thenatural habitat of their country of origin. Some willbe species commonly seen to colonize bare habi-tats. Most garden species will fall into the middlestages of succession. A few, whether they be trees,climbers or low-light-requirement annuals orperennials will be species of the climax succession.

The garden border contains plant species, whichcompete aggressively in their native habitat. Theartificial interplanting of species from differentparts of the world (the situation found in almostall gardens), may give rise to unexpected results as


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this competition continues year after year. Suchexperiences are part of the joys, and the heartachesof gardening.

Companion planting

An increasingly common practice in some areas ofhorticulture (usually in small-scale situations) isthe deliberate establishment of two or more plantspecies in close proximity with the intention ofderiving some cultural benefit from their associ-ation. Such a situation may seem at first sight toencourage competition rather than mutual bene-fit. Many supporters of companion planting replythat plant and animal species, in the natural worldshow more evidence of mutual cooperation thanof competition.

Some experimental results have given supportto the practice, but most evidence remains anec-dotal. It should be stated, however, that whilst mostcommercial producers in western Europe growblocks of a single species, in many other parts ofthe world two or three different species are inter-planted as a regular practice.

Several biological mechanisms are quoted insupport of companion planting:

• Nitrogen fixation. Legumes such as beans con-vert atmospheric nitrogen to useful plant nitro-genous substances (see page 188) by means ofRhizobium bacteria in their root nodules.Beans interplanted with maize are claimed toimprove maize’s growth by increasing its nitro-gen uptake.

• Pest suppression. Some plant species are claimedto deter pests and diseases. Some examples arelisted. Onions, sage and rosemary release chem-icals that mask the carrot crop’s odour thusdeterring the most serious pest, carrot fly frominfesting the carrot crop. African marigolds(Tagetes) deter glasshouse whitefly and soil-borne nematodes by means of the chemical,thiophene. Wormwood (Artemisia) releasesmethyl jasmonate as vapour that reduces cater-pillar feeding, and stimulates plants to resist diseases such as rusts. Chives and garlic reduceaphid attacks.

• Beneficial habitats. Some plant species present auseful refuge for beneficial insects (see page 139),such as ladybirds, lacewings and hoverflies. In thisway, companion planting may preserve a suffi-cient level of these predators and parasites toeffectively counter pest infestations. The follow-ing examples may be given: carrots attract lace-wings; yarrow (Achillea), ladybirds; goldenrod(Solidago), small parasitic wasps; poached-eggplant (Limnanthes) attracts hover flies. In addi-tion, some plant species can be considered astraps for important pests.Aphids are attracted tonasturtiums, flea beetles to radishes thus keepingthe pests away from a plant such as cabbage.

• Spacial aspects. A pest or disease, specific to aplant species will spread more slowly if the dis-tance between individual plants is increased.Companion planting achieves this goal. Forexample, potatoes interplanted with cabbageswill be less likely to suffer from potato blightdisease. The cabbages similarly would be lesslikely to be attacked by aphid.

ORGANISMS OCCURRING ON PLANT SURFACES

A further aspect of species interaction can be seenat the microscopic level in the soil and on the aer-ial parts of plants. The plant surfaces of leaves,stems and roots present an environment for bene-ficial and for damaging organisms. The lattergroup are described in Chapter 11.

Research has indicated the complex microbialcomposition of plant surfaces and their importanceto successful plant growth.The term rhizosphere isused to describe the environment for bacteria,fungi, mites and nematodes situated around theroot, whilst the comparable term phyllosphereapplies to the environment on the leaf and stem.

Rhizosphere

The term ‘rhizosphere’ refers to the environmentclosely adjacent to roots. It is relatively stablecompared to the leaf environment, particularly in


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terms of temperature and humidity. The compon-ents of this zone (soil mineral particles, water,gases, organic matter and micro-organisms) inter-act to influence root activity. The area of root nearthe tip produces numerous root hairs, which areimportant for absorption of water and nutrients.In the area behind this tip zone, however, there isoften a complex association of micro-organismswhose contribution to root activity may be equalto that of the root hairs. The most striking mem-bers of this community are the fungi that developclose associations with the root tissues, ofteninvading the root, but not damaging it.These fungiare known as mycorrhizae.The tiny strands spreadout into the surrounding soil and act as an add-itional absorption system for the plant.

Most plant families have been shown to utilizemycorrhizae. In some plant families, e.g. Rosaceae,there is a well-developed network of tiny fungalstrands (see Hyphae, page 122) inside the roots,whereas in others, such as the Ericaceae (heathers),the hyphae develop in masses outside but very closeto the root surface. Some mycorrhizae belong to the group of fungi that produces toadstools (the Basidiomycota group). The toadstool species areoften quite specific to the plant on which they havean association. For example, the fly agaric (Amanitaregalis), the red toadstool well known in fairy tales, isa mycorrhizal fungus found mainly on spruce roots.

Mycorrhizae have been shown in many speciesto have important roles in absorbing water andnutrients such as phosphate and nitrate. Phosphateis the least mobile major nutrient in the soil, andthe plant must ‘reach out’ to find it (see page 203).Mycorrhizae help plants to do this. An extremecase is the heather (Erica) genus which is oftenfound growing in acidic soils, where phosphate isinsoluble, where root growth is limited, and wherethere are very few bacteria involved in the break-down of organic matter. Under these conditions,there is a special need for mycorrhizae. Whilst themycorrhizae are responsible for assisting the root’sfunction, there is a reciprocal process whereby theplant, in return, provides the fungus with sugarsand other organic substances favourable for itsgrowth. This plant–fungus association is a goodexample of symbiosis (see page 186).

The use of sterile, compost-based growingmedia for propagation may deny the young plantsa source of mycorrhizal fungi. Consequently, spe-cially prepared cultures of these fungi are some-times introduced into growing media to stimulateplant growth, e.g in the production of youngconifers. The particular requirements of the myc-orrhizae have to be met where they are importantfor the plants success, e.g. many orchids are grownin translucent pots in order to provide for the lightrequirement of the species of fungus concerned.

Phyllosphere

Phyllosphere bacteria on the leaf may be ‘casual’or ‘resident’. Casual species, e.g. Bacillus mainlyarrive from soil, roots and water, and are more com-mon on the leaves close to the ground.These speciesare capable of rapid increase under favourableconditions, but then may decline. Resident species,e.g. Pseudomonas, may be weakly parasitic onplants, but more commonly persist (often for con-siderable periods) without causing damage, andon a wide variety of plants.

There is increasing evidence that phyllospherebacteria may reduce the infection of diseases suchas powdery mildews, Botrytis diseases on lettuceand onion, and turfgrass diseases. Practical diseasecontrol strategies by phyllosphere organisms havenot been developed, but there remains the generalprinciple that a healthy, well-nourished plant willbe more likely to have organisms on the leaf surface available to reduce fungal infection.

CONSERVATION

The ecological aspects of horticulture have beenhighlighted in recent years by the conservationmovement. One aim is to promote the growing ofcrops and maintaining of wildlife areas in such away that the natural diversity of wild species ofboth plants and animals is maintained alongsidecrop production, with a minimum input of fertil-izers and pesticides. Major public concern hasfocused on the effects of intensive production


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(monoculture) and the indiscriminate use by horticulturists and farmers of pesticides and quick-release fertilizers.

An example of wildlife conservation is the con-version of an area of regularly mown and ‘weed-killed’ grass into a wild flower meadow, providingan attractive display during several months of theyear. The conversion of productive land into wildflower meadow requires lowered soil fertility (inorder to favour wild species establishment andcompetition), a choice of grass seed species withlow opportunistic properties, and a mixture ofselected wild flower seed. The maintenance of thewild flower meadow may involve harvesting thearea in July, having allowed time for natural flowerseed dispersal. After a few years, butterflies andother insects become established as part of thewild flower habitat.

The horticulturist has three notable aspects ofconservation to consider. Firstly, there must be nowilful abuse of the environment in horticulturalpractice. Nitrogen fertilizer used in excess hasbeen shown, especially in porous soil areas, to bewashed into streams, since the soil has little abilityto hold on to this nutrient (see page 202). Thepresence of nitrogen in watercourses encouragesabnormal multiplication of micro-organisms (mainlyalgae). On decaying, these remove oxygen sourcesneeded by other stream life; particularly fish (aprocess called ‘eutrophication’).

Secondly, another aspect of good practiceincreasingly expected of horticulturists is the intel-ligent use of pesticides.This involves a selection ofthose materials least toxic to man and beneficialanimals, and particularly excludes those materialsthat increase in concentration along a food chain.Lessons are still being learned from the wide-spread use of dichlorodiphenyltrichloroethane(DDT) in the 1950s. Three of DDT’s propertiesshould be noted. Firstly, it is long lived (residual)in the soil. Secondly, it is absorbed in the bodies ofmost organisms with which it comes into contact,being retained in the fatty storage tissues. Thirdly,it increases in concentration approximately 10times as it passes to the next member of the foodchain.As a consequence of its chemical properties,

DDT was seen to achieve high concentrations inthe bodies of secondary (and tertiary) consumers,such as hawks, influencing the reproductive rateand hence causing a rapid decline in their num-bers in the 1960s. This experience rang alarm bellsfor society in general, and DDT was eventuallybanned in most of Europe.

The irresponsible action of allowing pesticidespray to drift onto adjacent crops, woodland orrivers has decreased considerably in recent years.This has in part been due to the Food and Envir-onment Protection Act (FEPA) 1985, which hashelped raise the horticulturist’s awareness of conservation.

A third aspect of conservation to consider is thedeliberate selection of trees, features and areaswhich promote a wider range of appropriatespecies in a controlled manner.A golf course man-ager may set aside special areas with wild flowersadjacent to the fairway, preserve wet areas andplant native trees. Planting bush species such ashawthorn, field maple and spindle together in ahedgerow provides variety and supports a mixedpopulation of insects for cultural control of pests.Tit and bat boxes in private gardens, an increas-ingly common sight, provide attractive homes forspecies that help in pest control. Continuoushedgerows will provide safe passage for mammals.Strips of grassland maintained around the edgesof fields form a habitat for small mammal speciesas food for predatory birds such as owls. Gardenerscan select plants for the deliberate encouragementof desirable species (nettles and Buddleia for but-terflies; Rugosa roses and Cotoneaster for winterfeeding of seed-eating birds; poached-egg plantsfor hoverflies).

It is emphasized that the development andmaintenance of conservation areas require con-tinuous management and consistent effort to main-tain the desired balance of species and requiredappearance of the area. As with gardens andorchards, any lapse in attention will result in inva-sion by unwanted weeds and trees.

In a wider sense, the conservation movement isaddressing itself to the loss of certain habitats andthe consequent disappearance of endangered


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species such as orchids from their native areas.Horticulturists are involved indirectly becausesome of the peat used in growing media is takenfrom lowland bogs much valued for their rich var-iety of vegetation. Considerable efforts have beenmade to find alternatives to peat in horticulture(see page 213) and protect the wetland habits ofthe British Isles.

Conservationists also draw attention to thethoughtless neglect and eradication of wild-ancestorstrains of present-day crops; the gene-bank onwhich future plant breeders can draw for furtherimprovement of plant species.There is also concernabout the extinction of plants especially those on the margins of deserts that are particularly vul-nerable if global warming leads to reduced water supplies. In situ conservation mainly applies to wildspecies related to crop plants and involves the cre-ation of natural reserves to protect habitats such aswild apple orchards and there is particular interestin preserving species with different ecological adaptions. Ex situ conservation includes whole plant collections in botanic gardens, arboreta, pineta and gene-banks where seeds, vegetative materials and tissue cultures are maintained. The botanic gardens are coordinated by the Botanic GardensConservation International (BGCI), which is basedat Kew Gardens, London, and are primarily con-cerned with the conservation of wild species.

Large national collections include the NationalFruit Collection at Brogdale, Kent (administeredby Wye College) and the Horticultural ResearchInternational at Wellesbourne, Birmingham, holdsvegetables. The Henry Doubleday Heritage SeedScheme conserves old varieties of vegetableswhich were once commercially available but whichhave been dropped from the National List (and sobecome illegal to sell). They encourage theexchange of seeds. The National Council for theConservation of Plants and Gardens (NCCPG)was set up by the Royal Horticultural Society atWisley in 1978 and is an excellent example of pro-fessionals and amateurs working together to con-serve stocks of extinction-threatened garden plants,to ensure the availability of a wider range of plantsand to stimulate scientific, taxonomic, horticultural,

historical and artistic studies of garden plants.There are over 600 collections of ornamental plantsencompassing 400 genera and some 5000 plants. Athird of these are maintained in private gardensbut many are held in publicly funded institutionssuch as colleges, e.g. Sarcococca at Capel ManorCollege in North London, Escallonia at the DuchyCollege in Cornwall, Penstemon and Philadelphusat Pershore College and Papaver orientale at theScottish Agricultural College, Auchincruive. Rareplants are identified and classified as ‘pink sheet’plants.

ORGANIC GROWING

The organic movement broadly believes that cropsand ornamental plants should be produced with aslittle disturbance as possible to the balance ofmicroscopic and larger organisms present in thesoil, and also in the above-soil zone.This stance canbe seen as closely allied to the conservation pos-ition, but with the difference that the emphasishere is on the balance of micro-organisms. Organicgrowers maintain soil fertility by the incorporationof animal manures, or green manure crops such asgrass–clover leys. The claim is made that cropsreceive a steady, balanced release of nutrientsthrough their roots; in a soil where earthwormactivity recycles organic matter deep down, theresulting deep root penetration allows an effectiveuptake of water and nutrient reserves.

The use of most pesticides and quick-releasefertilizers is said to be the main cause of speciesimbalance, and formal approval for licensedorganic production may require soil to have beenfree from these two groups of chemicals for atleast 2 years. Control of pests and diseases isachieved by a combination of resistant cultivarsand ‘safe’ pesticides derived from plant extracts,by careful rotation of plant species, and by the useof naturally occurring predators and parasites.Weeds are controlled by mechanical and heat-producing weed-controlling equipment, and by theuse of mulches. The balanced nutrition of the cropis said to induce greater resistance to pests and


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diseases, and the taste of organically grown food isclaimed to be superior to that of conventionallygrown produce.

The organic production of food and non-ediblecrops at present represents about 5 per cent of theEuropean market. The European CommunityRegulations (1991) on the ‘organic production ofagricultural products’ specify the substances thatmay be used as ‘plant-protection products, deter-gents, fertilizers or soil conditioners’ (see pages148 and 194). ‘Conventional horticulture’ is, thus,still by far the major method of production andthis is reflected in this book. However, it should berealized that much of the subsistence croppingand animal production in the Third World couldbe considered ‘organic’.

FURTHER READING

Allaby, M. Concise Oxford Dictionary of Botany(Oxford University Press, 1992).

Ayres, A. Gardening without Chemicals (Which/Hodder & Stoughton, 1990).

Baines, C. and Smart, J. A Guide to Habitat Creation(Packard Publishing, 1991).

Blake, F. Organic Farming and Growing (CrowoodPress, 1987).

Carson, R. Silent Spring (Hamish Hamilton, 1962).Carr, S. and Bell, M. Practical Conservation (Open

University/Nature Conservation Council, 1991).Caplan, B. Complete Manual of Organic Gardening

(Headline Publishing, 1992).Dowdeswell, W.H. Ecology Principles and Practice

(Heinemann Educational Books, 1984).Innis, D.Q. Intercropping and the Scientific Basis of

Traditional Agricuture (Intermediate TechnologyPublications, 1997).

Lampkin, N. Organic Farming (Farming Press, 1990).Lisansky, S.G., Robinson, A.P. and Coombs, J. Green

Growers Guide (CPL Scientific Ltd, 1991).Mannion, A.M. and Bowlby (Editors). Environmental

Issues in the 1990s (John Wiley & Sons, 1992).Tait, J. et al. Practical Conservation (Open University,

1988).


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THE SUN’S ENERGY

The energy that drives our weather systems comesfrom the sun in the form of solar radiation. Thesun radiates waves of electro-magnetic energy and

high-energy particles into space.This type of energycan pass through a vacuum and through gases.TheEarth intercepts the radiation energy and, as theseenergy waves pass through the atmosphere, theyare absorbed, scattered and reflected by gases, airmolecules, small particles and cloud masses (seeFigure 2.1).

About a quarter of the total radiation enteringthe atmosphere reaches the Earth’s surface directly.Another 18 per cent arrives indirectly after beingscattered (diffused). The surface is warmed as themolecules of rock, soil and water at the surfacebecome excited by the incoming radiation; theenergy in the electro-magnetic waves is convertedto heat energy as the surface material absorbs theradiation. A reasonable estimate of energy can becalculated from the relationship between radi-ation and sunshine levels.The amounts received inthe British Isles are shown in Figure 2.2, where the differences between winter and summer areillustrated.

However, the nature of the surface has a signifi-cant effect on the proportion of the incoming radi-ation that is absorbed. The sea can absorb over 90 per cent of radiation when the sun is overhead,whereas for land it is generally between 60 and 90 per cent. Across the Earth darker areas tend toabsorb more energy than lighter ones; dark soilswarm up more quickly than light ones; afforestedareas more than lighter, bare areas with grass inbetween. Where the surface is white (ice or snow)nearly all the radiation is reflected.

2

Climate and Microclimate

Sunlight, temperature, rainfall, humidity, frostand wind affect plant growth directly by affect-ing its physiology or indirectly through influ-encing working conditions. In general, growerseither seek to modify conditions to enable themto grow the plants of their choice, or selectplants that will grow in a given location.

This chapter explains how weather systemsare created and outlines the climate of theBritish Isles including the nature of the growingseason.This is contrasted with some of the maintypes of climate from which plants have beencollected. Factors such as altitude, topographyand proximity to large bodies of water thatmodify the general climate to produce the localclimates are described. The microclimate thatthe plant actually experiences is described andmethods of modifying conditions are discussed.

The chapter ends by looking at the meas-urement of temperature, humidity, rainfall,wind and sunshine and how the automationof environmental control in protected crop-ping is achieved.

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Effect of latitude

Over the Earth’s surface some areas becomewarmed more than others because of the differ-ences in the quantity of radiation absorbed. Mostenergy is received around the Equator where thesun is directly overhead and the radiation hits thesurface at a right angle. In higher latitudes, such asthe UK, more of the radiation is lost as it travelsfurther through the atmosphere. Furthermore theenergy waves strike the ground at an acute angleleading to a high proportion being reflectedbefore affecting the molecules at the surface (seeFigure 2.3).

As a consequence of the above, more energy isreceived than lost over the span of a year in theregion on either side of the Equator between the

Tropic of Capricorn and Tropic of Cancer. In con-trast, to the north and south of these areas moreenergy radiates out into space, which would leadto all parts of this region becoming very cold.However, air and water (making up the Earth’satmosphere and oceans) are able to redistributethe heat.

Movement of heat and weather systems

Heat energy moves from warmer areas (i.e. thoseat a higher temperature) into cooler areas (i.e. thoseat a lower temperature), and there are three typesof energy movement involved. Radiation energymoves efficiently through air (or a vacuum), butnot through water or solids. Heat is transferred fromthe Earth’s surface to the lower layers by conduc-tion. As soil surfaces warm up in the spring, tem-peratures in the lower layers lag behind, but this isreversed in the autumn as the surface cools andheat is conducted upwards from the warmer lowerlayers. At about 1 m down the soil temperaturetends to be the same all the year round (about 10°Cin lowland Britain).

Heat generated at the Earth’s surface is alsoavailable for redistribution into the atmosphere.However, air is a poor conductor of heat (whichexplains its usefulness in materials used for insula-tion such as polystyrene foam, glass fibre andwool).This means that, initially, only the air imme-diately in contact with the warmed surface gainsenergy. Although the warming of the air layersabove would occur only very slowly by conduction,it is the process of convection that warms the atmos-phere above. As fluids are warmed they expand,take up more room and become lighter.Warmed airat the surface becomes less dense than that above,so air begins to circulate with the lighter air risingand the cooler denser air falling to take its place;just as like a convector heater warming up a room.This circulation of air is referred to as wind.

In contrast, the water in seas and lakes iswarmed at the surface making it less dense whichtends to keep it near the surface. The lower layersgain heat very slowly by conduction and generally


radiationentering the

Earth’s atmosphere

absorbed

25% 20% 25% 25%

reflected backfrom clouds andatmospheric particles

absorbed byclouds andatmospheric particles

scattered (diffuse)and reaches surfaceindirectly

reflected

Figure 2.1 Radiation energy reaching the Earth’s surfaceshowing the proportions that are reflected back andabsorbed as it passes through the atmosphere and thatwhich reaches plants indirectly. About 5 per cent of theradiation strikes the Earth’s surface but is reflected back(this is considerably more if the surface is light coloured,e.g. snow, and as the angle of incidence is increased).

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Climate and Microclimate 15

1.5

1.5

2.0

2.0

2.01.5

2.5

2.5

3.0

3.0

(a)

15

15

17

17

17

19

19

17

(b)

Figure 2.2 Radiation received in the British Isles; mean daily radiation given in megajoules per metre square:(a) January and (b) July.

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depend on gaining heat from the surface by turbu-lence. Large-scale water currents are created bythe effect of tides and the winds blowing over them.

On a global scale, the differences in temperatureat the Earth’s surface lead to our major weather

systems.Convection currents occur across the worldin response to the position of the hotter and colderareas and the influence of the Earth’s spin (theCoriolis Effect). These global air movements,known as the trade winds, set in motion the seacurrents, which follow the same path but are modi-fied as they are deflected by the continental landmasses (see Figure 2.4).

WEATHER AND CLIMATE

Weather is the manifestation of the state of theatmosphere. Plant growth and horticultural oper-ations are affected by weather; the influence of rainand sunshine is very familiar, but other factors suchas frost, wind and humidity have important effects.It is not surprising that growers usually have a keeninterest in the weather and often seek to modifyits effect on their plants.Whilst most people dependon the public weather forecasting, some growers areprepared to pay for extra information and othersbelieve in making their own forecasts, especially iftheir locality tends to have different weather from


Earth’s atmosphere

The Earth

Sun

Figure 2.3 Effect of angle of incidence on heating at the Earth’s surface. A higher proportion of the incomingradiation is reflected as the angle of incidence increases.Note also that a higher proportion of the incoming radi-ation is absorbed or reflected back as it travels longerthrough the atmosphere in the higher latitudes.

warmer currents colder currents

Kur

oSi

wo

Californian

Antactic Circular Current

Braz

ilian

Gulf stream

North

Atlant

icDrif

t

N. Pacific Drift

AntarticCircular Current

Equatorial Current

Per

uvi

an

Figure 2.4 Global sea and wind movements. Warmer and colder water currents set in motion by the wind circulationaround the world.

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the rest of the forecast area.Weather forecasting iswell covered in literature and only its componentparts are considered here.

Climate can be thought of as a description of theweather experienced by an area over a long periodof time. More accurately, it is the long-term stateof the atmosphere. Usually the descriptions applyto large areas dominated by atmospheric systems(global, countrywide or regional) but local climatereflects the influence of the topography (hills andvalleys), altitude and large bodies of water (lakesand seas).

Climate of the British Isles

The British Isles has a maritime climate, which ischaracterized by mild winters and relatively coolsummers, that is a consequence of its proximity tothe sea. This is because water has a much largerheat capacity than materials making up the land.Asa consequence, it takes more heat energy to raisethe temperature of water by 1°C, and there is moreheat energy to give up when the water cools by 1°Cwhen compared with rock and soil. Consequently,bodies of water warm up and cool down moreslowly than adjoining land. The nearby sea thusprevents coastal areas becoming as cold in the win-ter as inland areas and also helps maintain tem-peratures well in the autumn.

In contrast, inland areas on the great land massesat the same latitude have a more extreme climatewith very cold winters and hot summers: the

features of a continental climate. Whereas most ofthe British Isles lowland is normally above freezingfor most of the winter, average mid-winter tempera-tures for Moscow and Hudson Bay (both contin-ental climate situations) are nearer �15°C.

The North Atlantic Drift, the ocean current flow-ing from the Gulf of Mexico towards Norway, dom-inates the climate of the British Isles (see Figure 2.4).The effect of the warm water, and the prevailingsouth-westerly winds blowing over it, is particularlyinfluential in the winter. It creates mild conditionscompared with places in similar latitudes, such asLabrador and the Russian coast well to the north ofVladivostock, which are frozen in the winter.

The mixing of this warm moist air stream andthe cold air masses over the rest of the Atlantic leadsto the formation of a succession of depressions.These regularly pass over the British Isles bringingthe characteristic unsettled weather with cloudsand rain where cold air meets the moist warm airin the slowly swirling air mass. Furthermore, themoist air is also cooled as it is forced to rise overthe hills to the west of the islands giving rise to orographic rain. In both instances clouds form whenthe dew point is reached (see page 25). This resultsin much higher rainfall levels in the west and northcompared with the south and east of the BritishIsles. In contrast, a rain shadow is created on theopposite side of the hills because, once the air haslost water vapour and falls to lower, warmer levels,there is less likelihood of the dew point beingreached again (see Figure 2.5). Depressions arealso associated with windier weather.


(a)

cold front

cold air

cold air

warm front

warm air

air cools as itrises; clouds formas dewpoint isreached

rain shadow

(b)

Figure 2.5 Cloud formation and rainfall caused by (a) fronts and (b) higher ground (orographic rain). Note: warm aircaused to rise over cold air or higher ground forms cloud when the air reaches the dew point of the air mass.

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The sequence of depressions (low pressure areas)is displaced from time to time by the developmentof high-pressure areas (anti-cyclones). These usu-ally bring periods of settled drier weather. In thesummer, these are associated with hotter weatherwith air drawn in from the hot European land massor north Africa. In the winter, clear cold weatheroccurs as air is drawn in from the very cold, dry con-tinental land mass. In the spring, these anti-cyclonesoften lead to radiation frosts, which are damagingto young plants and top fruit blossom.

The growing season

The outdoor growing season is considered to bethe time when temperatures are high enough forplant growth. Temperate plants usually start grow-ing when the daily mean temperatures are above

6°C. Spring in the south-west of the British Islesusually begins in March, but there is nearly a 2-months difference between its start in this areaand the north-east (see Figure 2.6).

In contrast, as temperatures drop below 6°C the growing season draws to a close.This occurs inthe autumn, but in the south-west of England and the west of Ireland this does not occur untilDecember, and on the coast in those areas there canbe 365 growing days per year. Within the generalpicture, there are variations of growth periodsrelated to altitude, aspect, frost pockets, proximityof heat stores, shelter and shade: the so-called localclimates and microclimates (see page 22). However,for most of mainland UK, the potential growingseason spans between 8 and 9 months. Examplesare given in Table 2.1.

Although this length of growing period will be astraightforward guide to grass growing days and


March 15

March 1

March 1

March 15

February 15

February 15

April 1

April 1

April 1

Figure 2.6 Beginning of spring in the British Isles (average dates when soil reaches 6°C).

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the corresponding need for mowing, many otherplants will stop growing as they complete their lifecycle well before low temperatures affect them.Furthermore, there are plants whose growing sea-son is defined differently. For example, most plantsintroduced from tropical or sub-tropical areas donot start growing until a mean daily temperatureof 10°C is experienced. More significantly they arerestricted by their intolerance of cold, so for manytheir outdoor season runs from the last frost ofspring to the first frost of autumn.

Proximity to the sea not only increases the lengthof the growing season but also it reduces its inten-sity, i.e. the extent to which temperatures exceedthe minimum for growth. Although inland areashave a shorter season they become much warmermore quickly before cooling down more rapidly in the autumn. The differences in intensity can beexpressed in terms of accumulated temperatureunits (ATUs).

ATUs are an attempt to relate plant growth anddevelopment to temperature and to the durationof each temperature. There is an assumption thatthe rate of plant growth and development increaseswith temperature.This is successful over the normal

range of temperatures that affect most crops. Onthe basis that most temperate plants begin growingat temperatures above 6°C, the simplest methodaccredits each day with the number of degreesabove the baseline of 6°C and accumulates them(note that negative values are not included). A sec-ond method calculates ATUs from weather recordson a monthly rather than daily basis. Examples aregiven in Table 2.2.This method provides a basis forcomparing the growing potential of different areas(see Table 2.3 and Figure 2.7).

Methods such as these can be used to predictlikely harvest dates from different sowing dates.Growers may also use such information to calculatethe required sowing date that will achieve a desiredharvest date. In the production of crops for thefreezing industry, it has been possible to smooth outthe supply to the factory by this method. For exam-ple, a steady supply of peas over 6 weeks can beorganized by using the local weather statistics to cal-culate when a range of early to late varieties of peas(i.e. with different harvest ATUs) should be sown.

More accurate methods, such as the OntarioUnits, use day and night temperatures in the calculation. These have been used to study the


Table 2.1 Length of growing season in the British Isles

Area Length of growing* Time of yearseason (days)

Start Finish

South-west Ireland 320 February 15 January 7Cornwall 320 February 15 January 7Isle of Wight 300 March 1 January 1Anglesey 275 March 1 December 15South Wales 270 March 15 December 15East Lancashire 270 March 15 December 1East Kent 265 March 15 November 28North Ireland 265 March 15 November 28Lincolnshire 255 March 21 November 25Warwickshire 250 March 21 November 22West Scotland 250 March 21 November 20East Scotland 240 March 28 November 15North-east Scotland 235 April 1 November 10

*Length of season is given for lower land in the area; reduce by 15 days for each 100 mrise into the hills (approximately 5 days per 100 ft).

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growth of tropical crops such as sunflowers, toma-toes and sweetcorn grown in a temperate area.Using this approach the extent to which bushtomatoes could be grown in southern England

for an expected yield of 50 t/ha in 9 years out of 10 could be mapped (see Figure 2.7).

The accumulated heat unit (AHU) concept can also be used to estimate greenhouse-heatingrequirements by measuring the extent to which theoutside temperature falls below a base or controltemperature, called ‘degrees of cold’. In January, agreenhouse maintained at 18°C at Littlehampton


Table 2.2 Examples of ATUs calculated on (a) a daily basis and (b) monthlybasis

Date Average Temperature units ATUs temperature (°C) (day-degrees) (day-degrees)

(a)March 1 6 1 � 0 � 0 0March 2 7 1 � 1 � 1 0 � 1 � 1March 3 7 1 � 1 � 1 1 � 1 � 2March 4 5 1 � 0 � 0 2 � 0 � 2March 5 8 1 � 2 � 2 2 � 2 � 4March 6 7 1 � 1 � 1 4 � 1 � 5March 7 8 1 � 2 � 2 5 � 2 � 7

(b)February 5 28 � 0 � 0 0March 7 31 � 1 � 31 31April 8 30 � 2 � 60 91May 11 31 � 5 � 155 246June 13 30 � 7 � 210 456July 14 31 � 8 � 248 704

Areas where bush tomatoesare likely to yield more than 50 t/hain 9 years out of 10

Figure 2.7 Use of Ontario Units to determine the likelysuccess of bush tomato crops.

Table 2.3 AHUs for different places in Europe

Location AHUs

May to July to TotalJune September

Edinburgh 300 700 1000Glasgow 250 650 900Belfast 300 700 1000Manchester 425 875 1300Norwich 430 950 1380Birmingham 450 900 1350Amsterdam 480 980 1460Swansea 450 900 1350London 470 950 1420Littlehampton 450 950 1400Channel Isles 480 970 1450Paris 550 1100 1650Bordeaux 600 1200 1800Marseilles 800 1500 2300

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on the coast in Sussex accumulates, on average, 420cold-degrees C compared with 430 for the samestructure inland at Kew, near London. For a hectareof glass this difference of 10 cold-degrees C is theequivalent of burning an extra 5000 l of oil. Thisprovides a useful means of assessing possible horti-cultural sites when other data such as solar heatingand wind speed are all brought together. Othermethods based on this concept enable growers to cal-culate when different varieties of rhubarb will startgrowing and the energy requirements for chill storesand refrigeration units.

World climates

In addition to maritime and continental climatesalready mentioned, there are many others includingthe Mediterranean climate (as found in southernparts of Europe, California, South Africa, Australiaand central Chile) that is typified by hot, dry sum-mers and mild winters.The characteristics of a rangeof the world climate types are given in Table 2.4.Plants native to these other areas can present a chal-lenge for those wishing to grow them in the BritishIsles. To some extent plants are tolerant, but caremust be taken when dealing with the plant’s degreeof hardiness (its ability to withstand all the featuresin the climate to which it is exposed). Plant speciesthat originated in sub-tropical areas (such assouth-east China and USA) tend to be vulnerable

to frosts and those from tropical and equatorialregions are most commonly associated with growingunder complete protection such as in conservatoriesand hothouses.

LOCAL CLIMATE

Most people will be aware that even their regionalweather forecast does not do justice to the wholeof the area. The local climate reflects the influenceof the topography (hills and valleys), altitude andlakes and seas that modifies the general influenceof the atmospheric conditions.

Coastal areas are subject to the moderatinginfluence of the body of water (see page 17).Waterhas a large heat capacity compared with othermaterials and this modifies the temperature of thesurroundings.

Altitude. The climate of the area is affected byaltitude: there is a fall in temperature with increasein height above sea level of nearly 1°C for each100 m.The frequency of snow is an obvious manifes-tation of the effect. In the south-west of England,there are typically only 5 days of snow falling at sealevel each year, whereas there are 8 days at 300 m.At higher altitudes the effect is more dramatic; inScotland there are nearly 35 days per year at sealevel, 38 days at 300 m but 60 days at 600 m.

The colder conditions at higher altitudes have a direct effect on the growing season. On the


Table 2.4 A summary of some of the world climates

Climatic region Temperature Rainfall

Range Winter Summer Distribution Total

TemperateMaritime Narrow Mild Warm Even ModerateContinental Wide Very cold Very warm Summer maximum ModerateMediterranean Moderate Mild Hot Winter maximum Moderate

Sub-tropical Moderate Mild* Hot Summer maximum HighTropical maritime Narrow Warm** Hot Even ModerateEquatorial Narrow Hot** Hot Even High

*Frosts uncommon, **no frosts.

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south-west coast of England, there are nearly 365growing days per year but this decreases by 9 daysfor each 30 m above sea level. In northern Englandand Scotland there are only about 250 growingdays which are reduced by 5 days per 30 m rise, i.e.to just 200 days at 300 m (1000 ft) above sea levelin northern England.

Topography. The presence of slopes modifiesclimate by its aspect and its effect on air drainage.Aspect is the combination of the slope and thedirection that it faces. North-facing slopes offerplants less sunlight than a south-facing one. This isdramatically illustrated when observing the snowon opposite sides of an east–west valley (or roofsin a street) when the north-facing sides are leftwhite long after the snow has melted on the otherside (see Plate 14); much more radiation is inter-cepted by the surface on the south-facing slope.Closer examination reveals considerable differ-ences in the growth of the plants in these situationsand it is quite likely that different species grow bet-ter in one situation compared with the other. Plantson such slopes experience not only different levelsof light and heat but also different water regimes;south-facing slopes can be less favourable for someplants because they are too dry.

Air drainage. Cold air tends to fall, because itis denser than warm air, and collects at the bottomof slopes such as in valleys. Frost pockets occurwhere cold air collects; plants in such areas aremore likely to experience frosts than those in simi-lar land around them. This is why orchards, whereblossom is vulnerable to frost damage, are estab-lished on the slopes away from the valley floor.Cold air can also collect in hollows on the waydown slopes. It can also develop as a result of bar-riers such as walls and solid fences placed across theslope (see Figure 2.8). Permeable barriers such astrees making up shelterbelts are less of a problemas the cold air is able to leak through. Frost suscep-tible plants grown where there is good air drainagemay well experience a considerably longer growingseason. Gardens on slopes can be modified toadvantage by having a low-permeable hedge above(a woodland is even better) and a very permeableone on the lower boundary.

MICROCLIMATES

The features of the immediate surroundings of theplant can further modify the local climate to createthe precise conditions experienced by the plant.This is known as its microclimate. The significantfactors that affect plants include nearness to abody of water or other heat stores, shelter or expos-ure, shade, altitude, aspect and air drainage. Themodifications for improvement, such as barriersreducing the effect of wind, or worse, such as bar-riers causing frost pockets, can be natural or artifi-cial. The microclimate can vary over very smalldistances. Gardeners will be familiar with the differ-ences across their garden from the cool, shady areasto the hot, sunny positions and the implicationsthis has in terms of the choice of plants and theirmanagement.

Growers improve the microclimate of plantswhen they establish windbreaks, darken the soil,wrap tender plants in straw, etc. More elaborateattempts involve the use of fleece, cold frames,cloches, polytunnels, greenhouses and conservator-ies. Automatically controlled, fully equipped green-houses with irrigation, heating, ventilation fans,supplementary lighting and carbon dioxide areextreme examples of an attempt to create the idealmicroclimate for plants.


cold air heldin hollows

cold air

cold airsolidbarrier

(a)

(b)

Figure 2.8 The creation of frost pockets: (a) natural hol-lows on the sides of valleys and (b) effect of solid barri-ers preventing the drainage of cold air.

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Heat stores are materials such as water (see heatcapacity, page 17) and brickwork, which collect heatenergy and then release it to the immediate envi-ronment that would otherwise experience moresevere drops in temperature. Gardeners can makegood use of brick walls to extend the growing sea-son and to grow plants that would otherwise be vul-nerable to low temperatures.Water can also be usedto prevent frost damage when sprayed onto fruittrees. It protects the blossom because of its latentheat: the energy that has to be removed from thewater at 0°C to turn it to ice. This effect is consider-able, and until the water on the surface has frozen,the plant tissues below are protected from freezing.

Shelter that reduces the effect of wind comes inmany different forms. Plants that are grown ingroups, or stands, experience different conditionsfrom those that stand alone. As well as the self-sheltering from the effect of wind, the grouped

plants also tend to retain a moister atmosphere,which can be an advantage but can also createconditions conducive to pest and disease attack.Walls, fences, hedges and the introduction of shel-terbelts also moderate winds but there are someimportant differences in the effect they have. Thereduction in flow downwind depends on the heightof the barrier but there is a smaller but significanteffect on the windward side (see Figure 2.9). Thediagram also shows how turbulence can be cre-ated in the lee of the barrier, which can lead toplants being damaged by down forces. Solid mater-ials such as brick and wooden fences create themost turbulence. In contrast, hedges and meshes fil-ter the wind; the best effect coming from those withequal gap to solid presented to the wind. However,the introduction of a shelterbelt can bring prob-lems if it holds back cold air to create a frost pocket(see page 22).


20 mphwind

20 mphwind

� 2 h �10–15 h subject to eddies

h � height of barrier

10 mph �5 mph 5–10 mph 10–15 mph 15–20* mph

*effect of wind break can be up to �40 height

(a)

(b)

h

Figure 2.9 The effect of windbreaks: (a) solid barriers tend to create eddies to windward and, more extensively, to lee-ward and (b) a permeable barrier tends to filter the air and reduce its speed without setting up eddies.

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Shade reduces the radiation that the plant andits surroundings receive. This tends to produce acooler, moister environment in which some speciesthrive (see Ecology, page 4). This should be takeninto account when selecting plants for differentpositions outside in gardens.The grower will delib-erately introduce shading on propagation units oron greenhouses in summer to prevent plants beingexposed to high temperatures and to reduce waterlosses.

Plant selection. The horticulturist is always con-fronted with choices; plants can be selected to fit themicroclimate or attempts can be made to changethe microclimate to suit the plant that is desired.

Forecasting. Not only is there an interest inweather forecasting in order to plan operationssuch as cultivations, planting, frost protection, etc.but also for predicting pest and disease attacks,many of which are linked to factors such as tem-perature and humidity. Examples of outbreaks andmethods of predicting them, such as Critical Periodsthat are used to predict potato blight, are to befound on page 124.

MEASUREMENT

The range of instruments in an agro-meteorologicalweather station are used to measure precipitation,temperature, wind and humidity.The measurementsare normally made at 09.00 Greenwich Mean Time(GMT) each day. Most of the instruments arehoused in a characteristic Stevenson’s Screenalthough there are usually other instruments on thedesignated ground or mounted on poles nearby(see Plate 15).

Temperature

The normal method of measuring the air tempera-ture uses a vertically mounted mercury-in-glassthermometer that is able to read to the nearest0.1°C. In order to obtain an accurate result, therm-ometers used must be protected from direct radi-ation, i.e. the readings must be made ‘in the shade’.

In meteorological stations, they are held in theStevenson’s Screen (see Plates 15 and 16), which is designed to ensure that accurate results areobtained at a standard distance from the ground.The screen’s most obvious feature is the slattedsides, which ensure that the sun does not shinedirectly onto the instruments (see Radiation, page13) whilst allowing the free flow of air around theinstruments. The whole screen is painted white toreflect radiation that, along with its insulated topand base, keeps the conditions inside similar to thatof the surrounding air. In controlled environmentssuch as glasshouses the environment is monitoredby instruments held in an aspirated screen, whichdraws air across the instruments to give a moreaccurate indication of the surrounding conditions.

The dry bulb thermometer is paired with a wetbulb thermometer that has, around its bulb, a muslinbag kept wet with distilled water. In combination,they are used to determine the humidity (see page25). Robust mercury-in-glass thermometers set insleeves are also used to determine soil tempera-tures; temperatures both at the soil surface and at300 mm depth are usually recorded in agro-meterological stations. The highest and lowesttemperatures over the day (and night) are recordedon the Max–Min (maximum and minimum) ther-mometers (see Plate 16) mounted horizontally onthe floor of the screen.The maximum thermometeris a mercury-in-glass design, but with a constrictionin the narrow tube near the bulb that contains themercury. This allows the mercury to expand as itwarms up, but when temperatures fall the mercurycannot pass back into the bulb and so the highesttemperature achieved can be read off (‘today’shigh’). Shaking the contents back into the bulbresets it.The minimum thermometer contains alco-hol. This expands as it warms but as it contracts tothe lowest temperature (‘tonight’s low’) a thinmarker is pulled down by the retreating liquid. Asit is lightly sprung, the marker is left behind when-ever the temperature rises. Using a magnet, or tilt-ing, to bring the marker back to the surface of theliquid in the tube can reset the thermometer. Inaddition to the screen reading, there is anotherlowest-temperature thermometer placed at ground


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level giving ‘over bare soil’ and ‘grass’ temperatures(see Plate 15).

Precipitation

The term precipitation covers all the ways inwhich water reaches the ground as rain, snow andhail. It is usually measured with a rain gauge (seeFigure 2.10).

Simple rain gauges are based on straight-sidedcans set in the ground with a dipstick used to deter-mine the depth of water collected. Accurate read-ings to provide daily totals are achieved with adesign that maximizes collection, but minimizesevaporation losses by intercepting the precipitationwater in a funnel.This leads to a tapered measuringglass calibrated to 0.1 mm. These gauges are pos-itioned away from anything that affects the local air-flow, e.g. buildings, trees and shrubs.They are set inthe ground but with the rim above it to preventwater running in from the surroundings.

Recording rain gauges are available which alsogive more details of the pattern of rainfall within the24 h periods.The ‘tipping bucket’ type has two opencontainers on a see-saw mechanism so arrangedthat as one bucket is filled, it tips and this is recordedon a continuous chart; meanwhile the other bucketis moved into position to continue collection.

Humidity

Humidity is the amount of water vapour in theatmosphere, i.e. the quantity of water held in theair. It increases as the air is heated because warmair is able to hold more water, as water vapour.Saturated air at 0°C can hold 3 g of water per kilo-gram of air, whereas it can hold 7 g at 10°C, 14 g at20°C and 26 g at 30°C. The maximum figure foreach temperature is known as its saturation pointor dew point and if such air is cooled further, thenwater vapour condenses out to liquid water. Onekilogram of saturated air at 20°C would give up 7 gof water as its temperature falls to 10°C. Indoors,this is seen as ‘condensation’ on the coolest sur-faces in the vicinity;outdoors it happens when warmair mixes with cold air. Droplets of water form asclouds, fog and mist; dew forms on cool surfacesnear the ground. If the air is holding less than themaximum amount of water it has drying capacity,i.e. it can take up water from its surroundings.

One of the most commonly used measurementsof humidity is relative humidity (RH) which is theratio, expressed as a percentage, of the actual quan-tity of water vapour contained in a sample of air tothe amount it could contain if saturated at the drybulb temperature.This is usually estimated by usingthe wet and dry bulb temperatures (see above) inconjunction with hygrometric tables. If the absolutehumidity for air at 20°C (on the dry bulb) is foundto be 14 g/kg this compares with the maximum of14 g that can be held when such air is saturated.Therefore, the RH is 100 per cent. It can be seen thatRH falls to 25 per cent when the wet bulb depres-sion shows only 3.5 g of water is present.This meansthat its drying capacity has increased (it can nowtake up 10.5 g of water before it becomes saturated).

10 1020 2030 3040 40

50 50

60 6070

mm

mm

level

125 mm 125 mm

funnelreduces

evaporation30

0 m

m

450

mm

ground

Figure 2.10 Rain gauges: a simple rain gauge consists of astraight-sided can in which the depth of water accumulatedeach day can be measured with a dipstick. An improveddesign incorporates a funnel, to reduce evaporation, and acalibrated collection bottle. A rain gauge should be setfirmly in the soil away from overhanging trees, etc. and therim should be 300 mm above ground to prevent waterflowing or bouncing in from surrounding ground.

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An example of working out the RH from wet anddry bulb measurements is given in Table 2.5.

The whirling hygrometer is the most accurateportable instrument used for taking air measure-ments (see Plate 17). The wet and dry bulb therm-ometers are mounted such that they can be rotated

around a shaft held in the hand rather like a footballrattle.The humidity is again calculated using hygro-metric tables after the full depression of the wetbulb temperature has been found. Hygrometersmade out of hair, which lengthens as the humidityincreases, are also used to indicate humidity levels.These can be connected to a pen that traces thechanges on a revolving drum carrying a hygrogramchart. Other hygrometers are based on the moistureabsorbing properties of different materials includingthe low technology ‘bunch of seaweed’ comparableto Humidity on page 25.

Wind

Wind speed is measured with an anemometer,which is made up of three hemispherical cups on avertical shaft ideally set 10 m above the ground(see Plate 15). The wind puts a greater pressure on the inside of the concave surface than on the

Table 2.5 Calculation of RH from wet and dry bulbmeasurements

Example

1 2 3 4

Dry bulb reading (A) (°C) 25 25 25 10Wet bulb reading (°C) 21 19.5 18 6.5Depression of the 4 5.5 7 3.5

wet bulb (B) (°C)

RH (%)* 70 60 50 60

*Found from tables supplied with the hygrometer byreading along the dry bulb reading row (A) then findwhere the column intersects the depression of the wetbulb figure (B).

Table 2.6 The Beaufort Scale

Force Description for use on land Equivalent wind speed

m/s Approximate miles/h

0 Calm: smoke rises vertically 0 0–11 Light air: wind direction seen

by smoke drift rather than by wind vanes 2 1–32 Light breeze: wind felt on face, vane

moves, leaves rustle 5 4–73 Gentle breeze: light flags lift, leaves

and small twigs move 9 8–124 Moderate breeze: small branches move,

dust and loose paper move 13 13–185 Fresh breeze: small leafy trees sway,

crested wavelets on lakes 19 19–246 Strong breeze: large branches sway,

umbrellas difficult to use 24 25–317 Near gale: whole trees move, difficult

to walk against 30 32–388 Gale: small twigs break off, impedes

all walking 37 39–469 Strong gale: slight structural damage 44 47–55

10 Storm: trees uprooted, considerablestructural damage 52 55–63

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convex one so that the shaft is spun round; therotation is displayed on a dial usually calibrated inknots (nautical miles per hour) or metres per second. An older and still much used visualmethod is the Beaufort Scale; originally based onobservations made at sea, it is used to indicate thewind forces at sea or on land (see Table 2.6).

Wind direction is indicated with a wind vane,which is often combined with an anemometer.Decorative wind vanes are a familiar sight, but thestandard meteorological design comprises a pointerwith a streamlined vertical plate on one endmounted so that it can rotate freely. The arrowshape points into the wind and the movements overa minute or so are averaged.The direction the windis coming from is recorded as the number of degreesread clockwise from true north, i.e. a westerly windis given as 270, south-easterly as 135 and a northerlyone as 360 (000 is used for recording no wind).

Light

The units used when measuring the intensity of allwavelengths are watts per square metre (W/m2),whereas lux (lumens/m2) are used when only lightin the photosynthetic range is being measured.More usually in horticulture, the light integral isused. The light sensors used for this measure thelight received over a period of time and expressedas gram calories per square centimetre (gcals/cm2)or megajoules per square metre (MJ/m2). Theseare used to calculate the irrigation need of plantsin protected culture.

The usual method of measuring sunlight at a metrological station is the Campbell–StokesSunshine Recorder, a glass sphere that focuses thesun’s rays onto a sensitive card; the burnt trailindicates the periods of bright sunshine. Anotherapproach is to use a solarimeter, which convertsthe incoming solar radiation to heat and then toelectrical energy that can be displayed on a dial.

Automation

Increasingly, instrumentation is automated and, inprotected culture, linked to computers programmedto maintain the desired environment by adjustingthe ventilation and boiler settings. To achieve this,the computer is informed by external instrumentsmeasuring wind speed, air temperature and humid-ity, and internally by those measuring CO2 levels,ventilation settings, heating pipe temperature, airtemperature and humidity.

FURTHER READING

Bakker, J.C. Greenhouse Climate Control – An IntegratedApproach (Wageningen Press, 1995).

Barry, R. et al. Atmosphere, Weather and Climate, 6thedition (Routledge, 1992).

Kamp, P.G.H. and Timmerman, G.J. ComputerisedEnvironmental Control in Greenhouses (IPC Plant,1996).

Reynolds, R. Guide to Weather (Philips, 2000).Taylor, J. Weather in the Garden (John Murray, 1996).

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PRINCIPLES OF CLASSIFICATION

Any classification system involves the grouping oforganisms or objects using characteristics commonto members within the group.The organisms consti-tuting the plant kingdom are distinguishable fromanimals in having sedentary growth, cellulose cellwalls and polyploidy (see Chapter 8). They are ableto change energy from one form, light, into another,organic molecules (autotrophic nutrition; seePhotosynthesis, page 53). Animals, amongst otherthings, have no cell walls and rely on eating ready-made organic molecules (heterotrophic nutrition).

Terms that are used in classification are:

• taxonomy, which deals with the principles onwhich a classification is based,

• systematics, which identifies the groups to beused in the classification,

• nomenclature, which deals with naming.

Various systems have been devised throughouthistory, but a seventeenth-century Swedish botanist,Linnaeus, laid the basis for much subsequent workin the classification of plants, animals and alsominerals. The original divisions of the plant king-dom were the main groupings of organisms accord-ing to their place in evolutionary history. Simplesingle-celled organisms from aquatic environmentsevolved to more complex descendants, multicellularplants with diverse structures, which were able tosurvive in a terrestrial habitat and develop sophis-ticated reproduction mechanisms.

The world of living organisms is currently dividedinto five kingdoms including Plantae (plants),Animalia (animals) and fungi. Less familiar are thetwo other categories Prokaryotae (bacteria) andthe Protoctista (all other organisms that are not inthe other kingdoms including algae and protozoa).In order to produce a universally acceptable system,the International Code of Botanical Nomenclature(ICBN) was formulated, which includes both non-cultivated plants and details specific to cultivatedplants.

KINGDOM PLANTAE (PLANTS)

Plant divisions (the names ending in -phyta) are fur-ther subdivided into class (ending -psida), order

3

Classification and Naming

The pressures of evolution have producedwidely varying plant types, found in differinghabitats, and distinguished by characteristicstructures and modes of life. An orderly sys-tem of classification seeks to ensure that thehorticulturist, plant scientist and naturalist canidentify and name any plant, animal, fungusor bacteria without ambiguity.A detailed pro-cedure for naming organisms is thereforeessential and is outlined with examples fromthe plant kingdom. Features of plants that areused in identification are discussed.

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Classification and Naming 29

(ending -ales), family (ending -aceae), genus andspecies. Species is the basic unit of classification, andis defined as a group of individuals with the great-est mutual resemblance, which are able to breedamongst themselves. A number of species withbasic similarities constitute a genus (plural genera),a number of genera, a family and a number of fam-ilies, an order (see example given in Table 3.1).

Divisions of the plant kingdom

The division of the plant kingdom is given in Table 3.2.

Mosses and liverworts. Over 25 000 plantspecies which do not have a vascular system (seepage 42) are included in the divisions Bryophytaand Hepatophyta.They have distinctive vegetativeand sexual reproductive structures, the latter

producing spores that require damp conditions forsurvival. Many from both divisions are pioneerplants that play an important part in the earlystages of soil formation.The low spreading carpetsof vegetation also present a weed problem on thesurface of compost in container-grown plants, oncapillary benches,and around glazing bars on green-house roofs.

Ferns and horsetails, in the division Pterido-phyta and Equisetophyta,have identifiable leaf, stemand root organs, but produce spores rather thanseeds from the sexual reproduction process. Manyspecies of ferns, e.g. maidenhair fern (Adiantumcuniatum), and some tropical horsetails, are grownfor decorative purposes, but the common horsetail(Equisetum arvense), and bracken (Pteris aquilina)that spread by underground rhizomes, are difficultweeds to control.

Seed-producing plants (Superdivision: sper-matophyta) contain the most highly evolved andstructurally complex plants. There are speciesadapted to most habitats and extremes of environ-ment. Sexual reproduction produces a seed, whichis a small, embryo plant contained within a protect-ive layer.

Conifers (Coniferophyta) are a large division of many hundred species that include the pines(Order: Pinales) and yews (Order: Taxales).Characteristically they produce ‘naked’ seeds, usu-ally in cones, the female organ. They show someprimitive features, and often display structuraladaptations to reduce water loss (see Figure 5.3).There are very many important conifers. Some aremajor sources of wood or wood pulp, but within horticulture many are valued because of their inter-esting plant habits, and foliage shape and colours.For example, the Cupressaceae includes fast-growing species, which can be used as windbreaks,and small, slow-growing types very useful for rockgardens. The yews are a highly poisonous group of plants that includes the common yew (Taxusbaccata) used in ornamental hedges and mazes.Thedivision Ginkgophyta is represented by a singlesurviving species, the maidenhair tree (Ginkgobiloba), which has an unusual slit-leaf shape, anddistinctive bright yellow colour in autumn.

Table 3.1 Classification.The lettuce cultivar ‘Little Gem’used to illustrate the hierarchy of the classification up tokingdom

Kingdom Plantae PlantsSub-kingdom Tracheobionta Vascular plantsSuperdivision Spermatophyta Seed plantsDivision Magnoliophyta Flowering plantsClass Magnoliopsida DicotyledonsSubclass AsteridaeOrder AsteralesFamily Asteraceae Aster familyGenus Lactuca LettuceSpecies L. sativa Garden lettuceCultivar L. sativa ‘Little Gem’

Table 3.2 Divisions of the plant kingdom

Divisions Common name

Bryophyta MossesHepatophyta Liverworts

Vascular plants Equisetophyta HorsetailsPteridophyta Ferns

Seed plants Coniferophyta ConifersGinkgophyta GinkgoMagnoliophyta Flowering plants

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Flowering plants (Division: Magnoliophyta)have a flower structure for sexual reproductionproducing seeds protected by fruits. This charac-teristic structure is used as the basis of their classi-fication. There are estimated to be some 25 000species, occupying a very wide range of habitats.Many in the division are important to horticulture,both as crop plants and weeds. This division is split into two main classes the Liliopsida formerlythe Monocotyledonae and generally known as themonocotyledons and the Magnoliopsida, thedicotyledons.

Monocotyledons include some important horti-cultural families, e.g. Arecaceae, the palms;Musaceae, the bananas; Cyperaceae, the sedges;Juncaceae, the rushes; Poaceae (formerly Grami-nae), the grasses; Iridaceae, the irises; Liliaceae,the lilies and the Orchidaceae, the orchids.

Dicotyledons has many more families signifi-cant to horticulture, including Magnoliaceae, themagnolias; Caprifoliaceae, the honeysuckles;Cactaceae, the cactuses; Malvaceae, the mallows;Ranunculaceae, the buttercups;Theaceae, the teas;Lauraceae, the laurels; Betulaceae, the birches,Fagaceae, the beeches; Solanaceae, the potatoes andtomato; Nymphaeaceae, the water lilies and Cras-sulaceae, the stonecrops. Four of the biggest andmost economically important families in this classhave had a change of name. Fabaceae (formerlythe Leguminosae), the pea and bean family, havefive-petalled asymmetric or zygomorphic (havingonly one plane of symmetry) flowers, which developinto long pods (legumes) containing starchy seeds.The characteristic upturned umbrella-shaped flowerhead or umbel is found in the Apiaceae (formerlythe Umbelliferae), the carrot family, and bears smallwhite five-petalled flowers, which are wind polli-nated. Asteraceae (formerly Compositae), the members have a characteristic flower head withmany small florets making up the composite, regular(or actinomorphic) structure, e.g. chrysanthemum,groundsel. Members of the Brassicaceae (formerlyCruciferae) are characterized by their four-petalledflower and contain the genus Brassica with a number of important crop plants such as cabbage,cauliflowers, swedes, brussels sprouts as well as

the wallflower (Cheiranthus cheiri). Most of thebrassicas have a biennial growth habit producingvegetative growth in the first season, and flowers inthe second, usually in response to a cold stimulus(see vernalization). A number of weed species arefound in this family, including shepherd’s purse(Capsella bursapastoris), which is an annual. Manyimportant genera, e.g. apples (Malus), pear (Pyrus)and rose (Rosa), are found within the Rosaceaefamily, which generally produces succulent fruitfrom a flower with five petals and often many maleand female organs. Many species within this familydisplay a perennial-growth habit (see Table 3.3).

NAMING OF CULTIVATED PLANTS

The binomial system

The name given to a plant species is very important.It is the key to identification in the field or garden,and also an international form of identity, whichcan lead to much information from books and theInternet. The common names, which we use forplants, such as potato and lettuce, are, of course,acceptable in English, but are not universally used.Linnaeus, a Swedish biologist in the eighteenthcentury, formulated a system that he claimed shouldidentify an individual plant type, by means of thecomposed genus name, followed by the speciesname. For example, the chrysanthemum used forcut flowers is Chrysanthemum genus and mori-folium species; note that the genus name beginswith a capital letter, while the species has a smallletter. Other examples are Ilex aquifolium (holly),Magnolia stellata (star-magnolia) and Ribes san-guineum (redcurrant).

The genus and species names must be written initalics, or underlined where this is not possible, toindicate that they are internationally acceptedterms. However, these two words may not encom-pass all possible variations, since a species can giverise to a number of naturally occurring varietieswith distinctive characteristics. In addition, culti-vation, selection and breeding have produced vari-ation in species referred to as cultivated varieties

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or cultivars.The two terms, variety and cultivar, areexactly equivalent, but the botanical variety nameis referred to in Latin, beginning with a small let-ter, while the cultivar is given a name often relat-ing to the plant breeder who produced it. There isno other significant difference in the use of thetwo terms, and therefore either is acceptable.However, the term cultivar will be used through-out this text. A cultivar name should be written ininverted commas and begin with a capital letter,after the binomial name or, when applicable, thecommon name. Examples include Prunus padus‘Grandiflora’, tomato ‘Ailsa Craig’, apple ‘Bramley’sSeedling’. If a cultivar name has more than oneacceptable alternative, they are said to be synonyms(sometimes written syn.).

Hybrids

When cross pollination occurs between two plants,hybridization results, and the offspring usuallybear the characteristics distinct from either parent.Hybridization can occur between different cultivarswithin a species, sometimes resulting in a new and distinctive cultivar (see Chapter 8), or betweentwo species, resulting in an interspecific hybrid,e.g. Prunus � yedoensis and Erica � darleyensis.A much rarer hybridization between two differ-ent genera results in an intergeneric hybrid,e.g. �Cupressocyparis leylandii and �Fatshederalizei.The names of the resulting hybrid types includeelements from the names of the parents, con-nected or preceded by a multiplication sign (�).A chimaera, consisting of tissue from two distinctparents, is indicated by a ‘plus’ sign, e.g.�Laburnocystisus adamii, the result of a graft.

Identifying plants

A flora is a text written for the identification offlowering plant species. Some floras use only pic-tures to classify plants. More detailed texts use amore systematic approach where reference ismade to a key of features that, by elimination, willlead to the name of a plant. Species are described

in terms of their flowers, inflorescences, stems,leaves and fruit.This description will often includedetails of shape, size and colour of these plant parts.

Flowers. The number and arrangement of flowerparts are the most important features for classifica-tion and is a primary feature in plant identification.It can be described in shorthand using a floral for-mula or a floral diagram (see Figure 3.1). For exam-ple, the floral formula, with the interpretation, forwallflower (Cheiranthus cheiri), a member of theCruciferae family is given.

Other examples of floral formulae include:

Sweet pea K(5) C5 A(9) � 1 G1(Leguminosae)

Buttercup K5 C5 A G(Ranunculaceae)

Dead nettle K(5) C(5)A4 G(2)(Labiatae)

Daisy K C(5)A(5) G(2)(Asteraceae)

The way that flowers are arranged on the plant isalso distinctive in different families: raceme,corymb, umbel and capitulum (see Figure 3.2).

K4Four

sepalsin calyx

C4Four

petals incorolla

G(2)Two ovaries

joinedtogether

A2 � 4Six

anthers

⊕Symmetrical

flower

Figure 3.1 Floral diagram for wallflower.

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Leaf form (see Figure 3.3) is a useful indicatorwhen attempting to identify a plant and descrip-tions often include specific terms; a few aredescribed below but many more are used in flora.Simple leaves have a continuous leaf blade, e.g. lin-ear, lanceolate, ovate, obovate, orbicular and oval.The margins of leaves can be described: entire, sin-uous, serrate and crenate. The arrangement of theleaf veins also characterizes the plant: reticulate,parallel, pinnate and palmate. Compound leaves,such as compound palmate and compound pin-nate, have separate leaflets each with an individualbase on one leaf stalk (see page 57 for leaf struc-ture), but the axillary bud is at the base of the mainleaf stalk.

Most horticulturists yearn for stability in thenaming of plants. Changes in names confuse manypeople who do not have access to up-to-date litera-ture. On the other hand, the reasons for change arejustifiable. New scientific findings may have shownthat a genus or species belongs to a different sectionof a plant family, and that a new name is the correctway of acknowledging this fact. Alternatively, aplant introduced from abroad, maybe many yearsago, may have mistakenly been given the incorrectname, along with all the cultivars derived from it.

Evidence from biochemistry, microscopy anddeoxyribonucleic acid (DNA) analysis is provingincreasingly important in adding to the more conventional plant structural evidence for plant

naming. There may be differing views whether agenus or species should be ‘split’ into smallerunits, or several species be ‘lumped’ into an exist-ing species or genus, or left unchanged. It wouldseem likely that changes in plant names wouldcontinue to be a fact of horticultural life.

There has been a massive increase in communi-cation across the world, especially as a result of theInternet. The level of information about plantnames has improved. The ICBN has laid down aninternational system. Within Britain, the RoyalHorticultural Society (RHS) has an advisory panelto help resolve problems in this area. An invalu-able reference document ‘Index Kewensis’ ismaintained by Kew Gardens listing the first publi-cation of the name for each plant species not hav-ing specific horticultural importance. Cultivatedspecies are listed in the ‘RHS Plant Finder’, whichalso indicates where they can be sourced.

Geographical origins of plants

Gardens and horticultural units, from the tropicsto more temperate climates, contain an astonish-ing variety of plant species from the different con-tinents. Below is a brief selection of well-knownplants, grown in Britain, illustrating this diversityof origins. It is salutary, when considering thesefar-flung places, to reflect on the sophisticated

racemee.g. foxglove

corymbe.g. Achillea

umbele.g. fennel

capitulume.g. Chrysanthemum

Figure 3.2 Flower forms. Four examples of inflorescence – the arrangement of flowers on the stem.

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lineare.g. carnation

lanceolatee.g. lilac

ovale.g. Garrya

peltatee.g. nasturtium

hastatee.g. Arum

lobede.g. Ribes

palmatee.g. lupin

pinnatee.g. rose

entire sinuous crennate serrate

parallel veinse.g. most monocotyledons

reticulate (net) veinse.g. most dicotyledons

Figure 3.3 Leaf forms. From top: five examples of simple leaf shapes; lobed simple leaf alongside two examples of com-pound leaves (with leaflets on petiole); four descriptors of leaf margins; the commonest forms of vein pattern.

cultures, with skills in plant breeding and a passionfor horticulture over the centuries that have takenwild plants and transformed them into the won-ders that we now see in our gardens:

• Far East (China and Japan): cherry, cucumber,peach, walnut, Clematis, Forsythia, hollyhock,Azalea, rose;

• India and South-East Asia: mustard, radish;

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• Australasia: Acacia, Helichrysum, Hebe;• Africa: Phaseolus, pea,African violet, Strelitzia,

Freesia, Gladiolus, Impatiens, Pelargonium,Plumbago;

• Mediterranean: asparagus, celery, lettuce, onion,parsnip, rhubarb, carnation, hyacinth, Antir-rhinum, sweet pea;

• Middle East and Central Asia: apple, carrot,garlic, grape, leek, pear, spinach;

• Northern Europe: cabbage, Campanula, Crocus,forget-me-not, foxglove, pansy, Primula, rose,wallflower;

• North America: Aquilegia, Ceonothus, lupin,Aster, Penstemon, Phlox, sunflower;

• Central and South America: Capsicum, maize,

potato, tomato, Fuchsia, nasturtium, Petunia,Verbena.

Further classifications of plants

Plants can be grouped into other useful categories.A classification based on their life cycle (ephem-erals, annuals, biennials and perennials) has longbeen used by growers who also distinguish betweenthe different woody plants such as trees and shrubs.Growers distinguish between those plants that areable to withstand a frost (hardy) and those thatcannot (tender); plants can be grouped accordingto their degree of hardiness. Table 3.3 brings

Table 3.3 Some commonly used terms that describe the life cycles, size and survival strategies of plants

Description Examples

Life cyclesEphemeral A plant that has several life cycles in a growing season Groundsel (Senecio vulgaris)

and can increase in numbers rapidly.

Annual A plant that completes its life cycle within a Poached-egg flower (Limnanthes growing season. douglasii)

Biennial A plant with a life cycle that spans two growing seasons. Foxglove (Digitalis purpurea)

Perennial A plant living through several growing seasons.

Herbaceous A perennial that loses its stems and foliage at the Michaelmas daisy (Aster spp.)perennial end of the growing season. and hop (Humulus lupulus)

Woody plantsWoody perennial A perennial that maintains live woody stem growth Bush fruit, shrubs, trees, climbers

at the end of the growing season. (e.g. grape)

Shrub A woody perennial plant having side branches emerging Lilac (Syringa vulgaris)from near ground level. Up to 5 m tall.

Tree A large woody perennial unbranched for some distance Horse chestnut (Aesculusabove ground. Usually more than 5 m. hippocastanum)

Deciduous A plant that sheds all its leaves at once. Mock orange (Philadelphusdelavayi)

Evergreen A plant retaining leaves in all seasons. Aucuba (Aucuba japonica)

HardinessVery hardy A plant able to survive �18°C. Kerria japonica

Moderately hardy A plant able to survive �15°C. Camellia japonica

Semi-hardy A plant able to survive �6°C. Pittosporium crassifolium

Tender A plant not hardy below �1°C. Pelargonium cvs

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together these useful terms, provides some defin-itions and gives some plant examples.

KINGDOM FUNGI

Some fungi are single celled (such as yeasts), butothers are multicellular such as the moulds andthe more familiar mushrooms and toadstools. Mostare made up of a mycelium, which is a mass ofthread-like filaments (hyphae).Their cell walls aremade of chitin.Their energy and supply of organicmolecules are obtained from other organisms(heterotrophic nutrition). They achieve this bysecreting digestive enzymes onto their food sourceand absorbing the soluble products. They obtaintheir food directly from other living organisms,possibly causing disease (see Chapter 11), or fromdead organic matter, so contributing to its break-down in the soil (see Chapter 15).

Fungi are classified into three divisions:

• Zygomycota (mitosporic fungi) have simpleasexual and sexual spore forms. Damping off,downy mildew and potato blight belong to thisgroup.

• Ascomycota have chitin cell walls, and show,throughout the group, a wide variety of asexualspore forms. The sexual spores are consistentlyformed within small sacs (asci), numbers ofwhich may themselves be embedded withinflask-shaped structures (perithecia), just visibleto the naked eye. Black spot of rose, applecanker, powdery mildew and Dutch elm diseasebelong to this group.

• Basidiomycota have chitin cell walls, and mayproduce, within one fungal species (e.g. cerealrust), as many as five different spore forms,involving more than one plant host. The fungiwithin this group bear sexual spores (basidio-spores) from a microscopic club-shaped struc-ture (basidium). Carnation rust, honey fungusand silver leaf diseases belong to this group.

An artificially derived fourth grouping of fungiis included in the classification of fungi. The

Deuteromycota include species of fungi that onlyvery rarely produce a sexual spore stage. As withplants, the sexual structures of fungi form the mostreliable basis for classification. But, here, the mainbasis for naming is the asexual spore andmycelium structure. Grey mould (Botrytis), Fusar-ium patch of turf and Rhizoctonia rot are placedwithin this group.

KINGDOM ANIMALIA (ANIMALS)

The animal kingdom includes a very large numberof species that have a significant influence on horti-culture mainly as pests (see Chapter 10) or as con-tributors to the recycling of organic matter (seeChapter 15).

Some of the most familiar animals are in thephylum Chordata that includes mammals, birds,fish, reptiles and amphibians. Mammal pest speciesinclude moles (see page 99), rabbits (see page 97),deer, rats and mice. Bird pest species are numer-ous including pigeons and bullfinches, but thereare very many that are beneficial in that they feedon harmful organisms such as tits that eat greenfly.Less familiar are important members of the phy-lum Nematoda (the roundworms) that includes avery large number of plant-disease-causing organ-isms including stem and bulb eelworm (see page118), root knot eelworms (see page 119), chrysan-themum eelworm and potato root eelworm (seepage 117). Phylum Arthropoda are the mostnumerous animals on Earth and includes insects,centipedes, millipedes and spiders; many of theseare dealt with in the chapter on plant pests(Chapter 10), but it should be noted that there aremany that are beneficial, e.g. honey bees (see page77) and centipedes, which are carnivorous andmany live on insect species that are harmful.Phylum Annelida (the segmented worms) includesearthworms, which are generally considered to beuseful organisms especially when they are helpingto decompose organic matter (see page 185) orwhen improving soils structure (see page 165), butsome species cause problems in fine turf whenthey produce worm casts. Phylum Mollusca is best

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known for the major pests slugs and snails (seepage 100).

KINGDOM PROKARYOTAE (MONERA)

Bacteria, the sole members of this kingdom, aresingle-celled organisms sometimes arranged inchains or groups (colonial). They are autotrophic(can produce their own energy supply and organicmolecules); some photosynthesize (see page 53)but others are able to make organic moleculesusing the energy released from chemical reactionsusually involving simple inorganic compounds.Theyhave great importance to horticulture by their bene-ficial activities in the soil (see page 185), and ascausative organisms of plant diseases (see page 129).

KINGDOM PROTOCTISTA (PROTISTA)

The algae, comprising some 18 000 species, are trueplants, since they use chlorophyll to photosynthesize(see Chapter 6). The division Chlorophyta (greenalgae) contains single-celled organisms that requirewater for reproduction and can present problemswhen blocking irrigation lines and clogging watertanks. Marine algal species in Phaeophyta (brownalgae) and Rhodophyta (red algae) are multicellu-lar, and have leaf-like structures. They include theseaweeds, which accumulate mineral nutrients, andare therefore a useful source of compound fertilizeras a liquid feed. (The blue-green algae, which cancause problems in water because they produceunsightly blooms but are also toxic, have beenrenamed Cyanobacteria and placed in KingdomProkaryotae.)

Lichen classification is complex since each lichenconsists of both fungal and algal parts. Both organ-isms are mutually beneficial or symbiotic. The sig-nificance of lichens to horticulture is not great. Ofthe 15 000 species, one species is considered a fooddelicacy in Japan. However, lichens growing ontree bark or walls are very sensitive to atmos-pheric pollution, particularly to the sulphur diox-ide content of the air. Different lichen species canwithstand varying levels of sulphur dioxide, and a

survey of lichen species can be used to indicatelevels of atmospheric pollution in a particulararea. Many contribute to the weathering of rock inthe initial stages of soil formation (see page 152).Lichens are also used as a natural dye, and canform an important part of the diet of some deer.

VIRUSES

Viruses are not included in any of the kingdoms.They are visible only under an electron microscope,and do not have a cellular structure, but consist ofnucleic acid surrounded by an outer protein coat(a capsid).They do not have cytoplasm, organellesand internal membranes found in the cells of livingorganisms (see page 40). They cannot grow, moveor reproduce without access to the cells of a hostcell, so they are not included in the classification ofliving things. Viruses survive by invading the cellsof other organisms, modifying their behaviour andoften causing disease: e.g. arabis mosaic, chrysan-themum stunt, cucumber mosaic, leaf mosaic,plum pox, reversion, tomato mosaic and tulipbreak (see pages 131–4).

FURTHER READING

Allaby, M. Concise Oxford Dictionary of Botany (OxfordUniversity Press, 1992).

Ayres,A.Gardening without Chemicals (Which/Hodder &Stoughton, 1990).

Baines, C. and Smart, J. A Guide to Habitat Creation(Packard Publishing, 1991).

Brown, L.V. Applied Principles of Horticulture, 2nd edi-tion (Butterworth-Heinemann, 2002).

Carson, R. Silent Spring (Hamish Hamilton, 1962).Caplin, B. Complete Manual of Organic Gardening

(Headline Publishing, 1992).Dowdeswell, W.H. Ecology, Principles and Practice

(Heinemann Educational Books, 1984).Ingram, D.S. et al. (Editors) Science and the Garden

(Blackwell Science Ltd., 2002).Lampkin, N. Organic Farming (Farming Press, 1990).Mannion, A.M. et al. (Editors). Environmental Issues in

the 1990s (John Wiley & Sons, 1992).Tait, J. et al. Practical Conservation (Open University,

1998).

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PLANT FORM

Most plant species at first sight appear very simi-lar, since all four organs, the root, stem, leaf andflower, are present in approximately the sameform and have the same major functions. The gen-eralized plant form for a dicotyledonous and amonocotyledonous plant can be seen in Figure 4.1.

In most species the functions of the root systemare to take up water and minerals from the grow-ing medium and to anchor the plant in the growingmedium.Two types of root system are produced; ataproot is a single large root which usually main-tains a direction of growth in response to gravity(see geotropism) with many small lateral rootsgrowing from it, e.g. in chrysanthemums, brassicasand dock. In contrast, a fibrous root system con-sists of many roots growing out from the base ofthe stem, as in grasses and groundsel (see page 47for root structure).

The leaf, consisting of the leaf blade (lamina)and stalk (petiole), carries out photosynthesis, itsshape and arrangement on the stem depending onthe water and light energy supply in the species’habitat (see page 57 for leaf structure).

The stem’s function is physically to support theleaves and the flowers, and to transport water,minerals and food between roots, leaves and flow-ers (see page 41 for stem structure). The leaf joinsthe stem at the node and has in its angle (axil) withthe stem an axillary bud, which may grow out toproduce a lateral shoot. The distance between onenode and the next is termed the internode.

Sexual reproduction is carried out in the flower,and therefore its appearance depends principallyon the agents of pollination (see page 76 for gen-eral flower structure).

Adaptation to plant organs have enabled plantsto compete and survive in their habitat. Plantsadapted to dry areas (xerophytes) such as cacti

4

Plant Organization

The main interest of many of those growingplants is the nature of the plant’s externalappearance, its form or morphology. Thischapter begins by describing the external fea-tures and discusses ways in which plants areused in gardens and landscapes.

A multicellular organism such as the plant,which carries out many complex processesinvolved in its growth and development,requires a complex organization to carry outits functions. To be efficient, the plant’s struc-tural unit must be subdivided so that a particu-lar area in the plant, i.e. an organ, carries outeach major function. The individual units ofthe plant, the cells, are grouped together intotissues of similar cell types, and each tissuecontributes to the activities of the whole organ.These structures and the nature of plantgrowth are explored and explained in the con-text of the stem.

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have leaves reduced to protective spines and stemscapable of photosynthesis (see Plate 4). Thorns,which are modified branches growing from axillarybuds, also have a protective function, e.g. hawthorn(Crataegus). Prickles are specialized outgrowthsof the stem epidermis, which not only protect butalso assist the plant in scrambling over other vege-tation, as in wild roses (see Plate 13).

Several species possess leaves modified specific-ally for climbing in the form of tendrils, as in manymembers of the Leguminosae family; and Clematisclimb by means of a sensitive, elongated leaf stalk,which twist round their support. In runner beansand honeysuckle (Lonicera), twining stems windaround other uprights for support. Others areable to climb with the help of adventitious rootssuch as ivies and virginia creeper (Parthenocissus).Epiphytes are physically attached to aerial parts ofother plants for support: they absorb and sometimesstore water in aerial roots, as in some orchids.

To survive an environment with very low nutri-ent levels, such as the sphagnum peat moor (seepage 190), some plants have evolved methods oftrapping insects and utilize the soluble products oftheir decomposed prey.These insectivorous plantsinclude the native sundew (Drosera) and butter-wort (Pinguicula spp.), which trap their prey withsticky glands on their leaves. Pitcher plants(Sarracenia spp.) have leaves that form containersinto which insects are able to enter but are pre-vented from escaping by slippery surfaces or bar-riers of stiff hairs. The venus flytrap (Dionaeamuscipula) has leaves that are hinged so that theycan snap shut on their prey when it alights on oneof the trigger hairs.

Plants found growing in coastal areas have adap-tations that allow them to withstand high salt lev-els, e.g. salt glands as found in the cord grass(Spartina spp.) or succulent tissues in ‘scurvy grass’(Cochlearia), both inhabitants of coastal areas.


flower bud

flower

bractpedicel

stempetiolelamina

midrib

internodeaxillary bud

taproot

spikelet

inflorescence

node

leaf blade

tiller emerging from nearground level

fibrous roots

(a) (b)

internode with a leaf steathenclosing the stem inside

Figure 4.1 Generalized plant form: (a) monocotyledon and (b) dicotyledon.

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Plant Organization 39

Other modifications in plants are dealt withelsewhere.This includes the use of stems and rootsas food and water storage organs (see vegetativepropagation).

Plant form in design

Plant form, as individual plants or in groups, is themain interest for many in horticulture who useplants in the garden or landscape. Contrasts inplant shapes and sizes can be combined to pleasethe eye of the observer.

The dominant plant within a garden feature isusually a tree or shrub chosen for its special strik-ing appearance. In a large feature, it may be aBetula pendula (silver birch) tree growing up to20 m in height with a graceful form, striking whitebark and golden autumn colour. In a smaller fea-ture, Euphorbia characias provides a very specialeffect with its 1 m high evergreen foliage, andspringtime yellow blooms. Such plants can form afocal point in a garden or landscape.

Spaced around these special plants, there maybe included species providing a visually support-ive background or skeletal form to the decorativefeature. Garrya elliptica, a 4 m shrub with ellip-tical, wavy edged evergreen leaves and mid-wintercatkins fits naturally into this category against alarger special plant. At 2.5 m, the evergreen shrubChoisya ternata (Mexican orange blossom) bearingfragrant white flowers in spring is a popular back-ground species in decorative borders. Jasminumnudifolium (winter jasmine) is an example of aclimber fulfilling this role. Such framework plantsnot only provide a suitable background, but alsocan provide continuity or unity through the gar-den or landscape and ensure interest all the yearround.

Fitting further into the mosaic of plantings are the numerous examples of decorative species,exhibiting particular aspects of general structureor flowering, and often having a deciduous growth.An example is the 0.3 m tall Cytisus � kewensis(broom) with its downy arching stems and profusecreamy-white spring flowers.A contrasting example

is the 2 m clump-forming grass species, Cortadieriaselloana, producing narrow leaves and featherylate summer flowering panicles. Climbing speciesfrom the Rosa and Clematis genera also fit into thedecorative category. Garden designers are alsoable to call on a very wide range of leaf forms tocreate textural or architectural interest in the bor-der (see Plate 2).

A host of deciduous pretty herbaceous andevergreen perennials is available for filling the dec-orative feature, fitting around the above-mentionedthree categories. Delphiniums (up to 2 m), Lupins(up to 1.5 m), Asters (up to 1.5 m), Sedums (up to0.5 m) and Alchemillas (up to 0.5 m) are five exam-ples illustrating a range of heights.

Finally, infill species either as bulbs (e.g. Tulipa,Narcissus, or Lilium), perennials (e.g. Saxifraga orCampanula) or annuals (e.g. Nicotiana or Begonia)may be placed within the feature, sometimes for arelatively short period whilst other perennials aregrowing towards full size. They are also used incolourful bedding displays.

Plant size and growth rate

It is important for anyone planning a garden thatthey recognize the eventual size (both in terms ofheight and of width) of trees, shrubs and perennials.This vital information is quite often ignored orforgotten at the time of purchase. The impressiveGinkgo (Maidenhair tree) really can grow up to30 m in height (at least twice the height of a nor-mal house) and is, therefore, not the plant to put ina small bed. Similarly, �Cupressocyparis leylandii(Leyland cypress), seemingly so useful in rapidlycreating a fine hedge, can also grow up to 30 m,and reach 5 m in width, to the consternation ofeven the most neighbourly of neighbours.

The eventual size of a plant is recorded in plantencyclopaedias, which should be carefully scrutin-ized for this vital statistic. It may also be wise tocontact a specialist nursery which deals with thisimportant aspect on a day-to-day basis, and willgive advise to potential buyers. It should beremembered that the eventual size of a tree or

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shrub may vary considerably in different parts ofthe country, and may be affected within a gardenby factors such as aspect, soil, shade and wind.

Attention should also be given to the rate atwhich a plant grows; Taxus (yew) or Magnolia stel-lata (star magnolia) are two notable examples ofslow growing species.

THE ANATOMY OF THE PLANT

A close examination of the internal structure(anatomy) of the plant with a microscope will revealhow it is made up of different tissues. Each tissueis a collection of specialized cells carrying out onefunction such as xylem conducting water.An organis made up of a group of tissues carrying out a spe-cific function such as a leaf producing sugars forthe plant. In the following section, the anatomy ofthe stem is illustrated and there is a comparable dis-cussion of root and leaf structures in the next twochapters.

The cell

Without the use of a microscope, the horticulturistwill not be able to see cells, since they are verysmall (about a twentieth of a millimetre in size).They are very complex and scientific studies con-tinued to discover more of the organization dis-played in this fundamental unit.

A simple, unspecialized cell of parenchyma (seeFigure 4.2) consists of a cellulose cell wall, andcontents (protoplasm) enclosed in a cell membrane,which is selective for the passage of materials in andout of the cell. The cellulose in the cell wall is laiddown in a mesh pattern, which allows for stretch-ing as the cell expands. Within the mesh frame-work are many apertures that, in active cells suchas parenchyma, allow for strands of cytoplasm(called plasmodesmata) connecting between adja-cent cells. These strands carry nutrients and hor-mones between cells, and are able to control thespeed at which this movement takes place.When aplant wilts, its cells become smaller, but the plas-modesmata normally retain their links with adjacent

cells. In the situation of ‘permanent wilting’ (seepage 174) or plasmolysis (see page 47), there is abreakage of these strands, and the plant is not ableto recover.

Suspended in the jelly-like cytoplasm are smallstructures (organelles) each enclosed within amembrane and having specialized functions withinthe cell. In all tissues, the cell walls of adjoiningcells are held together by calcium pectate (a glue-like substance which is an important setting ingre-dient in ‘jam-making’). Some types of cell (e.g.xylem vessels) do not remain biochemically active,but die in order to achieve their usefulness. Here,the first wall of cellulose becomes thickened byadditional cellulose layers and lignin, which is astrong, impervious substance.

The cell is made up of two parts, the nucleus andthe cytoplasm. The nucleus coordinates the chem-istry of the cell.The long chromosome strands thatfill the nucleus (see also page 80) are made up of the complex chemical deoxyribosenucleic acid,usually known as DNA. In addition to its ability toproduce more of itself for the process of cell div-ision, DNA is also constantly manufacturing smallerbut similar ribosenucleic acid (RNA) units, whichare able to pass through the nucleus membraneand attach themselves to other organelles. In thisway, the nucleus is able to transmit instructions forthe assembling, or destruction, of important chem-icals within the cell.


chloroplast

vacuole

mitochondrion

nucleus

cytoplasm enclosedin a cell membranecell wall

Figure 4.2 An unspecialized plant cell showing theorganelles responsible for the life processes.

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There are six main types of organelle in thecytoplasm.The first, the vacuole is a sac containingdilute sugar, nutrients and waste products. It mayoccupy the major volume of a cell, and its mainfunctions are storage and maintaining cell shape.The ribosomes make proteins from amino acids.Enzymes, which speed up chemical processes, aremade up of protein. The Golgi apparatus isinvolved in modifying and storing chemicals beingmade in the cell before they are transported wherethey are required. Mitochondria release energy,in a controlled way, by the process of respiration,to be used by the other organelles. The energy is transferred via a chemical called adenosinetriphosphate (ATP). The meristem areas of thestem, root and flower have cells with the highestnumber of mitochondria in order to help the rapidcell division and growth in these areas. Plastidssuch as the chloroplasts are involved in the pro-duction of sugar by the process of photosynthesis,

and in the short-term storage of condensed sugar(in the form of starch). Lastly, the endoplasmicreticulum is a complex mesh of membranes thatenables transport of chemicals within the cell, andlinks with the plasmodesmata at the cell surface.Ribosomes are commonly located on the endo-plasmic reticulum. The whole of the living matterof a cell, nucleus and cytoplasm, is collectivelycalled protoplasm.

TISSUES OF THE STEM

Dicotyledonous stem

The internal structure of a dicotyledonous plant,as viewed in cross-section, is shown in Figure 4.3.Three terms, epidermis, cortex and pith, are usedto broadly describe the distribution of tissuesacross the stem. The epidermis is present as an


epidermisepidermis

cortex

some collenchyma tissues

sclerenchyma fibressclerenchyma fibres

phloemphloem

vascularbundle

vascularbundle

cambiumxylem

xylem

cortex (parenchyma)

pith cavity

(a) (b)

Figure 4.3 Cross-sections of (a) monocotyledonous stem showing scattered vascular bundles with associated supporttissue and (b) dicotyledonous stem showing a ring of vascular bundles.

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outer protective layer of the stem, leaves androots. It consists of a single layer of cells; a smallproportion of them are modified to allow gasesto pass through an otherwise impermeable layer(see stomata). The second general term, cortex,describes the zone of tissues found inside the epi-dermis and reaching inwards to the the inner edgeof the vascular bundles. A third term, pith, refersto the central zone of the stem, which is mainlymade up of parenchyma cells.

Collenchyma and sclerenchyma cells are usu-ally found to the inside of the epidermis and areresponsible for support in the young plant. Bothtissues have cells with specially thickened walls.When a cell is first formed it has a wall composedmainly of cellulose fibres. In collenchyma cells theamount of cellulose is increased to provide extrastrength, but otherwise the cells remain relativelyunspecialized. In sclerenchyma cells, the thicknessof the wall is increased by the addition of a sub-stance called lignin, which is tough and causes theliving contents of the cell to disappear.These cells,which are long and tapering and interlock foradditional strength, consist only of cell walls.

The cortex of the stem contains a number of tis-sues. Many are made up of unspecialized cells suchas parenchyma. In these tissues, the cells are thinwalled and maintained in an approximately sphericalshape by osmotic pressure (see page 47). The massof parenchyma cells (surrounding the other tissues)combine to maintain plant shape. Lack of waterresults in the partial collapse of the parenchymacells, which is seen as wilting. Parenchyma cells alsocarry out other functions, when required. Many ofthese cells contain chlorophyll (giving the stemstheir green colour) and so are able to photosynthe-size. They release energy, by respiration, for use inthe surrounding tissues. In some plants such as thepotato they are also capable of acting as food stores(the potato tuber which stores starch).They are alsoable to undergo cell division, a useful property whena plant has been damaged.This property has practi-cal significance when plant parts such as cuttingsare being propagated, since new cells can be createdby the parenchyma to heal wounds and initiate rootdevelopment.

Contained in the cortex are vascular bundles, sonamed because they contain two vascular tissuesthat are responsible for transport. The first, xylem,contains long, wide, open-ended cells with verythick lignified walls, able to withstand the highpressures of water with dissolved minerals whichthey carry. The second vascular tissue, phloem,consists again of long, tube-like cells, and isresponsible for the transport of food manufac-tured in the leaves carried to the roots, stems orflowers (see translocation). The phloem tubes, incontrast to xylem, have fairly soft cellulose cellwalls. The end walls are only partially brokendown to leave sieve-like structures (sieve plates)at intervals along the phloem tubes. Alongsideevery phloem tube cell, there is a small companioncell, which regulates the flow of liquids down thesieve tube.The phloem is seen on either side of thexylem in the marrow stem, but is found to the out-side of the xylem in most other species. Phloem ispenetrated by the stylets of feeding aphids (seeChapter 10).

Also contained within the vascular bundles isthe cambium tissue, which contains actively divid-ing cells producing more xylem and phloem tis-sues as the stem grows.

Monocotyledonous stem

This has the same functions as those of a dicoty-ledon; therefore the cell types and tissues are simi-lar. However, the arrangement of the tissues doesdiffer because increase in diameter by secondarygrowth does not take place. The stem relies onextensive sclerenchyma tissue for support that, inthe maize stem shown in Figure 4.3, is found as asheath around each of the scattered vascular bun-dles. Monocotyledonous stem structures are seenat their most complex in the palm family. From theoutside, the trunk would appear to be made ofwood, but an internal investigation shows thatthe stem is a mass of sclerified vascular bundles.The absence of secondary growth in the vascularbundles makes the presence of cambium tissueunnecessary.


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STEM GROWTH

Growth of stems is initiated in the apical, or ter-minal, bud at the end of the stem (the apex). Deepinside the apical bud lies a tiny mass of small,delicate jelly-like cells, each with a conspicuousnucleus but no cell vacuole. This mass is the apicalmeristem (see Figure 4.4). Here, cells divide fre-quently to produce four kinds of meristematic tis-sues.The first, at the very tip, continues as meristemcells. The second (protoderm) near the outsidedevelops into the epidermis. The third (procam-bium) becomes the vascular bundles. The fourth(ground meristem) turns into the parenchyma,collenchyma and sclerenchyma tissues of the cor-tex and pith. In addition to its role in tissue forma-tion, the apical meristem also gives rise to smallleaves (bud scales) that collectively protect themeristem. These scales and the meristem togetherform the bud. It should be noted that any damageto the sensitive meristem region by aphids, fungi,bacteria or herbicides would result in distortedgrowth. A fairly common example of such a dis-tortion is fasciation, a condition that resembles anumber of stems fused together. Buds locatedlower down the stem, in the angle of the leaf (theaxil), are called axillary buds, and these often giverise to side branches.

In some plant families, e.g. the Graminae, themeristem remains at the base of the leaves, whichare therefore protected against some herbicides,e.g. 2,4-D (see Chapter 9). This also means thatgrasses re-grow from their base after animals havegrazed them. The new blades of grass grow frommeristems between the old leaf and the stem. Thismeans grasses can be mown which enables us tocreate lawns. The process of cutting back the grassalso leads it to sending up several shoots instead ofjust one. This process of tillering helps thicken upthe turf sward to make it such a useful surface forsport as well as decoration. Mowing kills thedicotyledonous plants that have their stems cut-off at the base and lose their meristems. However,many species are successful lawn weeds by grow-ing in prostrate form; the foreshortened stem(very short internodes) creates a rosette of leaves

that helps to conserve water, shades out surround-ing plants and the growing point stays below thecutting height of the mower.

Elongation of the plant stem takes place in twostages. Firstly, cell division, described above, con-tributes a little.The second phase is cell expansion,which occurs at the base of the meristem. Here,the tiny unspecialized meristem cells begin to takein water and nutrients to form a cell vacuole. As aresult, each cell elongates, and the stem rapidlygrows. In the expansion zone, other developmentsbegin to occur. Most importantly, the cells begin tocreate their cell walls, and the connections betweencells (plasmodesmata).The exact shape and chem-ical composition of the wall is different for eachtype of tissue cell, since it has a particular functionto perform. This whole process of cell maturationis referred to as ‘cell differentiation’.

As the stem length increases, so the width alsoincreases to support the bigger plant and supplythe greater amount of water and minerals required.This process in dicotyledons is called secondarygrowth (see Figure 4.5). Additional phloem andxylem are produced on either side of the cambiumtissue, which now forms a complete ring. As thesetissues increase towards the centre of the stem, sothe circumference of the stem must also increase.Therefore a secondary ring of cambium (corkcambium) is formed, just to the inside of the epi-dermis, the cells of which divide to produce a layerof corky cells on the outside of the stem.This layerwill increase with the growth of the tissue insidethe stem, and will prevent loss of water if cracksshould occur. As more secondary growth takesplace, so more phloem and xylem tissues are


protective leaves

apical meristemprotoderm

ground meristemprocambium

Figure 4.4 The tip of a dicotyledonous stem showing thefour meristematic areas.

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produced but the phloem tubes, being soft, aresquashed as the more numerous and very hardxylem vessels occupy more and more of the cross-section of the stem. Eventually, the majority of thestem consists of secondary xylem that forms thewood.

The central region of xylem sometimesbecomes darkly stained with gums and resins(heartwood) and performs the long-term functionof support for a heavy trunk or branch. The outerxylem, the sapwood, is still functional in transport-ing water and nutrients, and is often lighter incolour. The xylem tissue produced in the springhas larger diameter vessels than autumn-producedxylem, due to the greater volume of water that mustbe transported; a distinct ring is therefore producedwhere the two types of tissue meet. As these ringswill be formed each season, their number can indi-cate the age of the branch or trunk; they are calledannual rings. The phloem tissue is pushed againstthe cork layers by the increasing volume of xylemso that a woody stem appears to have two distinctlayers, the wood in the centre and the bark on theoutside.

If bark is removed, the phloem also will be lost,leaving the vascular cambium exposed.The stem’sfood transport system from leaves to the roots is

thus removed and, if a trunk is completely ringed(or ‘girdled’), the plant will die. Rabbits or deer inan orchard may cause this sort of damage. ‘Partialringing’, i.e. removing the bark from almost thewhole of the circumference can achieve a deliber-ate reduction in growth rate of vigorous tree fruitcultivars and woody ornamental species. Initially,the bark is smooth and shiny but with age it thick-ens and the outer layers accumulate chemicals(including suberin) that make it an effective pro-tection against water loss and pest attack.This partof the bark (called cork) starts to peel or flake off.This is replaced from below and the cork graduallytakes on its characteristic colours and textures.Many trees such as silver birch, London plane,Prunus serrula, Acer davidii and many pines andrhododendrons have attractive bark and are partic-ularly valued for winter interest (see Plate 13).

Since the division of cells in the cambium pro-duces secondary growth, it is important that whengrafting a scion (the material to be grafted) to astock, the vascular cambium tissues of both com-ponents be positioned as close to each other aspossible.The success of a graft depends very muchon the rapid callus growth derived from the cam-bium, from which new cambial cells form and sub-sequently from which the new xylem and phloem


pith

annual rings

xylem

vascular cambium

phloem

cork cambium

cork

medullary ray

lenticel

Figure 4.5 Cross-section of woody stem of lime (Tilia europea) showing large central woody area and development ofbark to the outside.

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vessels form to complete the union. The two partsthen grow as one to carry out the functions of theplant stem.

A further feature of a woody stem is the mass oflines radiating outwards from the centre, mostobvious in the xylem tissues. These are medullaryrays, consisting of parenchyma tissue linking upwith small areas on the bark where the corky cellsare less tightly packed together (lenticels). Theseallow air to move into the stem and across thestem from cell to cell in the medullary rays. Theoxygen in the air is needed for the process of res-piration, but the openings can be a means of entryof some diseases, e.g. fireblight. Other external fea-tures of woody stems include the leaf scars, whichmark the point of attachment of leaves fallen atthe end of a growing season, and can be a point ofentry of fungal spores such as apple canker.

Secondary thickening is found not only in treesand shrubs, but also in many herbaceous perennialsand annuals that have woody stem. However,trees and shrubs do exhibit this feature to thegreatest extent.

Note that trees are large woody plants that havea main stem with branches appearing some dis-tance above ground level. Shrubs are smaller, usu-ally less than 3 m in height, but with branchesdeveloping at or near ground level to give a bushyappearance to the plant.

FURTHER READING

Bowes, B.G. A Colour Atlas of Plant Structure (MansonPublishing, 1996).

Brown, L.V. Applied Principles of Horticulture, 2nd edition (Butterworth-Heinemann, 2002).

Clegg, C.J. and Cox, G. Anatomy and Activities of Plants(John Murray, 1978).

Cutler, D.F. Applied Plant Anatomy (Longman, 1978).Ingram, D.S. et al. (editors). Science and the Garden

(Blackwell Science Ltd, 2002).Lewes, D. Plants and Nitrogen – New Studies in Biology

(Edward Arnold, 1986).Mauseth, J.P. Botany – An Introduction to Plant Biology

(Saunders, 1998).


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WATER

Functions of water

The plant consists of about 95 per cent water,which is the main constituent of protoplasm or liv-ing matter. When the plant cell is full of water, orturgid, the pressure of water enclosed within amembrane or vacuole acts as a means of supportfor the cell and therefore the whole plant, so thatwhen a plant loses more water than it is taking up,the cells collapse and the plant may wilt. Aquaticplants are supported largely by external water andhave very little specialized support tissue. In orderto survive, any organism must carry out complexchemical reactions, which are explained, and theirhorticultural application described, in Chapters 6and 7. Raw materials for chemical reactions mustbe transported and brought into contact with each

other by a suitable medium; water is an excellentsolvent. One of the most important processes inthe plant is photosynthesis, and a small amount ofwater is used up as a raw material in this process.

MOVEMENT OF WATER

Water moves into the plant through the roots, thestem and into the leaves, and is lost to the atmos-phere. By the process of diffusion, molecules of agas or liquid move from an area of high concen-tration to an area where there is a relatively lowerconcentration of the diffusing substance. Thus,water vapour moves through the stomata (see page57) from an area of high concentration inside theleaf into the air immediately surrounding the leafwhere there is a lower relative humidity. The path-way of water movement through the plant fallsinto three distinct stages: water uptake, movementup the stem and transpiration loss from the leaves.

Water uptake

The movement of water into the roots is by a specialtype of diffusion called osmosis. Soil water entersroot cells through the cell wall and membrane. Thecell wall is permeable to both soil water and thedissolved inorganic minerals, but the cell mem-brane, although permeable to water, allows onlythe smallest molecules to pass through, somewhat

5

Water and Minerals in the Plant

Water is the major constituent of any livingorganism and the maintenance of a plant withoptimum water content is a very importantpart of plant growth and development (seeSoil Water, Chapter 14). Probably more plantsdie from lack of water than from any othercause. Minerals are also raw materials essen-tial to growth, and are supplied through theroot system.

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Water and Minerals in the Plant 47

like a sieve, and is therefore described as a par-tially permeable membrane.

The minerals move into the cells by a processthat requires energy to ‘push’ the molecules intothe cell, and a greater concentration of minerals isusually maintained inside the cell compared withthat in the soil water. This means that, by osmosis,water will move from the soil into the cell wherethere is relatively less water, as there are moreinorganic salts and sugars. The greater the differ-ence in concentration of inorganic salts either sideof the cell membrane, the greater the osmoticpressure and the faster the water moves into theroot cells, also affected by increased temperature.Osmosis can therefore be defined as the movementof water from an area of low salt concentrationto an area of relatively higher salt concen-tration, through a partially permeable membrane.If there is a build-up of salts in the soil, either overa period of time or, e.g. where too much fertilizeris added, water may move out of the roots byosmosis, and the cells are then described as plasmo-lysed. Cells that lose water this way can recovertheir water content if the conditions are rectifiedquickly, but it can lead to permanent damage tothe cell interconnections (see page 40). Such situa-tions can be avoided by correct dosage of fertilizerand by monitoring of conductivity levels in green-house soils and nutrient film technique (NFT) systems (see Chapter 17).

Root structure (see Figure 5.1). The function ofthe root system is to take up water and mineralnutrients from the growing medium and to anchorthe plant in that medium.Its major function involvesmaking contact with the water in the growingmedium. To achieve this it must have as a largesurface area as possible. The root surface near tothe tip where growth occurs (cell division in themeristem, see page 43) is protected by the rootcap. The root zone behind the root tip has tinyprojections called root hairs reaching numbers of 200–400/mm2, which greatly increase the sur-face area in this region (see Figure 5.2). Plantsgrown in hydroculture, e.g. NFT (page 218), pro-duce considerably fewer root hairs. The loss ofroot hairs during transplanting can check plant

growth considerably, and the hairs can be points ofentry of diseases such as club root (see Chapter 11).The layer with the root hairs, the epidermis, iscomparable with the epidermis of the stem (seestem structure); it is a single layer of cells whichhas a protective as well as an absorptive function.

Inside the epidermis is the parenchymatous cor-tex layer. The main function of this tissue is respir-ation to produce energy for growth of the root andfor the absorption of mineral nutrients. The cortexcan also be used for the storage of food where theroot is an overwintering organ (see page 68). The

Cross-section

thickenedepidermiscortex(parenchyma)endodermis withCasparian stripphloemxylem

stele

Figure 5.1 Cross section of lily root showing thickenedouter region, large area of cortex and central vascularregion enclosed in a single-celled endodermis.

root hairzone

root tip

root cap

Long view ofroot tip region

Figure 5.2 Root tip showing the tip protected by rootcap and the root hair zone.

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cortex is often quite extensive and water mustmove across it in order to reach the transportingtissue that is in the centre of the root. This centralregion, called the stele, is separated from the cortexby a single layer of cells, the endodermis, which hasthe function of controlling the passage of waterinto the stele. A waxy strip forming part of the cellwall of many of the endodermal cells (the Caspar-ian strip) prevents water from moving into the cellby all except the cells outside it, called passagecells. In this way, the volume of water passing intothe stele is restricted. If such control did not occur,more water could move into the transport systemthan can be lost through the leaves. In some condi-tions, such as in high air humidity (see page 25),more water moves into the leaves than is being lostto the air, and the more delicate cell walls in the leafmay burst. This condition is known as oedema, andcommonly occurs in Pelargonium as dark greenpatches becoming brown, and also weak-celledplants such as lettuce, when it is known as tipburn,because the margins of the leaves in particular willappear scorched. Guttation may occur when liquidwater is forced onto the leaf surface.

Water passes through the endodermis to thexylem tissue, which transports the water and dis-solved minerals up to the stem and leaves. Thearrangement of the xylem tissue varies betweenspecies, but often appears in transverse section as astar with varying numbers of ‘arms’. Phloem tissueis responsible for transporting carbohydrates fromthe leaves as a food supply for the production ofenergy in the cortex.

A distinct area in the root inside the endoder-mis, the pericycle, supports cell division and produces lateral roots, which push through to themain root surface from deep within the structure.

Roots age and become thickened with waxysubstances, and the uptake rate of water becomesrestricted.

Movement of water up the stem

Three factors contribute to water movement.Osmotic forces, previously described, push water

up the stem to a height of about 30 cm. This effect,called root pressure, can provide a large propor-tion of the plant’s water needs in smaller annualspecies. A second effect is the capillary action(attraction of the water molecules for the sides ofthe xylem vessels), which may lift water a few centi-metres, but is not considered a significant factor inwater movement. The third factor, transpirationpull, is the major process that moves soil water toall parts of the plant.

Transpiration

Transpiration is the loss of water vapour from theleaves of the plant. Any plant takes up a lot ofwater through its roots; e.g. a tree can take upabout 1000 litres (about 200 gallons) a day.Approximately 98 per cent of the water taken upmoves through the plant and is lost by transpira-tion; only about 2 per cent is retained as part of theplant’s structure, and a yet smaller amount is usedup in photosynthesis.

The seemingly extravagant loss through leavesis due to the unavoidably large pores in the leafsurface (stomata) essential for carbon dioxide dif-fusion (see Figure 6.1). Two other points, however,should be considered here. Water vapour diffusesoutwards through the leaf stomata more quicklythan carbon dioxide (to be used for photosynthesis)

reducedsurfacearea

sunkenstomata

thickcuticle

Figure 5.3 Cross-section of pine (Pinus) leaf showingsome adaptations to reduce excessive water loss.

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entering. However, the plant is able to partiallyclose the stomata to reduce water loss withoutcausing a carbon dioxide deficiency in the leaf. Sec-ondly, the diffusion rate of water vapour throughthe stomata leads to a leaf cooling effect enablingthe leaf to function whilst being exposed to highlevels of radiation. A remarkable aspect of tran-spiration is that water can be pulled (‘sucked’)such a long way to the tops of tall trees. Engineershave long known that columns of water breakwhen they are more than about 10 m long; and yeteven tall trees such as the giant redwoods pullwater up a 100 m from ground level.This apparentability to flout the laws of nature is probably dueto the small size of the xylem vessels, which greatlyreduce the possibility of the water columns col-lapsing. A further impressive aspect of the plantstructure is seen in the extreme ramifications ofthe xylem system in the veins of the leaf. This finenetwork ensures that water moves by transpir-ation pull right up to the spongy mesophyll spacesin the leaf (see page 57), and avoids any watermovement through living cells, which would slowdown the process many thousand times.

The potential for the entry of other factors, e.g.fungi, is also due partly to the existence of thestomata. If the air surrounding the leaf becomesvery humid, then the diffusion of water vapourwill be much reduced and the rate of transpirationwill decrease. Application of water to greenhousepaths during the summer, damping down, increasesrelative humidity and reduces transpiration rate. Ifthe air surrounding the leaf is moving, humidity ofair around the leaf is low, so that transpiration ismaintained. Windbreaks, e.g. some woody species,reduce the risk of desiccation of crops. Ambienttemperatures affect the rate at which liquid waterin the leaf evaporates and thus determines thetranspiration rate. The plant is able to control itstranspiration rate because the cuticle (a waxywaterproof layer) protects most of its surface andthe stomata are able to close up as the cells in theleaf start to lose their turgor (see leaf structure,page 57). The stomatal pore is bordered by twosausage-shaped guard cells, which have thick cellwalls near to the pore. When the guard cells are

fully turgid, the pressure of water on the thinnerwalls causes the cells to buckle and the pore toopen. If the plant begins to lose more water, theguard cells lose their turgidity and the stomataclose to prevent any further water loss.Stomata alsorespond to light and dark, respectively, by openingand closing. The mechanism of this action is notfully understood. The very quick response of thechloroplast-containing guard cells is unlikely to be due to cell turgor induced by photosynthesis.Stomata also close if carbon dioxide concentrationin the air rises above optimum levels.

A close relationship exists between the dailyfluctuation in the rate of transpiration and thevariation in solar radiation. This is used to assessthe amount of water being lost from cuttings inmist units (see misting); a light-sensitive cell auto-matically switches on the misting. In artificial con-ditions, e.g. in a florist shop, transpiration rate canbe reduced by providing a cool, humid and shadedenvironment.

Plasmolysed leaf cells can occur if highly con-centrated sprays cause water to leave the cells andresult in scorching (see page 47).

The evaporation of water from the cells of theleaf means that in order for the leaf to remainturgid, which is important for efficient photosyn-thesis, the water lost must be replaced by water inthe xylem. Pressure is created in the xylem by theloss from an otherwise closed system and watermoves up the petiole of the leaf and stem of theplant by suction (see transpiration pull). If the waterin the xylem column is broken, e.g. when a stem ofa flower is cut, air moves into the xylem and mayrestrict the further movement of water when thecut flower is placed in water. However, once thecolumn is restored water enters by the cut surfaceat a faster rate than if the plant was intact with aroot system.

Anti-transpirants are plastic substances which,when sprayed onto the leaves, will create a tempor-ary barrier to water loss over the whole leaf sur-face, including the stomata. These substances areuseful to protect a plant during a critical period inits cultivation; e.g. conifers can be treated whilethey are moved to another site.


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Structural adaptations to the leaf occur in somespecies to enable them to withstand low watersupplies with a reduced surface area, a very thickcuticle and sunken guard cells protected below theleaf surface Figure 5.3. Compare this cross sectionwith that of a more typical leaf shown in Figure6.1. In extreme cases, e.g. cacti, the leaf is reducedto a spine, and the stem takes over the function ofphotosynthesis and is also capable of water stor-age, as in the stonecrop (Sedum). Other adapta-tions are described on page 58.

MINERALS

Essential minerals are those inorganic substancesnecessary for the plant to grow and develop normally. They can be conveniently divided intotwo groups. The major nutrients (macronutrients)are required in relatively large quantities whereasthe micronutrients (trace elements) are needed inrelatively small quantities, usually measured inparts per million (ppm), and within a narrow con-centration range to avoid deficiency or toxicity.The list of essential nutrients is given in Table 5.1.

Non-essential minerals, such as sodium andchlorine, have important functions in some plantsbut are not constantly required for plant growthand development.

Functions and deficiency symptoms of minerals in the plant

Many essential minerals have very specific func-tions in the plant cell processes. When in short sup-ply (deficient) the plant shows certain characteristicsymptoms, but these symptoms tend to indicate anextreme deficiency. To ensure optimal mineral supplies, growing media analysis (see Chapter 16)or plant tissue analysis can be used to forecast low nutrient levels, which can then be suitablyincreased.

Nitrogen is a constituent of proteins, nucleicacids and chlorophyll and, as such, is a majorrequirement for plant growth. Its compounds com-prise about 50 per cent of the dry matter of proto-plasm, the living substance of plant cells. Deficiencycauses slow, spindly growth in all plants and yel-lowing of the leaves (chlorosis) due to lack ofchlorophyll. Stems may be red or purple due to theformation of other pigments. The high mobility ofnitrogen in the plant to the younger, active leavesleads to the old leaves showing the symptoms first.

Phosphorus is important in the production ofnucleic acid and the formation of adenosinetriphosphate (ATP) (see page 41). Large amountsare therefore concentrated in the meristem.Organic phosphates, so vital for the plant’s respir-ation, are also required in active organs such asroots and fruit, while the seed must store adequatelevels for germination. Phosphorus supplies at theseedling stage are critical; the growing root has ahigh requirement and the plant’s ability to estab-lish itself depends on the roots being able to tapinto supplies in the soil before the reserves in theseed are used up (see 202). Deficiency symptomsare not very distinctive. Poor establishment ofseedlings results from a general reduction in growthof stem and root systems. Sometimes a generaldarkening of the leaves in dicotyledonous plantsleads to brown leaf patches, while a reddish tingeis seen in monocotyledons. In cucumbers grown indeficient peat composts or NFT, characteristicstunting and development of small young leaveslead to brown spotting on older leaves.


Table 5.1 Nutrient requirements of plants

Macronutrients (major nutrients)N NitrogenP PhosphateK PotassiumMg MagnesiumCa CalciumS Sulphur

Micronutrients (trace elements)Fe IronBo BoronMn ManganeseCu CopperZn ZincMo Molybdenum

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Potassium. Although present in relatively largeamounts in plant cells, this mineral does not haveany clear function in the formation of importantcell products. It exists as a cation and acts as anosmotic regulator, e.g. in guard cells (see osmosis,page 47), and is involved in resistance to chillinginjury, drought and disease.

Deficiency results in brown, scorched patches onleaf tips and margins, especially on older leaves,due to the high mobility of potassium towards grow-ing points. Leaves may develop a bronzed appear-ance and roll inwards and downwards (see Plate 9).

Magnesium is a constituent of chlorophyll. It isalso involved in the activation of some enzymesand in the movement of phosphorus in the plant.Deficiency symptoms appear initially in olderleaves because magnesium is mobile in the plant.A characteristic interveinal chlorosis appears,which subsequently become reddened and even-tually necrotic (dead) areas develop (see Plate 10).

Calcium is a major constituent of plant cell wallsas calcium pectate, which binds the cells together.It also influences the activity of meristems espe-cially in root tips. Calcium is not mobile in theplant so the deficiency symptoms tend to appear inthe younger tissues first. It causes weakened cellwalls, resulting in inward curling, pale youngleaves and sometimes death of the growing point.Specific disorders include ‘topple’ in tulips, whenthe flower head cannot be supported by the top ofthe stem, ‘blossom end rot’ in tomato fruit, and‘bitter pit’ in apple fruit.

Sulphur is a vital component of many proteinsthat includes many important enzymes. It is alsoinvolved in the synthesis of chlorophyll. Conse-quently a deficiency produces a chlorosis that, dueto the relative immobility of sulphur in the plant,is shown in younger leaves first.

Iron and manganese are involved in the syn-thesis of chlorophyll; although they do not formpart of the molecule they are components of someenzymes required in its synthesis. Deficiencies ofboth minerals result in leaf chlorosis. The immo-bility of iron causes the younger leaves to showinterveinal chlorosis first. In extreme cases, thegrowing area turns white.

Boron affects various processes, such as thetranslocation of sugars and the synthesis of gib-berellic acid in some seeds (see dormancy, page63). Deficiency causes a breakdown and disorgan-ization of tissues, leading to early death of thegrowing point. Characteristic disorders include‘brown heart’ of turnips, and ‘hollow stem’ in bras-sicas. The leaves may become misshapen, andstems may break. Flowering is often suppressed,while malformed fruit are produced, e.g. ‘corkycore’ in apples and ‘cracked fruit’ of peaches.

Copper is a component of a number of enzymes.Deficiency in many species results in dark greenleaves, which become twisted and may prema-turely wither.

Zinc, also involved in enzymes, produces char-acteristic deficiency symptoms associated with thepoor development of leaves, e.g. ‘little leaf ‘ in cit-rus and peach, and ‘rosette leaf’ in apples.

Molybdenum assists the uptake of nitrogen,and although required in very much smaller quan-tities, its deficiency can result in reduced plant nitro-gen levels. In tomatoes and lettuce, deficiency ofmolybdenum can lead to chlorosis in older leaves,followed by death of cells between the veins (inter-veinal necrosis) and leaf margins. Tissue browningand infolding of the leaves may occur. In brassicas,the ‘whiptail’ leaf symptom involves a dominantmidrib and loss of leaf lamina.

Mineral uptake

Minerals are absorbed in water by the root sys-tem, which obtains its supply from the growingmedium (see Chapter 17). The plants take up onlywater-soluble material. Therefore all supplies ofnutrients including fertilizers and manures mustbe in the form of ions (charged particles). Themovement of the elements in the form of ionsoccurs in the direction of root cells containing ahigher mineral concentration than the soil, i.e.against a concentration gradient. The passage inthe water medium across the root cortex is by sim-ple diffusion, but transport across the endodermisrequires a supply of energy from the root cortex.


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The process is therefore related to temperatureand oxygen supply (see respiration, page 59).

Nutrients are taken up predominantly by theextensive network of fine roots that grow in thetop layers of the soil (see Figure 5.4). Damage tothe roots near the soil surface by cultivations shouldbe avoided because it can significantly reduce theplant’s ability to extract nutrients. It is recom-mended that care should be taken to ensure thattrees and shrubs are planted so their roots are notburied too deeply and many advocate that thehorizontally growing roots should be set virtuallyat the surface to give the best conditions forestablishment.

The surface thickening that occurs in the ageingroot does not significantly reduce the absorptionability of most minerals, e.g. potassium and phos-phate, but calcium is found to be principally takenup by the young roots.

FURTHER READING

Bowes, B.G. A Colour Atlas of Plant Structure (Manson,1996).


Capon, B. Botany for Gardeners (Timber Press, 1990).Clegg, C.J. and Cox, G. Anatomy and Activities of Plants

(John Murray, 1978).Ingram, D.S. et al. (Editors) Science and the Garden

(Blackwell Science Ltd., 2002).MAFF. Diagnosis of Mineral Disorders in Plants, Vol. 1

Principles (HMSO, 1984), Vol. 2 Vegetables (HMSO,1984), Vol. 3 Glasshouse Crops (HMSO, 1987).

Mauseth, J.P. Botany – An Introduction to Plant Biology(Saunders, 1998).

Scott Russell, R. Plant Root Systems:Their Function andInteraction with the Soil (McGraw-Hill, 1982).

Sutcliffe, J. Plants and Water (Edward Arnold, 1971).Sutcliffe, J.F. and Baker, D.A. Plant and Mineral Salts

(Edward Arnold, 1976).

Figure 5.4 Apple (Cox on M1) excavated at 16 years to reveal distribution of roots. Note the vigorous main root systemnear the surface with some penetrating deeply (courtesy of Dr E.G. Coker).

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Growth is a difficult term to define because itreally encompasses the totality of all the processesthat takes place during the life of an organism.However, it is useful to distinguish between theprocesses which result in an increase in size andweight, and those processes which cause thechanges in the plant during its life cycle, which can usefully be called development, described inChapter 7.

PHOTOSYNTHESIS

Photosynthesis is the process in the chloroplastsof the leaf and stem cells by which a green plantmanufactures food in the form of high energy carbo-hydrates such as sugars and starch, using light asenergy. All living organisms require organic mat-ter as food to build up their structure and to pro-vide chemical energy to fuel their activities. Whilstphotosynthesis is the crucial process, it should beremembered that a multitude of other processes is occurring all over the plant. Proteins are beingproduced, many of which are enzymes necessaryto speed up chemical reactions in the leaf, thestem, the root, and later in the flower and fruit.The complex carbohydrate, cellulose is being built up as cell walls of almost every cell. Nucleo-proteins are being provided in meristematic areasto enable cell division. These are three examplesof the many, to show that growth involves muchmore than just photosynthesis and respiration.

All the complex organic compounds, based oncarbon, must be produced from the simple rawmaterials, water and carbon dioxide. Many organ-isms are unable to manufacture their own food,and must therefore feed on already manufacturedorganic matter such as plants or animals. As largeanimals predate on smaller animals, which them-selves feed on plants, all organisms depend directlyor indirectly on photosynthesis occurring in theplant as the basis of a food web or chain.

6

Plant Growth

In any horticultural situation, growers areconcerned with controlling and even manipu-lating plant growth.They must provide for theplants the optimum conditions to produce themost efficient growth rate and the end-productrequired.Therefore the processes that result ingrowth are explored in order that the mostsuitable or economic growth can be achieved.Photosynthesis is probably the single mostimportant process in plant growth. Respirationis the process by which the food matter produced by photosynthesis is converted into energy usable for growth of the plant.Photosynthesis and respiration make theseprocesses possible and the balance betweenthese results in growth. It must be emphasizedthat growth involves the plant in hundreds ofchemical processes, occurring in the differentorgans and tissues throughout the plant.

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A summary of the process of photosynthesis is given in Table 6.1 as a word formula and as achemical equation. This apparently simple equa-tion represents, in reality, two different stages inthe production of glucose. The first, the ‘light reac-tion’, occurs during daylight, and splits water intohydrogen and oxygen. The second, the ‘dark reac-tion’ occurring at night, takes the hydrogen andjoins it to carbon dioxide to make glucose.

Most plant species follow a ‘C-3’ process of photo-synthesis where the intermediate chemical com-pound contains three-carbon atoms (C-3) beforeproducing the six-carbon glucose molecule (C-6).Many C-3 plants are not able to increase their rateof photosynthesis under conditions of very highlight levels.

In contrast, a ‘C-4’ process is seen in many trop-ical families, including the maize family, whereplants, which use an intermediate compound con-taining four-carbon atoms (C-4), are able to con-tinue to respond to very high levels of light, thusincreasing their productivity.

A third process, called ‘crassulacean acid metab-olism’ (CAM), was first discovered in the stonecropfamily. Here, the intermediate chemical is a differ-ent four-carbon compound, malic acid. This thirdprocess has more recently been found in severalother succulent plant families (including cacti), allof which need to survive conditions of drought.Suchplants need to keep their stomata closed duringthe heat of the day, but this prevents the entry ofcarbon dioxide. During the night, carbon dioxide isabsorbed and stored as malic acid, ready for con-version to glucose, the next day. CAM plants do not normally grow very fast because they are notable to store large quantities of this malic acid,

and thus their potential for glucose production islimited.

Requirements for photosynthesis

Carbon dioxideIn order that a plant may build up organic com-pounds such as sugars, it must have a supply of car-bon which is readily available. Carbon dioxide ispresent in the air in concentrations of 330 ppm (partsper million) or 0.03 per cent, and can diffuse into theleaf through the stomata, as described in Chapter 5.Carbon dioxide gas moves 10 000 times faster than itwould in solution through the roots.The amount ofcarbon dioxide in the air immediately surroundingthe plant can fall when planting is very dense, orwhen plants have been photosynthesizing rapidly,especially in an unventilated greenhouse.

This reduction will slow down the rate of photo-synthesis, but a grower may supply additional car-bon dioxide inside a greenhouse or polythenetunnel to enrich the atmosphere up to about threetimes the normal concentration, or an optimum of1000 ppm (0.1 per cent) in lettuce. Such practiceswill produce a corresponding increase in growth,provided other factors are available to the plant. Ifany one of these is in short supply, then the processwill be slowed down. This principle, called the lawof limiting factors, states that the factor in least sup-ply will limit the rate of the process, and applies toother non-photosynthetic processes in the plant. Itwould be wasteful, therefore, to increase the carbondioxide concentration artificially, e.g. by burningpropane gas, or releasing pure carbon dioxide gas,if other factors were not proportionally increased.


Table 6.1 Two ways to represent the chemistry involved in photosynthesis

(a) Written in a conventional way, the process can be expressed in the following way:

Carbon dioxide � water � light → glucose � oxygen (when in the presence of chlorophyll)

(b) Written in the form of a chemical equation, which represents molecular happenings at the, sub-microscopic level; theabove sentence becomes:

6CO2 molecules � 6H2O molecules � light → 1C6H12O6 molecule � 6O2 molecules(when in the presence of chlorophyll)

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Plant Growth 55

LightLight is a factor required in order that photosyn-thesis can occur. In any series of chemical reactionswhere one substance combines with another toform a larger compound, energy is needed to fuelthe reactions. Energy for photosynthesis is pro-vided by light from the sun or from artificial lamps.As with carbon dioxide, the amount of light energypresent is important in determining the rate ofphotosynthesis – simply, the more light or greaterilluminance (intensity) absorbed by the plant, themore photosynthesis can take place. Light energyis measured in joules per square metre, but for prac-tical purposes the light for plant growth is meas-ured according to the light falling on a given area,i.e. lumens per square metre (lux). Radiant energy(irradiance) is a less useful method of measure-ment because it includes a significant quantity ofenergy from wavelengths that do not contribute tophotosynthesis. However, photosynthetically activeradiation (PAR) is the most useful method as it is the energy that can be used for photosynthesis(units � W/m2).Whilst the measurement of illumi-nance is a very useful tool for the grower, it is difficult to state the plant’s precise requirements,as variation occurs with species, age, temperature,carbon dioxide levels, nutrient supply and healthof the plant.

However, it is possible to suggest approximatelimits within which photosynthesis will take place;a minimum intensity of about 500–1000 lux enablesthe plant’s photosynthesis rate to keep pace withrespiration, and thus maintain itself.The maximumamount of light many plants can usefully absorb is approximately 30 000 lux, while good growth in many plants will occur at 10 000–15 000 lux.Plant species adapted to shade conditions, how-ever, e.g. Ficus benjamina, require only 1000 lux.Other shade-tolerant plants include Taxus spp.,Mahonia and Hedera. In summer, light intensitycan reach 50 000–90 000 lux and is therefore notlimiting, but in winter months, between Novemberand February, the low natural light intensity ofabout 3000–8000 lux is the limiting factor forplants actively growing in a heated greenhouse orpolythene tunnel. Care must be taken to maintainclean glass or polythene, and to avoid condensation

that restricts light transmission. Intensity can beincreased by using artificial lighting, which canalso extend the length of day, which is short duringthe winter, by supplementary lighting.This methodis used for plants such as lettuce, bedding plantsand brassica seedlings.

Total replacement lighting. Growing roomswhich receive no natural sunlight at all use con-trolled temperatures, humidities and carbon diox-ide levels, as well as light. Young plants which canbe grown in a relatively small area, and which arecapable of responding well to good growing condi-tions in terms of growth rate, are often raised in agrowing room.

The type of lamp. Lamps are chosen for increas-ing intensity, and therefore more photosynthesis.All such lamps must have a relatively high effi-ciency of conversion of electricity to light, and onlythe gas discharge lamps are able to do this. Light is produced when an electric arc is formed acrossthe gas filament enclosed under pressure inside aninner tube. Light, like other forms of energy, e.g.heat, X-rays and radio waves, travels in the form ofwaves, and the distance between one wave peakand the next is termed as the wavelength. Lightwavelengths are measured in nanometres (nm);1 nm � one thousandth of a micrometre. Visiblelight wavelengths vary from 800 nm (red light – inthe long wavelength area) to 350 nm (blue light –in the short wavelength area), and a combinationof different wavelengths (colours) appears as whitelight. Each type of lamp produces a characteristicwavelength range and, just as different colouredsubstances absorb and reflect varying colours oflight, so a plant absorbs and reflects specific wave-lengths of light.

Since the photosynthetic green pigment chloro-phyll absorbs mainly red and blue light andreflects more of the yellow and green part of thespectrum, it is important that the lamps used pro-duce a balanced wavelength spectrum to includeas high a proportion of those colours as possible,in order that the plant makes most efficient use ofthe light provided. The gas included in a lampdetermines its light characteristics. The two mostcommonly used gases for horticultural lighting aremercury vapour, producing a green blue light with

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no red, and sodium, producing yellow light. Thislimited spectrum may be modified by the inclusionof fluorescent materials in the inner tube, whichallow the tube to re-emit wavelengths more usefulto the plant emitted by the gas and re-emit theenergy as a shorter wavelength. Thus, modifiedmercury lamps produce the desirable red lightmissing from the basic emission.

Low-pressure mercury-filled tubes produce dif-fuse light and, when suitably grouped in banks,provide uniform light close to plants. These areespecially useful in a growing room, provided thatthey produce a broad spectrum of light as is seen in the ‘full spectrum fluorescent tubes’. Gasenclosed at high pressure in a second inner tubeproduces a small, high intensity source of light.These small lamps do not greatly obstruct naturallight entering a greenhouse and, while producingvaluable uniform supplementary illumination at adistance, cause no leaf scorch. Probably the mostuseful lamp for supplementary lighting in a green-house is a high-pressure sodium lamp, which pro-duces a high intensity of light, and is relativelyefficient (27 per cent).

Carbon dioxide enrichment should be matchedto artificial lighting in order to produce the great-est growth rate and most efficient use of both factors.

TemperatureThe complex chemical reactions,which occur duringthe formation of carbohydrates from water andcarbon dioxide requires the presence of chemicalscalled enzymes to accelerate the rate of reactions.Without these enzymes, only little chemical activ-ity would occur. Enzyme activity in living thingsincreases with temperature from 0°C to 36°C, andceases at 40°C. This pattern is mirrored by theeffect of air temperature on the rate of photosyn-thesis. But here, the optimum temperature varieswith plant species from 25°C to 36°C as optimum.It should be borne in mind that at very low lightlevels, the increase in photosynthetic rate withincreased temperature is only limited. This meansthat any input of heating into the growing situation

during cold weather will be largely wasted if thelight levels are low.

Integrated environmental control in a green-house is a form of computerized system developedto maintain near-optimum levels of the main environmental factors (light, temperature and carbon dioxide) necessary for plant growth. Itachieves this by frequent monitoring of the green-house using carefully positioned sensors. Such asystem is able to avoid the low temperature/lightinteraction described above. The beneficial effectsto plant growth of lower night temperatures com-pared with day are well known in many species,e.g. tomato. The explanation is inconclusive,but the accumulation of sugars during the nightappears to be greater, suggesting a relationshipbetween photosynthesis and respiration rates.Such responses are shown to be related to temper-ature regimes experienced in the areas of origin ofthe species.

Temperature adaptations. Adaptation to extre-mes in temperature can be found in a number ofspecies; e.g. resistance to high temperatures above40°C in thermophiles; resistance to chilling injuryis brought about by lowering the freezing point ofcell constituents. Both depend on the stage ofdevelopment of the plant, e.g. a seed is relativelyresistant, but the hypocotyl of a young seedling is particularly vulnerable. Resistance to chillinginjury is imparted by the cell membrane, whichcan also allow the accumulation of substances toprevent freezing of the cell contents. Hardeningoff of plants by gradual exposure to cold tempera-tures can develop a change in the cell membraneas in bedding plants and peas. Examples of planthardiness are found in Table 3.3.

WaterWater is required in the photosynthesis reactionbut this represents only a very small proportion ofthe total water taken up by the plant (see transpi-ration). Water supply through the xylem is essen-tial to maintain leaf turgidity and retain fully openstomata for carbon dioxide movement into the leaf.In a situation where a leaf contains only 90 per cent


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of its optimum water content, stomatal closure willprevent carbon dioxide entry to such an extentthat there may be as much as 50 per cent reductionin photosynthesis. A visibly wilting plant will notbe photosynthesizing at all.

MineralsMinerals are required by the leaf to produce thechlorophyll pigment that absorbs most of the lightenergy for photosynthesis. Production of chloro-phyll must be continuous, since it loses its efficiencyquickly. A plant deficient in iron, or magnesiumespecially, turns yellow (chlorotic) and loses muchof its photosynthetic ability. Variegation similarlyresults in a slower growth rate.

The leafThe leaf is the main organ for photosynthesis in theplant, and its cells are organized in a way that pro-vides maximum efficiency. The upper epidermisis transparent enough to allow the transmission oflight into the lower-leaf tissues.The sausage-shapedpalisade-mesophyll cells are packed together,

pointing downwards, under the upper epidermis.The sub-cellular chloroplasts within them carryout the photosynthesis process. The absorption oflight by chlorophyll occurs at one site and theenergy is transferred to a second site within thechloroplast where it is used to build up carbohy-drates, usually in the form of insoluble starch. Thespongy mesophyll, below the palisade mesophyll,has a loose structure with many air spaces. Thesespaces allow for the two-way diffusion of gases.The carbon dioxide from the air is able to reachthe palisade mesophyll; and oxygen, the wasteproduct from photosynthesis, leaves the leaf. Thenumerous stomata on the lower-leaf surface arethe openings to the outside by which this gas move-ment occurs.The numerous small vascular bundles(veins) within the leaf structure contain the xylemvessels that provide the water for the photosyn-thesis reaction. The phloem cells are similarlypresent in the vascular bundles, for the removal ofsugar to other plant parts. Figure 6.1 shows thestructure of the leaf and its relevance to theprocess of photosynthesis. A newly expanded leafis most efficient in the absorption of light, and thisability reduces with age.

Plant Growth 57

CuticleEpidermis

Palisade

Epidermis

Stoma

XYLEMPHLOEMFIBRES

VASCULAR BUNDLE(Transport system)

STOMATA

�

�

Spongymesophyll

Figure 6.1 Cross-section of privet (Ligustrum) leaf showing features for efficient photosynthesis. Inset shows the tissuesmaking up the leaf blade in more detail.

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Movement of sugarThe product of photosynthesis in most plants isstarch (some plants produce sugars only), which isstored temporarily in the chloroplast or moved inthe phloem to be more permanently stored in theseed, the stem cortex or root, where specializedstorage organs such as rhizomes and tubers mayoccur.The movement or translocation of materialsaround the plant in the phloem and xylem is a complex operation, and does not have a full scientific explanation. The phloem is principallyresponsible for the transport of the products ofphotosynthesis as soluble sugars, usually sucrose,which move under pressure to areas of need, suchas roots, flowers or storage organs. Each phloemsieve-tube cell has a smaller companion cell thathas a high metabolic rate. Energy is thus madeavailable to the protoplasm at the end of eachsieve plate, which is able to ‘pump’ dilute sugarsolutions around the plant. The flow can be inter-rupted by the presence of disease organisms suchas club root.

Leaf adaptationsWhilst remaining essentially the organ of photo-synthesis and transpiration, the leaf takes on otherfunctions in some species. The most notable ofthese is the climbing function. Tendrils are slenderextensions of the leaf, and are of three types. InClematis spp., the leaf petiole curls round thestems of other plants or garden structures in orderto support the climber. In sweet pea (Lathyrusodoratus), the plant holds on with tendrils modi-fied from the end-leaflets of the compound leaf. Inthe monocotyledonous climber (Smilax china),the support is provided by modified stipules(found at the base of the petiole). In cleavers(Galium aparine), both the leaf and stipules, bornein a whorl, have prickles that allow the weed tosprawl over other plant species.

Buds and bulbs are composed mainly of leaf tis-sue. In the former, the leaves (called scales) arereduced in size, hard and brown rather than green.They tightly overlap each other, giving protectionto the delicate meristematic tissues inside the

bud. In a bulb, the succulent, light-coloured scaleleaves contain all the nutrients and moisture necessary for the bulbs emergence. The scales arepacked densely together around the terminal bud,minimizing the risk that might be caused byextremes of climate, or by pests such as eelwormsor mice.

In the houseplant, Bryophyllum daigremon-tianum, the succulent leaf bears adventitious budsthat are able to develop into young plantlets (seealso page 69).

Leaf formThe novice gardener may easily overlook the impor-tance that the shape, texture, venation, colour andsize of leaves can contribute to the general appear-ance of a garden, as they focus more on the floralside of things. Flowers are the most striking feature,but they are often short lived. It should be empha-sized that the dominant theme in most gardens isthe foliage and not the flowers (see Plates 1, 2 and5).The possibilities for contrast are almost endlesswhen these five leaf aspects are considered.

In Chapter 3 (see page 33) the range of leafforms is described. Consider leaf shape first.The large linear leaves of Phormium tenax (NewZealand Flax) are a well-known striking example.In contrast are the large palmate leaves ofGunnera manicata. On a smaller scale, the shade-loving Hostas, with their lanceolate leaves, mixwell with the pinnate-leaved Dryopteris filix-mas(male fern).

Secondly, leaf texture is also important. Mostspecies have quite smooth textured leaves. Notablydifferent are Verbascum olympicum, Stachys byzan-tina (lamb’s tongue) and the alpine Leontopodiumalpinum (edelweiss) which all are woolly in tex-ture. Glossy-leaved species such as Ilex aquifolium(holly) and Pieris japonica provide a strikingappearance.

Thirdly, the plant kingdom exhibits a wide vari-ety of leaf colour tones (see Plate 5). The coniferJuniperus chinensis (Chinese juniper), shrubs of theCeanothus genus and Helleborus viridus (Christmasrose) are examples of dark-leaved plants. Notable


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examples of plants with light-coloured leaves are the tree Robinia pseudoacacia (false acacia),the climber Humulus lupulinus ‘Aureus’ (commonhop) and the creeping herbaceous perennialLysimachia nummularia ‘Aurea’ (Creeping Jenny).Plants with unusually coloured foliage may also be briefly mentioned: the small tree Prunus‘Shirofugen’ (bronze-red), the sub-shrub Seneciomaritima (silver-grey) and the shade perennialAjuga reptans ‘Atropurpurea’ (bronze-purple).

Variegation (the presence of both yellow and green areas on the leaf) gives a novel appear-ance to the plant (see Plate 8). Example speciesare Aucuba japonica (Laurel), Euonymus fortuneiand Glechoma hederacea (ground ivy). Fourthly,in autumn, the leaves of several tree, shrub andclimber species change from green to a strikingorange-red colour. Acer japonicum (Japanesemaple), Euonymus alatus (Winged spindle) andParthenocissus tricuspidata (Boston ivy) areexamples.

PollutionGases in the air, which are usually products ofindustrial processes or burning fuels, can causedamage to plants, often resulting in scorchingsymptoms of the leaves. Fluoride can accumulatein composts and be present in tap water, so causingmarginal and tip scorch in leaves of susceptiblespecies such as Dracaena and Gladiolus. Sulphurdioxide and carbon dioxide may be produced byfaulty heat exchangers in glasshouse burners, espe-cially those using paraffin. Scorch damage over thewhole leaf is preceded by a reddish discoloration.

RESPIRATION

Respiration is the process by which sugars andrelated substances are broken down to yield energy,the end-products being carbon dioxide and water.In order that growth can occur, the food must bebroken down in a controlled manner to releaseenergy for the production of useful structural sub-stances such as cellulose, the main constituent ofplant cell walls, and proteins for enzymes. Thisenergy is used also to fuel cell division and themany chemical reactions that occur in the cell.

The energy requirement within the plant varies,and reproductive organs can respire at twice therate of the leaves. Also, in apical meristems, theprocesses of cell division and cell differentiationrequire high inputs of energy. In order that thebreakdown is complete, oxygen is required in theprocess of aerobic respiration. A summary of the process is given in Table 6.2.

It would appear, at first sight, that respiration isthe reverse of photosynthesis (see page 54). Thissupposition is correct in the sense that photosyn-thesis creates glucose as an energy-saving strategy,and respiration breaks down glucose as an energy-releasing mechanism. It is also correct in the sensethat the simple equations representing the twoprocesses are mirror images of each other.

It should, however, be emphasized that the twoprocesses have two notable differences.The first isthat respiration in plants (as in animals) occurs inall living cells of all tissues at all times (in leaves,stems, flowers, roots and fruits). Photosynthesisoccurs predominantly in the palisade-mesophylltissue of leaves. Secondly, respiration takes place

Plant Growth 59

Table 6.2 A summary of the process of aerobic respiration

(a) Written in a conventional way, the process can be expressed in the following way:

Glucose � oxygen → carbon dioxide � water � energy (in the mitochondria of the cell)

(b) Written in the form of a chemical equation, which represents molecular happenings at the sub-microscopic level; theabove sentence becomes:

1C6H12O6 molecule � 6O2 molecules → 6CO2 molecules � 6H2O molecules � energy(in the mitochondria of the cell)

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in the torpedo-shaped organelles of the cell calledmitochondria. Photosynthesis occurs in the oval-shaped chloroplasts. Details of biochemistry, beyondthe scope of this book, would reveal how differentthese processes are, in spite of their superficialsimilarities. In the absence of oxygen, inefficientanaerobic respiration takes place and incompletebreakdown of the carbohydrates produces alcoholas a waste product, with energy still trapped in themolecule. If a plant or plant organ such as root is supplied with low oxygen concentrations in awaterlogged or compacted soil, the consequentalcohol production may prove toxic enough tocause root death. Over-watering, especially of pot plants, leads to this damage and encouragesdamping-off fungi.

Storage of plantsThe actively growing plant is supplied with thenecessary factors for photosynthesis and respira-tion to take place. Roots, leaves or flower stemsremoved from the plant for sale or planting willcease to photosynthesize, though respiration con-tinues. Carbohydrates and other storage products,such as proteins and fats, continue to be brokendown to release energy, but the plant reserves aredepleted and dry weight reduced. A reduction inthe respiration rate should therefore be consideredfor stored plant material, whether the period ofstorage is a few days, e.g. tomatoes and cut flowers,or several months, e.g. apples.Attention to the fol-lowing factors may achieve this aim.

Temperature. The enzymes involved in res-piration become progressively less active with areduction in temperatures from 36°C (optimum)to 0°C.Therefore, a cold store employing tempera-tures between 0°C and 10°C is commonly used for the storage of materials such as cut flowers,e.g. roses; fruit, e.g. apples; vegetables, e.g. onions;and cuttings, e.g. chrysanthemums, which rootmore readily later. Long-term storage of seeds ingene banks (see Chapter 8) uses liquid nitrogen at �20°C.

Oxygen and carbon dioxide. Respirationrequires oxygen in sufficient concentration; if oxy-gen concentration is reduced, the rate of respirationwill decrease. Conversely, carbon dioxide is aproduct of the process and, as with many processes,a build-up of a product will cause the rate of theprocess to decrease. A controlled environmentstore for long-term storage, e.g. of top fruit, ismaintained at 0–5°C according to cultivar, and isfed with inert nitrogen gas to exclude oxygen.Carbon dioxide is increased by up to 10 per centfor some apple cultivars.

Water loss. Loss of water may quickly desic-cate and kill stored material, such as cuttings.Seeds also must not be allowed to lose so muchwater that they become non-viable, but too humidan environment may encourage premature germi-nation with equal loss of viability.

FURTHER READING

Attridge, T.H. Light and Plant Responses (EdwardArnold, 1991).

Bickford, E.D. et al. Lighting for Plant Growth (KentState University Press, 1973).

Bleasdale, J.K.A. Plant Physiology in Relation toHorticulture (Macmillan, 1983).

Bowes, B.G. A Colour Atlas of Plant Structure (Manson,1996).

Brown, L.V. Applied Principles of Horticulture, 2nd edition (Butterworth-Heinemann, 2002).

Capon, B. Botany for Gardeners (Timber Press, 1990).Grow Electric Handbook No. 2. Lighting in Greenhouses,

Part 1 (Electricity Council, 1974).Hopkins, W.G. Introduction to Plant Physiology (Wiley

and Sons, 1995).Ingram, D.S. et al. (editors). Science and the Garden

(Blackwell Science Ltd, 2002).MAFF. Carbon Dioxide Enrichment for Lettuce, HPG51

(HMSO, 1978).Moorby, J. Transport Systems in Plants (Longman, 1981).Sutcliffe, J. Plants and Temperature (Edward Arnold,

1977).Thompson, A.K. Postharvest Technology of Fruit and

Vegetables. (Blackwell Science Ltd, 1996).


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The characteristics of the plant are under geneticcontrol (see Chapter 8). However, the developmentof the plant, from one stage of the life cycle to thenext, takes place in response to a number of stim-uli, many of which are changes in factors occurringin the plant’s environment.

The plant’s most reliable indicator of season,especially in temperate regions, is the daylength,which shortens and lengthens with the time of year.Annual temperature fluctuations are a valuable, ifless precise, indicator of seasonal occurrence. Theplant responds to these environmental factorsby breaking seed or bud dormancy in spring, by

flowering at appropriate seasons and by the drop-ping (abscission) of leaves in autumn.

PLANT HORMONES

In order that the genetic potential of a plant can bestimulated by factors of the environment, it mustpossess a system that, having perceived the stimu-lus, is able to activate the response. Such control isachieved by chemicals produced by the plant.These chemicals are the plant hormones. The con-centration of hormone may vary according to thestage of plant development. Equally important,the balance of hormones and their relative con-centrations will determine, to a large extent, thekind of response required. The concentration ofthese chemicals, required to cause an effect onplant development, is extremely low (measured inparts per million). The chemicals are all relativelysimple in structure and can be moved around theplant quite easily. The plant’s development is bestachieved when the hormone is at the optimal con-centration.Any decrease or increase in concentra-tion away from the optimum level at the site ofactivity may result in delayed development ordistorted growth.

There are five main types of plant hormones:

• Auxin (indoleacetic acid) is primarily producedin the dividing cells of the apical and leaf meri-stems. Its role in the plant is varied. It induces

7

Plant Development

The life cycle of the flowering plant contains anumber of identifiable stages, each with a dis-tinct significance to horticulture. The seed isthe means by which a new generation begins,usually resulting in great variation in theplants produced.This is followed by the sensi-tive seedling, vulnerable to diseases, pestattack and physiological disorders, but highlyresponsive to good growing conditions. Thevegetative stage may be manipulated to therequired size and shape or used for propaga-tion. The flowering stage is often the desiredobjective, while the formation of fruit may bean important horticultural aim, whether in anedible form or as the precursor of seeds.

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decreased cell division and increased cellelongation in the tip of the stem, inhibits growthof side shoots, promotes secondary growth invascular tissues, inhibits leaf fall, promotes fruitdevelopment whilst inhibiting fruit drop andstimulates cell differentiation in wounded tissue.

• Gibberellin is produced in young leaves, seedembryos and root tips. It is involved in stemelongation, increased cell division and initiatingseed germination.

• Cytokinin is produced in dividing cells of roottips, seeds and fruits. It is involved in stimulatingcell division in meristems, promoting side-budgrowth on stems, delaying leaf ageing and sti-mulating fruit development. It may travel in thetranspiration stream from root to leaf.

• Abscisic acid and ethylene contribute to theprecision achieved by the three above-describedhormones. Abscisic acid is produced in ageingleaves, and in stems, fruits and seeds. It inducesdormancy in buds, halts cell elongation in roots,promotes leaf fall and fruit fall, and promotesclosure of stomata in drought conditions. Ethyl-ene (ethene) is produced in all plant parts, buthas its greatest effect in promoting the ripeningof fruits.

The horticulturist is able to both modify the plantenvironment and also utilize synthetic chemicalssimilar in action to those produced by the plant,called plant growth regulators, to manipulate thegrowth and development of the plant. The rele-vance of these applications will be discussed ateach stage of development.

The life cycle of plants is described and includesseveral of the vast number of variations on the basicpattern. One of the most obvious differences is thelength of the life cycle and the terms ‘ephemeral’,‘annual’, ‘biennial’, ‘perennial’, ‘shrub’ and ‘tree’are described in Table 3.3 along with other differ-ences which are usefully adopted by growers.

THE SEED

The seed, resulting from sexual reproduction,creates a new generation of plants that bear

characteristics of both parents.The plant must sur-vive often through conditions that would be dam-aging to a growing vegetative organism. The seedis a means of protecting against extreme conditionsof temperature and moisture, and is thus the over-wintering stage. The seed structure may be spe-cialized for wind dispersal, e.g. members of theAsteraceae family, including groundsel, dandelionand thistle, that have parachutes, as does Clematis(Ranunculaceae). Many woody species such aslime (Tilia), ash (Fraxinus) and sycamore (Acer)produce winged fruit.Other seedpods are explosive,e.g. balsam and hairy bittercress. Organisms suchas birds and mammals distribute hooked fruitssuch as goosegrass and burdock, succulent types(e.g. tomato, blackberry and elderberry) or thosethat are filled with protein (e.g. dock). Dispersalmechanisms are summarized in Table 7.1.

Seed structure

The basic seed structure is shown in Figure 7.1. Inorder to survive, the seed must contain a smallimmature plant (embryo) protected by a seed coator testa, which is formed from the outer layers ofthe ovule after fertilization. A weakness in thetesta, the micropyle, marks the point of entry ofthe pollen tube prior to fertilization, and the hilumis the point of attachment to the fruit.The embryoconsists of a radicle, which will develop into theprimary root of the seedling and a plumule, whichdevelops into the shoot system, the two beingjoined by a region called the hypocotyl. A singleseed leaf (cotyledon) will be found in monocoty-ledons, while two are present as part of the embryoof dicotyledons.The cotyledons may occupy a largepart of the seed, e.g. in beans, to act as the foodstore for the embryo.

In some species, e.g. grasses and Ricinus (castoroil plant), the food of the seed is found in a differ-ent tissue from the cotyledons. This tissue is calledendosperm and is derived from the fusion of extracell nuclei, at the same time as fertilization. Plantfood is usually stored as the carbohydrate, starch,formed from sugars as the seed matures, e.g. in


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Plant Development 63

peas and beans. Other seeds, such as sunflowers,contain high proportions of fats and oils, and pro-teins are often present in varying proportions.The seed is also a rich store of nutrients such asphosphorus (see page 202) that it requires when itgerminates as a seedling.

Seed dormancy

As soon as the embryo begins to grow out of theseed, i.e. germinates, the plant is vulnerable todamage from cold or drought. Therefore, the seed

must have a mechanism to prevent germinationwhen poor growing conditions prevail. Dormancyis a period during which very little activity occurs inthe seed, other than a very slow rate of respiration.Seeds will not germinate until dormancy is broken.

A thick testa prevents water and oxygen,essentialin germination, from entering the seed. Gradualbreakdown of the testa, occurring through bacter-ial action or freezing and thawing, eventually per-mits germination following unsuitable conditions.The passing of fruit through the digestive systemof an animal such as a bird may promote germina-tion, e.g. in tomato, cotoneaster and holly. Many

Table 7.1 Fruits and the dispersal of seeds

Fruits

True fruits (formed from the ovary wall after fertilization)Succulent (indehiscent) Drupes Cherry, plum

Blackberry (collection of drupes)Berries Gooseberry, marrow, banana

Dry indehiscent SchizocarpsSamara SycamoreLomentum TrefoilCremocarb HogweedCarcerulus HollyhocksAchenes (nuts) Acorn, rose, strawberry

Dry dehiscent Capsules Poppy, violet, campanulaSiliquas Wallflowers, stocksSiliculas Shepherd’s purse, honestyLegumes Pea, bean, lupin,Follicles Delphinium, monkshood

False fruits (formed from parts other than, or as well as, the ovary wall)From inflorescence Pineapple, mulberryFrom receptable Apple

Seed dispersal

Method of seed dispersal Type of fruit Examples

Animals Succulent Elderberry, blackberry – eaten by birdsMistletoe, yew – stick to beaks

Hooked Burdock, goosegrass – catch on fur

Wind Winged Ash, sycamore, lime, elmParachutes Dandelion, clematis, thistlesCenser (dry capsules) Poppy, campion, antirrhinum

Explosion Pods Peas, lupins, gorse, vetches, geranium

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species, e.g. fat hen, produce seed with variabledormancy periods, to spread germination time overa number of growing seasons.Spring soil cultivationscan break the seed coat and induce germination ofweed seeds (see page 88). This structural dormancy,in horticultural crops, may present germinationproblems in plants such as rose rootstock speciesand Acacia. Physical methods using sandpaper orchemical treatment with sulphuric acid (collec-tively known as scarification) can break down theseed coat and therefore the dormancy mechanism.

Chemical inhibitors may occur in the seed to pre-vent the germination process. Abscisic acid at highconcentrations helps maintain dormancy while, asdormancy breaks, progressively lower levels occur,with a simultaneous increase in concentrations ofgrowth promotors such as gibberellic acid andcytokinins. Inhibitory chemicals located just belowthe testa may be washed out by soaking in water.

Cold temperatures cause similar breaks of dor-mancy in other species (stratification), the exacttemperature requirement varying with the periodof exposure and the plant species. Many alpineplants require a 4°C stratification temperature whileother species, e.g. Ailanthus, Thuja, ash, and manyother trees and shrubs, require both moisture andthe chilling treatment. The chemical balanceinside the seed may be changed in favour of ger-mination by treatment with chemicals such as gibberellic acid and potassium nitrate.

An undeveloped embryo in a seed is incapableof germinating until time has elapsed after the seedis removed from the parent plant, i.e. the afterripening period has occurred, as in the tomato andmany tropical species such as palms. Some seedssuch as Acacia are recorded to have a dormancy ofmore than a 100 years.

Seed germination

Seed germination is the emergence of the radiclethrough the testa, usually at the micropyle. Thereare a number of essential germination requirementsin order that successful seedling emergence occurs.

A viable seed has the potential for germinationgiven the required external conditions. Its viabil-ity, therefore, indicates the activity of the seed’sinternal organs, i.e.whether the seed is ‘alive’ or not.Most seeds remain viable until the next growingseason, a period of about 8 months, but many canremain dormant for a number of years until condi-tions are favourable for germination. In general,viability of a batch of seed diminishes with time, itsmaximum viability period depending largely onthe species. For example, celery seed quickly losesviability after the first season, but wheat has beenreported to germinate after scores of years. Thegermination potential of any seed batch will dependon the storage conditions of the seed, which shouldbe cool and dry, slowing down respiration, andmaintaining the internal status of the seed. Theseconditions are achieved in commercial seed storesby means of sensitive control equipment. Packagingof seed for sale takes account of these require-ments and often includes a waterproof lining ofthe packet, which maintains a constant water con-tent in the seeds. The main requirements for thesuccessful germination of most seed are as follows.

Water supply to the seed is the first environ-mental requirement for germination. The watercontent of the seed may fall to 10 per cent duringstorage, but must be restored to about 70 per centto enable full chemical activity. Water initially isabsorbed into the structure of the testa in a waysimilar to a sponge taking up water into its air


seed coat (testa)

hilum

plumule

radicle

cotyledon

hypocotyl

Figure 7.1 Long section of a broad bean seed showing thestructure of seed coat and embryo essential for germination.

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space, i.e. by imbibition. This softens the testa andmoistens the cell walls of the seed in order that thenext stages can proceed. The cells of the seed takeup water by osmosis often assuming twice the sizeof the dry seed. The water provides a suitablemedium for the activity of enzymes in the processof respiration. A continuous water supply is nowrequired if germination is to proceed at a consistentrate, but the growing medium, whether it is out-door soil or compost in a seed tray, must not bewaterlogged, because oxygen essential for aerobicrespiration would be withheld from the growingembryo. In the absence of oxygen, anaerobic res-piration occurs and eventually causes death of thegerminating seed, or suspended germination, i.e.induced dormancy.

Temperature is a very important germinationrequirement, and is usually specific to a givenspecies or even cultivar. It acts by fundamentallyinfluencing the activity of the enzymes involvedin the biochemical processes of respiration, whichoccurs between 0°C and 40°C. However, speciesadapted to specialized environments respond to anarrow range of germination temperatures. Forexample, cucumbers require a minimum tempera-ture of 15°C and tomatoes 10°C. On the otherhand, lettuce germination may be inhibited by tem-peratures higher than 30°C, and in some cultivars,at 25°C, a period of induced dormancy occurs.Some species, such as mustard, will germinatein temperatures just above freezing and up to40°C, provided they are not allowed to dry out.

Light is a factor that may influence germinationin some species, but most species are indifferent.Seeds of Rhododendron, Veronica and Phlox areinhibited in its germination by exposure to light,while that of celery, lettuce, most grasses, conifersand many herbaceous flowering plants is sloweddown when light is excluded. This should be takeninto account when the covering material for aseedbed is considered (see tilth).The colour (wave-length) of light involved may be critical in the par-ticular response created. Far red light (720 nm),occurring between red light and infra-red lightand invisible to the human eye, is found to inhibitgermination in some seeds, e.g. birch, while red light

(660 nm) promotes it. A canopy of tall deciduousplants filters out red light for photosynthesis.Seeds of species growing under this canopy receivemainly far red light, and are prevented from germi-nating.When the leaves fall in autumn, these seedswill germinate both in response to the now avail-able red light and to the low winter temperatures.

The Seeds Acts

In the UK the Seeds Acts control the quality ofseed to be used by growers. A seed producer mustsatisfy the minimum requirements for species ofvegetables and forest tree seed by subjecting a seedbatch to a government testing procedure.A sampleof the seed is subjected to standardized ideal ger-mination conditions, to find the proportion that isviable (germination percentage). The germinationand emergence under less ideal field conditions(field emergence) where tilth and disease factorsare variable may be much lower than germinationpercentage.

The sample is also tested for quality which pro-vides information, available to the purchaser of theseed, covering trueness to type; i.e. whether thecharacteristics of the plants are consistent withthose of the named cultivar; the percentage of non-seed material, such as dust; the percentage of weedseeds, particularly those of a poisonous nature (seeWeeds Act, Chapter 12). The precise regulationsfor sampling and testing, and requirements forspecific species, have changed slightly since the1920 Act, the 1964 Act (which also included thedetails of plant varieties) and entry of Britain intothe European Community (EC). Some controlunder EC regulations is made of the provenanceof forestry seed, as the geographical location of itssource is important in relation to a number of factors, including response to drought, cold, dor-mancy, habit, and pest and disease susceptibility.

THE SEEDLING

Within the seed is a food store that provides themeans to produce energy for germination. Once the


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food store has been exhausted, the seedling mustrapidly become independent in its food supplyand begin to photosynthesize. It must thereforerespond to stimuli in its environment to establishthe direction of growth. Such a response is termedas tropism, and is very important in the early sur-vival of the seedling (see Figure 7.2).

Geotropism is a directional growth response togravity.The emergence of the radicle from the testais followed by growth of the root system,which mustquickly take up water and minerals in order thatthe shoot system may develop.A seed germinatingnear the surface of a growing medium must notput out roots that grow onto the surface and dry out,but the roots must grow downwards to tap watersupplies. Conversely, the plumule must grow awayfrom the pull of gravity in order that the leavesdevelop in the light.

Etiolation is the type of growth which the shootproduces as it moves through the soil in responseto gravity. The developing shoot is delicate andvulnerable to physical damage, and therefore oftenthe growing tip is protected by being bent intoplumular hook. The stem grows quickly, is sup-ported by the structure of the soil and therefore isvery thin and spindly, stimulated by friction in thesoil which causes release of ethylene.The leaves areundeveloped, as they do not begin to function untilthey move into the light. Mature plants that aregrown in dark conditions also appear etiolated.

Seedling development

The emergence of the plumule above the growingmedium is usually the first occasion that theseedling is subjected to light.This stimulus inhibitsthe extension growth of the stem so that it becomesthicker and stronger, but the seedling is still verysusceptible to attack from pests and damping-offdiseases. The leaves unfold and become green inresponse to light, which enables the seedling tophotosynthesize and so supports it.The first leavesto develop, the cotyledons, derive from the seedand may emerge from the testa while still in thesoil, as in peach and broad bean (hypogeal germin-ation), or be carried with the testa into the air,where the cotyledons then expand (epigeal germin-ation), e.g. in tomatoes and cherry.

Phototropism occurs in order that the shootgrows towards a light source that provides theenergy for photosynthesis.A bend takes place in thestem just below the tip as cells in the stem awayfrom the light grow larger than those near to thelight source.A greater concentration of auxin in theshaded part of the stem causes the extended growth(Figure 7.2). Roots display a negative phototropicresponse, growing away from light when exposedat the surface of the growing medium, e.g. on a steepbank. The growth away from light may supersedethe root’s geotropic response, and will cause theroots to grow back into the growing medium.


Direction of lightgreater amount ofauxin in shadedhalf of stem shoot grows towards

light source

roots grow awayfrom light source

down in responseto gravity

then SOIL LEVEL

� positive phototropism

� positive geotropism

� negative phototropism

Figure 7.2 Geotropism and phototropism shown as mechanisms assisting the survival of a seedling.

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Hydrotropism is the growing of roots towards asource of water.The explanation of this tropism hasnot been found, but it can be shown to occur.

The cotyledons that emerge from the testa con-tribute to the growth of the seedling in photosyn-thesis, but the true leaves of the plant, which oftenhave a different appearance to the cotyledon, veryquickly unfold.

Conditions for early plant growth

Many plant species are propagated in glasshouses.A few principles are described here to help ensuresuccess. Seed trays should be thoroughly cleanedto prevent the occurrence of diseases such as ‘damp-ing off’ (see page 127). Fresh growing mediumshould be used for these tiny plants that have littleresistance to disease. Compost low in soluble fer-tilizer is less likely to scorch young plants. Compostshould be firmed down in containers to providecloser contact with the developing root system.

With very small seed, there is a danger that toomany seeds are sown together, with the result thatthe seedlings intertwine and are hard to separate.This problem can often be avoided by dilutingbatches of very small seed in some fine sand beforesowing. Small seed samples need to be sown onthe surface of the compost, and then covered withonly a fine sprinkling of compost. In this way, theirlimited food reserves are not overtaxed as theystruggle through the compost to reach the light.

Water quality is important with young plants.Mains water is recommended, as it will be freefrom diseases. Water butts and reservoirs needparticular scrutiny to avoid this problem. Waterthat has been left to reach ambient temperature ofthe glasshouse is less likely to harm seedlings. Thecompost in seed trays should be kept permanentlymoist (but not waterlogged), as seedling roots eas-ily dry out. Glass or plastic covers placed over seedtrays will help prevent moisture loss, and these canbe removed when root establishment has occurredand when seedlings are pushing against the covers.

As soon as seedlings have expanded their cotyle-don leaves, they should be carefully transferred

(‘pricked off’) from the seed tray and placed inanother tray filled with compost having higherlevels of fertility. The seedlings are spaced atapproximately 2.5 cm intervals, thus providing aroot volume for increased growth. Later, plantswill be transferred to pots (‘potted on’) to allow forfurther growth.

Plants growing in glasshouses are tender. Thecuticle covering leaves and stems is very thin.Growth is rapid and the stem’s mechanical strengthis likely to be dependent on tissues such as col-lenchyma and parenchyma rather than the stur-dier xylem vessels (see page 42). When a plant istransferred from a glasshouse to cooler, windieroutside conditions (e.g. in spring), it may becomestressed, lose leaves and stop growing. It is advisableto ‘harden off’ plants before this stressful expos-ure. Reducing heat and increasing ventilation inthe glasshouse are two ways of achieving this aim.Traditionally, plants were moved out into coldframes to gradually expose them to the conditionsinto which they are to be planted. Moving plantsout during the day, and back inside overnight, fora number of weeks, is another strategy.

THE VEGETATIVE PLANT

The role of the vegetative stage in the life cycle ofthe plant is to grow rapidly and establish the indi-vidual in competition with others. It must there-fore photosynthesize effectively and be capable ofresponding to good growing conditions. Growingrooms with near-ideal conditions of light, tempera-ture and carbon dioxide utilize this capacity thatwill reduce with the ageing of the plant (seeChapter 6).

Juvenility

The early growth stage of the plant, the juvenilegrowth, is characterized by certain physical appear-ances and activities that are different from thosefound in the later stages or in adult growth. Oftenleaf shapes vary; e.g. the juvenile ivy leaf is three


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lobed while the adult leaf is more oval, as shown inPlate 8. The habit of the plant is also different; thejuvenile stem of ivy tends to grow horizontally andis vegetative in nature, while the adult growth isvertical and bears flowers. Other examples arecommon in conifer species where the completeappearance of the plant is altered by the change in leaf form, e.g. Chamaecyparis fletcheri and manyJuniperus species such as J. chinensis. In the generaChamaecyparis and Thuja, the juvenile conditioncan be achieved permanently by repeated vege-tative propagation producing a plant called aretinospore, which is used as decorative feature.

Leaf retention is also a characteristic of juvenil-ity. It can be significant in species such as beech(see Plate 7), where the phenomenon is exagger-ated, where the trees can be pruned back to thevegetative growth. This can create additional pro-tection in windbreaks although the barrier createdtends to be too solid to provide the ideal wind pro-tection (see page 23).

The juvenile stage is a period after germinationthat is capable of rapid vegetative growth and isunlikely to flower. Many species that require anenvironmental change to stimulate flower initiation,such as the Brassicas that require a cold period,will not respond to the stimulus until the juvenileperiod is over; about 11 weeks in Brussels sprouts.

The adult stage essential for sexual reproduc-tion is less useful for vegetative propagation thanthe responsive juvenile growth, a condition prob-ably due to the hormonal balance in the tissues.Plate 8 shows the spontaneous production ofadventitious roots on the ivy stem. Adult growthshould be removed from stock plants to leave themore successful juvenile growth for cutting.

Vegetative propagation

Although the life cycle of most plants leads to sex-ual reproduction, all plants have the potential toreproduce asexually or by vegetative propagation,when pieces of the parent plant are removed anddevelop into a wholly independent plant.All livingcells contain a nucleus with a complete set of

genetical information (see genetic code, Chapter 8),with the potential to become any specialized celltype. Only part of the total information is broughtinto operation at any one time and position in theplant.

If parts of the plant are removed, then cells losetheir orientation in the whole plant and are able toproduce organs in positions not found in the usualorganization. These are described as adventitiousand can, for example, be roots on a stem cutting,buds on a piece of root, or roots and buds on a pieceof leaf used for vegetative propagation. Manyplant species use the ability for vegetative propa-gation in their normal pattern of development, inorder to increase the number of individuals of thespecies in the population. The production of thesevegetative propagules, as with the production ofseed, is often the means by which the plant sur-vives adverse conditions (see overwintering), actingas a food store which will provide for the renewedgrowth when it begins. The stored energy in theswollen taproots of dock and dandelion enablesthese plants to compete more effectively with seed-lings of other weed and crop species, which wouldalso apply to roots of Gypsophila paniculata,carrots and beetroot.

Stems are telescoped in the form of a corm infreesia and cyclamen, or swollen into a tuber inpotato, or a horizontally growing underground rhi-zome in iris and couch grass. Leaves expandedwith food may form a large bud or offset found inlilies.A bulb, as seen in daffodils, tulips and onions,is largely composed of succulent white leavesenveloping the much reduced stem, found at thebase of the bulb (Figure 7.3).

Other natural means of propagation include lat-eral stems, which grow horizontally on the soil sur-face to produce nodal, adventitious roots andsubsequently plantlets, e.g. runners or stolons ofstrawberries and yarrow. The adventitious natureof stems is exploited when they are deliberatelybent to touch the ground, or enclosed in compost,in the method known as layering, used in carnations,some apple rootstocks, many deciduous shrubssuch as Forsythia, and pot plants such as Ficus andDieffenbachia. The roots of species, especially in


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the Rosaceae family, are able to produce under-ground adventitious buds that grow into aerialstems or suckers, e.g. pears and raspberries. By allthese methods of runners, layering and suckers,the newly developing plant (propagule) will subse-quently become detached from the parent plant bythe disintegration of the connecting stem or root.

These natural methods are used in horticultureto produce number of plants from a single parent

plant. This group of plants, or clone, strictly speak-ing, is an extension of the parent plant, and there-fore all have the same genetical characteristics.The horticulturist is able to reproduce a cultivar,by this means, in which all the resulting plantsexhibit consistent characteristics. Seed productionis likely to result in variation of characteristics.Even in vegetatively propagated cultivars, changescan occur (see mutations), and differing clonalcharacteristics within the same cultivar can be dis-tinguished in some conifer species, e.g. the leafcolour and plant habit of � Cupressocyparisleylandii. Seeds produced without fertilization (i.e.by apomixis) found in Rosaceae and Graminae pre-sent a special case of natural clonal propagation.

Artificial methods of propagation

The artificial methods of vegetative propagationencompass most organs of the plant. Cuttings areparts of plants that have been carefully cut awayfrom the parent plant, and which are then used toproduce a new plant. Many species can be propa-gated in this way. Different methods may be neces-sary for different species. Only healthy parentplants should be used. Hygienic use of knives,compost and containers is strongly recommended.Cuttings are normally taken from parts of the plantexhibiting juvenile growth. Below is a brief descrip-tion of the most common methods used for cuttings.

Stem cuttings can be taken from stems that haveattained different stages of maturity. Hardwoodcuttings are from pieces of dormant woody stemcontaining a number of buds, which grow out intoshoots when dormancy is broken in spring. Thebase of the cutting is cut cleanly to expose thecambium tissue from which the adventitious rootswill grow (e.g. in rose rootstocks, Forsythia andmany deciduous ornamental shrubs). In Hydrangeaand currant the stems show evidence of pre-formed adventitious roots (root initials), which aidthe process of root establishment. Hardwood cuttings are normally taken in late autumn (theyare 15–25 cm in length), and are often placed withhalf their length immersed in a growing medium


CROCUS CORM

TULIP BULB

COUCH GRASS RHIZOME

developing leaf

fibrous roots

developing flower

swollen base of stem(new corm)

old corm

swollen leaves

developing flower

developing shoot

axillary bud(next year’s bulb)

stem

adventitious root

adventitious roots

dormant bud

Figure 7.3 Structure of organs responsible for over-wintering and vegetative propagation.

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containing half compost and half sand.A 12 monthsperiod is often necessary before the cuttings canbe lifted.

Semi-ripe cuttings are taken from stems, whichare just becoming woody.They are normally takenfrom mid-summer to early autumn. Most cuttingsof this kind are 5–10 cm long. Rooting in asand/compost mixture may be achieved in coldframes or, more quickly, in a heated structure atabout 18°C.Eleagnus (oleaster) will root only if heatis provided. Many shrub and tree species, e.g. hollyand conifers, are propagated as ‘heeled’ cuttings.Here, the semi-ripe cutting is taken in such a waythat a 1 cm sliver of last year’s wood (the heel) is stillattached.The heel cambium facilitates root forma-tion and, hence, easier establishment of cuttings.

Stems without a woody nature are used for thepropagation of species such as of Fuchsia, Pelargo-nium and Chrysanthemum. These are called soft-wood cuttings, and they are most often taken inlate spring and early summer. The area of leaf onthese cuttings should be kept to a minimum toreduce water loss. Misting (which is spraying theplants with fine droplets of water to increase humid-ity and reduce temperature) can further reducethis risk by slowing down the transpiration rate.

Automatic misting employs a switch attached toa sensitive device used for assessing the evaporationrate from the leaves. The cool conditions favour-ing the survival of the aerial parts of the cutting,however, do not encourage the division of cells inthe cambium area of the root initials. Therefore,the temperature in the rooting medium may beincreased with electric cables producing bottomheat. These special conditions for the encourage-ment of the success of cuttings are provided inpropagation benches in a greenhouse.

Leaf cuttings are also susceptible to wilting beforethe essential roots have been formed, and willbenefit from mist, provided the wet conditions donot encourage rotting of the plant material. Leavesof species such as Begonia, Streptocarpus andSansevieria are divided into pieces from whichsmall plantlets are initiated, while leaves plus peti-oles are used for Saintpaulia propagation. Nurserystock species, e.g. Camellia and Rhododendron,

require a complete leaf and associated axillarybud in a leaf-bud cutting.

Root cuttings may be an option when othermethods are not seen to succeed. This method isused for species such as Phlox paniculata andAnchusa azurea (alkanet). Roots about a centi-metre in thickness are taken in winter and cut into5 cm lengths. They are inserted vertically into asand/compost mixture in most species but thinner-rooted species such as Phlox are placed horizon-tally. It is important that root cuttings are not,inadvertently, placed upside-down, as this willprevent establishment.

Tissue culture

Tissue culture is a method used for vegetativepropagation, employing a small piece of plant tissue, the explant, grown in a sterile artificialmedium that supplies all vitamins, mineral andorganic nutrients. The medium and explant areenclosed in a sterile jar or tube and subjectedto precisely controlled environmental conditions(Figures 7.4 and 7.5). This method has advantagesover conventional propagation techniques, sincelarge numbers of propagules can be produced fromone original plant. This method has particularvalue with rare or novel plants. An added advan-tage is the reduced time taken for bulking up plantstocks. Some species that traditionally propagateonly by seed, e.g. orchids and asparagus, can nowbe grown by tissue culture. One of the problems of conventional vegetative propagation is that diseases and pests are passed onto the propagules.

Stock plants grown at high temperatures areable to greatly reduce levels of disease (particu-larly virus) in their growing tips. Following thisheat treatment, a meristem tip can be dissectedout of the stem and grown in a tissue culturemedium, to produce stock that is free from disease(e.g. chrysanthemum stunt viroid, see Chapter 11).This method of propagation is now used forspecies including Begonia, Alstroemeria, Figus,Malus, Pelargonium, Boston fern (Nephrolepsisexaltata), roses and many others.


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In all the methods described, cell division (seemitosis) must be stimulated in order to producethe new tissues and organs. The correct balance ofhormones produced by the cells triggers this initi-ation. Auxins are found to stimulate the initiationof adventitious roots of cuttings. In the propaga-tion of cuttings, the bases may be dipped in pow-der or liquid formulations of auxin-like chemicalssuch as naphthalene acetic acid to achieve thisresult. The number of roots is increased and pro-duction time reduced. The precise concentrationof chemical in the cells is critical in producing the

desired growth response. A large amount of hor-mone can bring about an inhibition of growthrather than promotion. For this reason, manufac-turers of hormone powders and dips produceseveral distinct formulations with differing hor-mone concentrations, relevant to the hardwood, thesemi-ripe and the softwood cutting situations.Alsodifferent organs respond to different concentra-tion ranges; e.g. the amount of auxin needed toincrease stem growth would inhibit the productionof roots. The same principle applies to anothergroup of chemicals important in cell division, thecytokinins, which can be applied to leaf cuttingsto increase the incidence of plantlet formation.

Both auxin and cytokinin must be included in atissue culture medium, at concentrations appropri-ate to the species and the type of growth required.The subsequent weaning of plantlets from theirprotected environment in tissue culture condi-tions requires care and usually conditions of highrelative humidity, shade and warmth.

Grafting

Grafting involves the union of a scion (portion ofstem) with a rootstock (root system) taken from


a

b

Figure 7.4 Tissue cultures of Sorghum bicolour show-ing: (a) early growth of callus tissue and (b) later plantletproduction.

Figure 7.5 Plantlets growing in tubs under tissue cultureconditions in a commercial laboratory.

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another plant (see also page 44). Grafted plantsare commonly used in top fruit, grapes, roses andamenity shrubs with novel shapes and colours.Rootstocks resistant to soil-borne pests and dis-ease are sometimes used when the desired culti-vars would succumb if grown on their own roots,e.g. grapevines, tomatoes and cucumbers grown inborder soils. Grafting is not usually attempted inmonocotyledons, as they do not produce continu-ous areas of secondary cambium tissue, suitablefor successful graft-unions.

In top fruit, grafting is used for several reasons.A grafted plant will establish more quickly than aseedling. Plants derived from seedlings will showdifferent (usually inferior) qualities of fruitingcompared with their commercially useful parentplants, so a means of vegetative propagation isadvantageous; the cultivars are, therefore, clonesderived from one original parent. Another reasonfor grafting is to control the size of the tree throughthe choice of dwarfing rootstock. The M27 applerootstock causes the grafted scion cultivar to bedwarfed. Reduced levels of auxin and cytokinin inthe rootstock possibly bring this about.

There are numerous grafting methods that havebeen developed for particular plant species.Several principles common to all methods can bebriefly mentioned. Firstly, the scion and stockshould be genetically very similar. Secondly, thescion and stock will need to have been carefullycut so that their cambial components are able tocome in contact. In this way, there will be a higherlikelihood of callus growth (resulting from cam-bial contact), which quickly leads to graft estab-lishment. Thirdly, the graft union should be sealedwith grafting tape to maintain the graft contact, toprevent drying out and to keep out disease organ-isms such as Botrytis. Fourthly, the buds on thestem taken as scion material should, ideally, bedormant (leafy material would quickly dry out).The rootstock should be starting active growth,and thus bring water, minerals and nutrients to thegraft area.

More details of common methods of graftingsuch as ‘whip and tongue’, ‘approach graft’ and‘bud graft’ are available in specialist textbooks.

Apical dominance

After the germination of the seed, the plumuleestablishes a direction of growth partly due to thegeotropic and phototropic forces acting on it. Oftenthe terminal bud of the main stem sustains the majorgrowth pattern, while the axillary buds are inhibitedin growth to a degree that depends on the species.

In tomatoes and chrysanthemums, the lateralshoots have the potential to grow out, but are inhib-ited by a high concentration of auxin, which accu-mulates in these buds. The source of the chemicalis the terminal bud, which maintains the inhibition.In commercial chrysanthemum production, theremoval of the main shoot (stopping) is a commonpractice. It takes away the auxin supply to the axil-lary buds, which are then able to grow out to createa larger, more balanced inflorescence. Conversely,the practice of disbudding in chrysanthemums andcarnations takes out the axillary buds to allow theterminal bud to develop into a bigger bloom thatbenefits from the greater food availability.

Pruning

Parts of plants can be pruned (removed) to reducethe competition within the plant for the availableresources. In this way, the plant is encouraged togrow, flower or fruit in a way the horticulturistrequires.A reduction in the number of flower budsof, for example, chrysanthemum will cause theremaining buds to develop into larger flowers; areduction in fruiting buds of apple trees will pro-duce bigger apples, and the reduction in branchesof soft fruit and ornamental shrubs will allow theplants to grow stronger when planted densely.Pruning will also affect the shape of the plant, asmeristems previously inhibited by apical dominancewill begin to develop. The success of such pruningdepends very much on the skill of the operator, asa good knowledge of the species habit is required.

A few general principles apply to most pruningsituations:

• Young plants should be trained in a way thatwill reflect the eventual shape of the more


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mature plant (formative pruning). For example,a young apple tree (called a ‘maiden’) canbe pruned to have one dominant ‘leader’shoot, which will give rise to taller, moreslender shape. Alternative pruning strategieswill lead to quite different plant shapes.Pruning back all branches in the first few yearsforms a bush apple.A cordon is a plant in whichthere is a leader shoot, often trained at 45° tothe ground, and where all side shoots arepruned back to one or two buds. Cordon fruitbushes are usually grown against walls orfences. Similarly, fans and espalier forms can bedeveloped.

• The pruning cut should be made just above abud that points in the required direction (usuallyto the outside of the plant). In this way, the plantis less likely to acquire too dense growth in itscentre.

• Pruning should remove any shoots that arecrossing, as they will lead to dense growth. Someplants such as roses and gooseberries are madeless susceptible to disease attack by the creationof an open centre to produce a more buoyant(less humid) atmosphere.

• Weak shoots should be pruned the hardestwhere growth within the plant is uneven, andstrong shoots pruned less, since pruning causesa stimulation of growth.

• Species that flower on the previous year’sgrowth of wood (e.g. Forsythia) should be prunedsoon after flowering has stopped. Conversely,species that flower later in the year on the pre-sent year’s wood, e.g. Buddleia davidii, shouldbe pruned the following spring.

Root pruning was used to restrict over-vigorouscultivars, especially in fruit species, but this tech-nique has been largely superseded by the useof dwarfing rootstock grafted onto commerciallygrown scions. Root pruning is still seen, however,in the growing of Bonsai plants.

Pruning is largely concerned with creating theshape of a plant, and controlling apical domin-ance. But the removal of dead, damaged and diseased parts is also an important aspect.

Growth retardation

Stem extension growth is controlled by auxinsproduced by the plant, and also by gibberellins thatcan dramatically increase stem length, especiallywhen externally applied. Growth retardation maybe desirable, especially in the production of com-pact pot plants from species that would normallyhave long stems, e.g. chrysanthemums, tulips andAzaleas. Therefore, artificial chemicals such asdaminozide or phosfon (Figure 7.6), which inhibitthe action of the growth promoting hormones, canretard the development of the main stem, and alsostimulate the growth of side shoots to produce amore bushy, compact plant. Flower production maybe inhibited, but this can be countered by theapplication of flower stimulating chemicals.

THE FLOWERING PLANT

The progression from a vegetative to a floweringplant involves profound physical and chemicalchanges. The stem apex displays a more complexappearance under the microscope as flower initi-ation occurs, and is followed, usually irreversibly, bythe development of a flower. The stimulus for thischange may simply be genetically derived, butoften an environmental stimulus is required whichlinks flowering to an appropriate season.


Untreated Treated

Figure 7.6 Chemical growth retardant is incorporatedinto compost used for pot plants such as chrysanthemum.

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Photoperiodism

Photoperiodism is a daylength-stimulated responseinvolved in the initiation of flowers (and also inbud and seed dormancy, and in leaf abscission). Ofthese the flower initiation process is probably themost important for manipulation by the horticul-turist. Repeated daily exposure to light for a givenperiod of time (critical period), the length of whichdepends on the species, will bring about flower ini-tiation, provided the light periods are separated byan appropriate period of darkness. Some speciesare adapted to flower after exposure to daylengthsshorter than a critical period. These are short dayplants, and this adaptation will ensure that theyproduce flowers at the most favourable time forsuccessful seed production and survival, which forthem is towards the autumn. Chrysanthemummorifolium, Poinsettia, and Kalanchoe speciesmust have short days, followed by relatively longnights in order that flower buds are produced.

Long day plants which require a daylengthlonger than a critical period include hostas,Campanula carpatica, Nigella damascena, radishand carnation. Although this latter species willflower under any photoperiodic conditions, flow-ers are produced more readily when treated withlong days. Many species previously considered tobe non-responsive or day neutral, e.g. Begonia elatior and tomato, have been shown to respond in some degree to a lighting period, especially in combin-ation with temperature.

Light is absorbed in the photoperiodic responseby a blue pigment called phytochrome (not pres-ent in sufficient amounts to contribute colour tothe plant). The switching mechanism between theplant’s vegetative growth and its need to initiateflowering, is controlled by phytochrome. Themechanism is a little complex. It involves a rapidreaction of a few seconds to produce ‘activatedphytochrome’ under the influence of red light.Red light represents a considerable proportion ofthe emission of tungsten filament bulbs, which areused in glasshouses for flower initiation. There is asimilar but much slower reversal from ‘activatedphytochrome’ back to phytochrome which occurs

in total darkness. A rapid reversal of this reactionfrom ‘activated phytochrome’ back to phytochromeoccurs when far red light is the dominant wave-length. Far red is just outside the visible spectrum(see example below).

Artificial control of lighting for flowering

Plants such as greenhouse chrysanthemumscan be artificially induced to flower even thoughthe natural daylength conditions are not suitable.The photoperiod can be extended by artificiallighting, or shortened by blacking out the daylight.The relatively low light intensity of approximately100–150 lux, needed to stimulate a response, isadequately provided by incandescent tungsten fila-ment lamps placed above the crop, while black-out can be provided using black polythene (seeFigure 7.7).

Night breaks. In a short day plant such aschrysanthemum, the night period must be relativelylong in order to bring about a flowering response.If a period of artificial lighting divides the nightinto two periods, i.e. a night break, then theflowering response does not occur. This practiceis used to maintain vegetative growth in order toproduce a full-length stem when a natural shortday would flower before this had been achieved.

Cyclic lighting. For chrysanthemums, the redlight effect results in ‘activated phytochrome’,which in turn results in flower initiation. Too longa period of darkness (15 min for chrysanthemums)results in a return to phytochrome, and this pre-vents flower initiation. Growers, therefore, use astrategy called cyclic lighting in which a repeatedpattern is maintained: 15 min of light is followedby 15 min of darkness. In this way, the plantresponds as if there was continuous daylight, andthe resulting ‘activated phytochrome’ initiatesflowering.

The development of night-break lighting (pre-venting flowering in immature plants) and cycliclighting (enabling flowering during the ‘unnatural’times of the year) has resulted in the ‘all-year-round’ (AYR or YR) chrysanthemum production.


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Flower initiation

Flower initiation can be stimulated largely by photo-periodic or temperature changes, or a complexinteraction between temperature and daylength.

Cold temperatures experienced during the winterbring about flower initiation (i.e. vernalization) inmany biennial species such as Brassica, lettuce,red beet, Lunaria and onion. The period for theresponse depends on the exact temperature, as


Figure 7.7 Chrysanthemum blooms with lighting and blackout for daylength control.

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with budbreak and seed dormancy (see stratifica-tion).The optimum temperature for many of theseresponses is about 4°C. Hormones are involved incausing the flower apex to be produced.The balanceof auxins, gibberellins and cytokinins is important,but some species respond to artificial treatment ofone type of chemical; e.g. the daylength require-ment for chrysanthemum plants can be partlyreplaced by gibberellic acid sprays.

The plant detects daylength in its leaves. Theexact nature of the chemical link between leaf andflower has been the subject of much speculation.In spite of name such as ‘florigen’ and ‘floral sti-mulus’ being given to the hypothetical substance,research has not yet provided a definitive answerto this question. Some scientists consider that amixture of auxin, gibberrelin and cytokinin movesfrom the leaf to initiate flowering.

Flower structure

The flower structure is shown in Figure 7.8. Theflower is initially protected inside a flower bud bythe calyx or ring of sepals, which are often greenand can therefore photosynthesize. The develop-ment of the flower parts requires large energyexpenditure by the plant, and therefore vegetativeactivities decrease. The corolla or ring of petalsmay be small and insignificant in wind-pollinatedflowers, e.g. grasses, or large and colourful ininsect-pollinated species. The colours and size ofpetals can be improved in cultivated plants bybreeding, and may also involve the multiplicationof the petals or petalody, when fewer male organsare produced.

The flowers of many species have both male andfemale organs (hermaphrodite), but some haveseparate male and female flowers (monoecious),e.g. Cucurbita, walnut, birch (Betula), whereas oth-ers produce male and female flowers on differentplants (dioecious), e.g. holly, willows, Skimmiajaponica and Ginkgo biloba.

Each male stamen bears an anther that pro-duces and discharges the pollen grains.The femaleorgan is positioned in the centre of the flower and

consists of an ovary containing one or more ovules(egg cells). The style leads from the ovary to astigma at its top where pollen is captured. Theflower parts are positioned on the receptacle, whichis at the tip of the pedicel (flower stalk).Associatedwith the flower head or inflorescence are leaf-likestructures called bracts, which can sometimesassume the function of insect attraction, e.g. inPoinsettia.

Extended flower life

The flower opens to expose the organs for sexualreproduction. The life of the flower is limited tothe time needed for pollination and fertilization,but it is often commercially desirable to extend thelife of a cut flower or flowering pot plant. In cutflowers, water uptake must be maintained and dis-solved nutrients for opening the flower bud aretermed as an opening solution.

Vase life can be extended by the addition ofsterilants and sugar to the water. A sterilant, e.g.silver nitrate, in the water can reduce the risk ofblockage of xylem by bacterial or fungal growth.Ethylene has a considerable effect on flowerdevelopment, and can bring about premature death(senescence) of the flower after it begins to open.


petal (corolla)

anther

ovaryovulesepal (calyx)receptaclepedicel

stigma

style

filamentstamen –male organ

femaleorgan

Figure 7.8 Cross-section of a flower showing the struc-tures involved in the function of sexual reproduction.

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Cut flowers should therefore never be stored nearto fruit, e.g. apples or bananas that produce ethyl-ene. Some chemicals, such as sodium thiosulphate,reduce the production of ethylene in carnationsand therefore extend their life.

Pollination and fertilization

The function of the flower is to bring about sexualreproduction, which is the production of offspringfollowing the fusion of male and female nuclei.The male and female nuclei are contained withinthe pollen grain and ovule respectively, and pol-lination is the transference of the pollen from theanther to the stigma. The source of pollen may bethe same flower in self-pollination or a differentflower in cross-pollination, although for practicalplant breeding purposes cross-pollination shouldinclude different plants. Cross-pollination ensuresthat variation is introduced into new generationsof offspring. Natural agents of cross-pollinationare mainly wind and insects.

Wind-pollinated flowers are usually small, green,lacking nectar and unscented, but produce largeamounts of pollen, e.g. the grasses and catkins ontrees such as Salix (willow), Betula (birch), Corylus(hazel), Fagus (beech), Quercus (oak), etc. Theyalso tend to have proportionally large stigma thatprotrude from the flower to maximize the chancesof intercepting pollen grains in the air. TheGymnosperma (conifers) also produces pollenfrom the small male cones.

Insect-pollinated flowers produce odours, havebrightly coloured petals to attract insects and pro-duce nectar to feed them. The insects such as beesand flies then collect pollen on their bodies andcarry it to other flowers.

Plant breeders may use houseflies, enclosed in aglasshouse, to carry out pollination, or mechanicaltransference of pollen, which may be achieved byhand using brushes. Some floral mechanisms, e.g.snapdragon and clover, physically prevent non-pollinating species by allowing only heavy bees to enter the flower. Others trap pollinators for aperiod of time to give the best chance of successful

fertilization, e.g. Arum lily. Certain Primula spp.have stigma and stamens of differing lengths toensure cross-pollination.

The pollen grain on arrival at the stigma absorbssugar and moisture from the stigma’s surface andthen produces a pollen tube. The male nuclei arecarried in the pollen tube, which grows downinside the style and into the ovary wall. Afterentering the ovule, male and female nuclei fusetogether in fertilization, resulting in the zygote,which divides and differentiates to become theembryo of the seed. The seed endosperm may beproduced by more nuclear fusion, and the testa ofthe seed forms from the outer layers of the ovule.The ovary develops into a protective fruit.

Bees in pollination

The well-known social insect, the honey bee (Apismellifera), is helpful to horticulturists. The femaleworker collects pollen and nectar in special pockets(honey baskets) on its hind legs. This is a supplyof food for the hive and, in collecting it, the beetransfers pollen from plant to plant. Several crops,e.g. apple and pear, do not set fruit when self-pollinated. Thus, this insect provides a useful function to the fruit grower. In large areas of fruit production the number of resident hives is insuffi-cient to provide effective pollination, and in cool,damp or windy springs, the flying periods of the beesare reduced. It may therefore be advantageous forthe grower to introduce beehives into the orchardsduring the blossom time, as an insurance againstbad weather. One hive is normally adequate toserve 0.5 ha of fruit. Blocks of four hives placed inthe centre of a 2 ha area require foraging bees totravel a maximum distance of 140 m. In addition tohoney bees, wild species, e.g. the potter flower bee(Anthophora retusa) and red-tailed bumble-bee(Bombus lapidarius), increase fruit set, but theirnumbers are not really high enough to dispensewith the honey bee. The pollination of tomatoes in greenhouses is commonly achieved by small in-house populations of a bumble-bee species(Bombus terrestris).


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All species of bee are killed by broad spectruminsecticides, e.g. deltamethrin, and it is importantthat spraying of such chemicals be restricted toearly morning or evening during the blossom timeperiod when hives have been introduced.

Colour in flowers

The use of different flower colours in the gardenhas been the subject of much discussion in Britainover the last 300 years. Many books have beenwritten on the subject, and authorities on the sub-ject will disagree about what combination of plantscreates an impressive border. Some combinationsare mentioned here, and Plate 1 illustrates oneexample of the harmony created by blue flowersplaced next to yellow ones. Other combinationssuch as blue and white,e.g.Ceanothus ‘Blue Mound’and Clematis montana; yellow and red, e.g.Euphorbia polychroma and Geum rivale; yellowand white, e.g. Verbascum nigrum and Tanacetumparthenium; purple and pale yellow, e.g. Salvia �superba and Achillea ‘lucky break’; and red andlavender, e.g. Rosa gallica and Clematis integrifolia.

Removal of dead flowers

The removal of dead flowers, an activity calleddead heading, is an effective way to help to main-tain the appearance of a garden border. Examplesof species needing this procedure are seen in bed-ding plants which flower over several months, e.g.African marigold (Tagetes erecta); in herbaceousperennials, e.g. Delphinium and Lupin; in smallshrubs, e.g. Penstemon fruticosus; and in climbers,e.g. sweet pea and Rosa ‘Pink Perpetue’.As flowersage, they begin to use up a considerable amount ofthe plant’s energy in the production of fruits.Also,hormones produced by the fruit inhibit flowerdevelopment. With species such as those men-tioned above, the maturation of fruits will consid-erably reduce the plant’s ability to continueproducing flowers. The act of dead heading, there-fore, will greatly improve subsequent flowering.

An added bonus is that plants that have been deadheaded may continue to flower many weeks longerthan those allowed to retain their dead flowers.

Many species such as wax begonia (Begonia �semperflorens cultorum) and busy lizzie (Impatienswallerana) used as bedding plants have been spe-cially bred as F1 hybrids (see page 84) where, inthis case, flowers do not produce fruits containingviable seed. In such cases, there is not such a greatneed to dead head,but this activity will help preventunsightly rotting brown petals from spoiling theappearance of foliage and newly produced flowers.

THE FRUITING PLANT

The development of the fruit involves either theexpansion of the ovary into a juicy succulent struc-ture, or the tissues becoming hard and dry, bothproviding protection and/or dispersal of the seeds.The succulent fruits are often eaten by animals,which help seed dispersal, and may also bringabout chemical changes to break dormancy mech-anisms (see page 63). Some fruits (described asbeing dehiscent) release their seeds into the air.They do this either by an explosive method as seenin the brooms and poppies, or by tiny featheryparachutes, seen in willow herb and groundsel.Dry fruits may rot away gradually to release theirseeds by an indehiscent action. Different adapta-tions of fruit, many of which are of economicimportance, and the methods by which seeds aredispersed are summarized in Table 7.1

Fruit set

The process of pollination, in most species, stimu-lates fruit set. The hormones, in particulargibberellins, carried in the pollen, trigger the pro-duction of auxin in the ovary, which causes thecells to develop. In species such as cucumber, thenaturally high content of auxin enables fruit pro-duction without prior fertilization, i.e. partheno-carpy, a useful phenomenon when the object ofthe crop is the production of seedless fruit. Such


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activity can be simulated in other species, espe-cially when poor conditions of light and tempera-ture have caused poor fruit set in species such astomato and peppers. Here, the flowers are sprayedwith an auxin-like chemical, but the quality of fruitis usually inferior. Pears can be sprayed with asolution of gibberellic acid to replace the need forpollination. Fruit ripening occurs as a result ofhormonal changes and involves a change in thesugar content in tomatoes, i.e. at the crucial stagecalled climacteric. After this point, fruit will con-tinue to ripen and also respire after removal fromthe plant. Ethylene is released by ripening fruit,which contributes to deterioration in store. Earlyripening can be brought about by a spray ofa chemical, e.g. ethephon, which stimulates therelease of ethylene by the plant, e.g. in the tomato.

THE AGEING PLANT

At the end of an annual plant’s life, or the growingseason of perennial plants, a number of changestake place. The changes in colour associated withautumn are due to pigments that develop in theleaves and stems and are revealed as the chlorophyll(green) is broken down and absorbed by the plant.

Pigments are substances that are capable ofabsorbing light; they also reflect certain wave-lengths of light, which determine the colour of thepigment. In the actively growing plant, chlorophyll,which reflects mainly green light, is produced inconsiderable amounts, and therefore the plant,especially the leaves, appears predominantly green.Other pigments are present; e.g. the carotenoids(yellow) and xanthophylls (red), but usually thequantities are so small as to be masked by thechlorophyll. In some species, e.g. copper beech(Fagus sylvatica), other pigments predominate,masking chlorophyll.These pigments also occur inmany species of deciduous plants at the end of thegrowing season, when chlorophyll synthesis ceasesprior to the abscission of the leaves. Many coloursare displayed in the leaves at this time in speciessuch as Acer platanoides, turning gold and red,Prunus cerasifera ‘pissardii’ with light purple

leaves, European larch with yellow leaves, virginiacreeper (Parthenocissus and Vitus spp.) with redleaves, beech with brown leaves, Cotoneaster andPyracantha with coloured berries and Cornusspecies, which have coloured stems.These are usedin autumn colour displays at a time when fewerflowering plants are seen outdoors.

In deciduous woody species, the leaves drop inthe process of abscission, which may be triggered byshortening of the daylength. In order to reduce riskof water loss from the remaining leaf scar, a corkylayer is formed before the leaf falls.Auxin produc-tion in the leaf is reduced, and this stimulates theformation of the abscission layer, and abscisic acid isinvolved in the process.Auxin sprays can be used toachieve a premature leaf fall in nursery stock plantsthus enabling the early lifting of bare-root plants.Ethylene inhibits the action of auxin, and can there-fore also cause premature leaf fall, e.g. in Hydrangeaprior to cold treatment for flower initiation.

FURTHER READING

Attridge, T.H. Light and Plant Responses (EdwardArnold, 1991).

Bleasdale, J.K.A. Plant Physiology in Relation toHorticulture (Macmillan, 1983).

Capon, B. Botany for Gardeners (Timber Press, 1990).Donnely, D.J. and Vidaver,W.E. Glossary of Plant Tissue

Culture (Cassell, 1995).Electricity Council. Grow Electric Handbook No. 2

Lighting in Greenhouses, Part 2, 1974.Gardner, R.J. The Grafter’s Handbook (Cassell, 1993).Hart, J.W. Plant Tropisms and Other Growth Movements

(Unwin Hyman, 1990).Hartman, H.T. et al. Plant Propagation, Principles and

Practice (Prentice-Hall, 1990).Hattatt, L. Gardening with Colour (Parragon, 1999).Leopold, A.C. Plant Growth and Development, 2nd edi-

tion (McGraw-Hill, 1977).Machin, B. Year-round Chrysanthemums (Grower

Publications, 1983).MAFF. Quality in Seeds, Advisory Leaflet 247.MAFF. Guide to the Seed Regulations (HMSO, 1979).Stanley, J. and Toogood, A. The Modern Nurseryman

(Faber, 1981).Wareing, P.F. and Phillips, I.D.J. The Control of Growth

and Differentiation in Plants (Pergamon, 1981).


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Plant breeding uses the basic principle that funda-mental characteristics of a species are passed onfrom one generation to the next (heredity). How-ever, sexual reproduction may produce differentcharacteristics in the offspring (variation). A plantbreeder relies on the principles of heredity toretain desirable characteristics in a breeding pro-gramme, and new characteristics are introduced inseveral ways to produce new cultivars.

In order to understand the principles of plantbreeding, the genetic make-up of the plant mustfirst be studied.

THE CELL

Every living plant cell contains a nucleus whichcontrols every activity occurring in the cell (seepage 40). Within the nucleus is the chemicaldeoxyribose nucleic acid (DNA), a very large mol-ecule made up of thousands of atoms. The DNAcontains hundreds of sub-units (nucleotides), eachof which contains a chemically active zone called a‘base’. There are four different bases: guanine,cytosine, thymine and adenine. The sequence ofthese bases is the method by which genetic infor-mation is stored in the nucleus, and also the meansby which information is transmitted from thenucleus to other cell organelles (this mechanism iscalled the genetic code). A change in the basesequencing of a plant’s code will lead to it develop-ing new characteristics. These very long moleculesof DNA are called chromosomes.

Each species of plant has a specific number ofchromosomes. The cells of tomato (Lycopersicumesculentum) contain 24 chromosomes, the cells ofPinus and Abies species 24 and onions 16 (humanbeings have 46). Each chromosome contains a suc-cession of units, called genes, containing many baseunits. Each gene usually is the code for a singlecharacteristic. Scientists have been able to relatethe many gene locations to plant characteristics.Microscopic observation of cells during cell div-ision reveals two similar sets of chromosomes,

8

Genetics and Plant Breeding

Ever since growers selected seed for their nextcrop, they have influenced the genetic make-up and therefore the potential of succeedingcrops. A basic understanding of plant breed-ing principles enables the grower to under-stand the potential and limits of plantcultivars and therefore make more realisticrequests to the plant breeder. Plant breedingnow supplies a wide range of plant types tomeet growers’ specific needs. The plantbreeder’s skill relies on his knowledge ofgenetics to manipulate inheritable characters,and the ability to recognize and select thedesirable characters when they occur.

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Genetics and Plant Breeding 81

e.g. in tomatoes, 12 different pairs.This condition istermed as diploid. A gene for a particular charac-teristic, such as flower colour, has a precise locationon one chromosome, on the same location of thepaired chromosome (an homologous pair). Foreach characteristic, therefore, there are at least twoalleles (gene alternatives), one on each chromo-some in the homologous pair, which providesgenetic information for that characteristic.

Cell division

When a plant grows, the cell number increases inthe growing points of the stems and roots, the div-ision of one cell producing two new ones. Geneticinformation in the nucleus must be reproducedexactly in the new cells to maintain the plant’s char-acteristics. The process of mitosis achieves this.Each chromosome in the parent cell produces aduplicate of itself producing sufficient material forthe two new daughter cells. A delicate, spindle-shaped structure ensures the separation of chromo-somes, one complete set into each of the new cells.A dividing cell wall forms across the old cell tocomplete the division.

Sexual reproduction involves the fusion ofgenetic material contained in the sex cells from bothparents (see fertilization). Half of the chromosomesin the cells of an offspring are therefore inheritedfrom the male parent, and half from the female. Toensure that the offspring chromosome numberequals that of the parents, contents of the nuclei ofmale and female sex cells (see pollen and ovule)must be halved. The cell division process (meiosis),occurring in the anthers and ovaries, produces thenuclei of the sex cells (gametes). It ensures the sep-aration of each chromosome from its partner so thateach sex cell contains only one complete set ofchromosomes. This condition is termed as haploid.

INHERITANCE OF CHARACTERISTICS

Genetic information is passed from parent to offspring when material from male and female

parent comes together by fusion of the sex cells.Genes from each parent can, in combination, pro-duce a mixture of the parents’ characteristics in theoffspring; e.g. a gene for red flowers inherited fromthe male parent, combined with a gene for whiteflowers from the female parent, could producepink-flowered offspring. If one of the genes, how-ever, completely dominated the other, e.g. if the redgene inherited from the male parent was dominantover the white female gene, all offspring would pro-duce red flowers (see Figure 8.1).

The recessive white gene will still be present aspart of the genetic make-up of the offspring cellsand can be passed on to the next generation. If itthen combines at fertilization with another whitegene, the offspring will be white-flowered offspring.

The example given considers the inheritance ofone pair of genes by a single offspring (mono-hybrid cross). However, many sex cells are pro-duced from one flower in the form of pollen grainsand ovules, which fuse to form many seeds of thenext generation. The plant breeder must know thegenetic composition of the whole population ofoffspring.

Consider now the same example of flowercolour. If a red-flowered plant, containing twogenes for red (described as pure), is crossed with apure white-flowered plant, the red-flowered plantsupplies pollen as the male parent, and the white-flowered plant produces seed as the female parent. As both parents are pure, the male parentcan produce only one type of sex cell, containingthe ‘red’ version (allele) of the gene, and the femaleparent only the white (allele). Since all pollengrains will carry ‘red’ genes and all ovules ‘white’,then in the absence of dominance, the only pos-sible combination for the first generation or F1yields pink offspring, each containing an allele forred and an allele for white (i.e. impure). Figure 8.2illustrates this inheritance by using letters todescribe genes, R to represent a red gene, and r torepresent a white gene. The genotype (the innergenetic make-up of a cell) can be represented byusing letters, e.g. Rr. The outward appearanceresulting from the genotype’s action (e.g. red, pinkor white flower) is called the phenotype.

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If the plants from the F1 generation were usedas parents and crossed (or perhaps self-pollinated), then the results in the second or F2generation would be as shown in Figure 8.3, i.e. 25per cent of the population, therefore, would havethe phenotype of red flowers, 50 per cent pinkflowers and 25 per cent white flowers (a ratio of1:2:1).The plant breeder by analysing the ratios ofeach colour would therefore be able to calculate

which colour genes were present in the parents,and whether the ‘colour’ was pure.

More usually, one of the alleles at the gene siteexhibits dominance and the other is recessive.When offspring inherit both alleles, there is nointermediate phenotype; only the dominant geneexpresses itself. It was the monk Gregor Mendelworking in a monastery in what is now Brno in theCzech Republic, in the middle of the eighteenth

NO DOMINANCE:

Red flower

Genes: red, red

Genes: red, white

Genes: red, red Genes: white, white

Genes: red, white

Red flowers

DOMINANCE:

Red flower White flower

OR

Genes: white, white

Pink flowers

�

�

White flower

Figure 8.1 The pattern of inheritance of genes.

Parents:

Sex cells:

F1 plants:All pink

Rr

RR rr

R R r r

Rr

M E I O S I S

�

F E RT I L I Z AT I O N

ovulespollen

Pure red flowers Pure white flowers

Figure 8.2 Simple inheritance: production of the F1generation.

Parents:(F1 plants)

Sex cells:

F2 generation

pink pinkred white

Rr

RrRr

RR rr

R R rr

Rr

�

ovulespollen

Impure pink Impure pink

Figure 8.3 Simple inheritance: production of the F2generation.

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century who laid the foundations of genetics with his breeding experiments. He used peas andworked with single characteristics including theheight of the plant. He found that in the second orF2 generation they were either tall or short; therewere no middle-sized phenotypes. They werefound to be in the ratio of three tall to one shortplant. Similarly, he crossed pure (homozygous)round pea parents (RR) with pure (homozygous)wrinkled pea parents (rr). Crossing the two pureparents (RR � rr) the genotype of the first gener-ation or F1 follows the pattern as shown in Figure8.2, but all the plants produced round peas, i.e. theround pea allele was dominant in peas. When thesecond or F2 generation was produced the geno-type followed the pattern shown in Figure 8.3, butthere were three round pea to every one wrinkledpea type. Both RR and Rr genotypes producedthe same phenotype; only a double recessive (rr)produced plants with wrinkled peas. He went onto look at other simple ‘single gene characteristics’including seed colour (yellow dominant and greenrecessive) and flower colour (purple dominantand white recessive). In all these cases he foundthat the ratio of phenotypes in the second or F2generation was 3:1. By observation he establishedthe Principle of Segregation; the phenotype isdetermined by the pair of alleles in the genotypeand only one allele of the gene pair can be presentin a single gamete (i.e. passed on by a parent).

Mendel then went on to investigate the crossingof plants that differed in two contrasting characters.The results of crossing tall purple-flowered peaswith short white-flowered ones were to produceall tall purple-flowered peas, they all had the samegenotype (TtPp) and the same phenotype. The F2generation produced from these parents is illus-trated in Figure 8.4, where the combinations areshown in a Punnet Square (or ‘checkerboard’ – auseful way of showing the genotypes produced ina crossing). Note that each gene has behaved inde-pendently; there are 12 tall to four short (ratio 3:1)and 12 purple to four white-flowered plants (3:1).This illustrates Mendel’s Principle of IndependentAssortment; the alleles of (unlinked) genes behaveindependently at meiosis. The cross involving

two independent (unlinked, i.e. not on the samechromosome) genes produces combinations ofphenotypes in the ratio 9:3:3:1. For the crossTtPp � TtPp, the genotypes produced are shownin Figure 8.4 and the phenotypes are as follows:

9 Tall, purple-flowered plants1 TTPP2 TTPp2 TtPP4 TtPp

3 Tall, white-flowered plants2 TTpp1 Ttpp

3 Short, purple-flowered plants2 ttPP1 ttPp

1 Short, white-flowered plants1 ttpp

TTPP

TP TP

TTPp

Tp

TtPp

tP

TtPp

tp

TTPp

Tp

TTpp

TtPp

Ttpp

TtPP

tP

TtPp

ttPP

ttPp

TtPp

tp

Ttpp

ttPp

ttpp

PARENTPHENOTYPES

Tall, purpleflowers

Tall, purple flowers

TTPP

TtPp � TtPp

ttpp

TP tp

Small, whiteflowers

PARENTGENOTYPES

GAMETES

OFFSPRINGGENOTYPESOFFSPRINGPHENOTYPES

GAMETESfrom MALE

OFFSPRINGSECOND (F2)GENERATIONGENOTYPESshown in thePunnet Square(checkerboard)

GAMETESfrom FEMALE

FIRST (F1)GENERATION

Figure 8.4 Punnet square showing a dihybrid cross.

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Not all genes act independently, as some arelinked by being on the same chromosome and willusually appear together. The number of groupedgenes is equal to the number of chromosome pairsfor the species (seven for peas).

F1 HYBRIDS

The importance of F1 hybrids to the grower isthat, given a uniform environment, plants of thesame cultivar that are F1 hybrids will produce auniform crop because they are all genetically iden-tical (Figure 8.2). Crops grown from F1 hybridseed such as cabbage, Brussels sprouts and carrotscan be harvested at one time and they have similarcharacteristics of yield; flower crops will have uniformity of colour and flower size.

Plants derived from crossing parent types withdifferent characteristics display hybrid vigour, andgenerally respond well to good growing conditions.The desirable characteristics of the two parents, suchas disease resistance, good plant habit, high yieldand good fruit or flower quality, may be incorp-orated along with established characteristics of successful commercial cultivars by means of the F1 hybrid breeding programme. F1 hybrid seedproduction first requires suitable parent stock,which must be pure for all characteristics. In thisway, genetically identical offspring are produced, asdescribed in Figure 8.2.The production of pure par-ent plants involves repeated self-pollination (self-ing) and selection, over eight to twelve generations,resulting in suitable inbred parent lines. During thisand other self-pollination programmes vigour islost but, of course, is restored by hybridization.

The parent lines must now be cross-pollinatedto produce the F1 hybrid seed. It is essential toavoid self-pollination at this stage, therefore oneof the lines is designated the male parent to supplypollen.The anthers in the flowers of the other line,the female parent, are removed, or treated to pre-vent the production of viable pollen. The growingarea must be isolated to exclude foreign pollen,and seed is collected only from the female parent.This seed is expensive compared with other

commercial seed, due to the complex programmerequiring intensive labour. Seed collected fromthe commercial F1 hybrid crop represents the F2,and will produce plants with very diverse charac-teristics (Figure 8.3). It is clear, therefore, that seedfrom an F1 should not be saved for sowing, if uni-form characteristics need to be retained. Some F2seed, however, is produced in flowering plantssuch as geraniums and fuchsias, where variablecolour and habit are required.

OTHER BREEDING PROGRAMMES

In addition to F1 hybrid breeding, where specificimprovements are achieved, plant breeders maywish to bring about more general improvementsto existing cultivars, or introduce characteristicssuch as disease resistance. Programmes are requiredfor crops which self-pollinate (inbreeders), or thosewhich cross-pollinate (outbreeders), and three ofthese strategies are described.

Pedigree breeding is the method for plantbreeding most widely used by both amateurs andprofessionals. Two plants with different desirablecharacteristics are crossed to produce an F1 popu-lation.These are selfed and the offspring with use-ful characteristics is selected for further selfing toproduce a line of plants.After repeated selfing andselection, the characteristics of the new lines arecompared with existing cultivars and assessed forimprovements. Further field trials will determine anew type’s suitability for submission and potentialregistration as a new cultivar. If a plant breederwishes to produce a strain adapted to particularconditions, such as hardiness, exposure of plants ofa selected cultivar to the desired conditions willeliminate unsuitable plants, thus allowing thehardy plants to set seed. Repetition of this processgradually adapts the whole population, while othercharacteristics, such as earliness, may be selectedby harvesting seed early.

Disease resistance (see Chapter 12), a geneticcharacteristic, enables the plant to combat fungalattack. The disease organism may itself develop a corresponding genetical capacity to overcome


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the plant’s resistance by mutation.The introductionof disease resistance into existing cultivars requiresa backcross breeding programme, involving acommercial cultivar lacking resistance, crossedwith a wild plant which exhibits resistance. Forexample, a lettuce cultivar may lack resistance to downy mildew (Bremia lactucae), or a tomato cultivar lack tomato mosaic virus resistance. Thiscommercial cultivar is crossed with a resistant wildplant to produce an F1, then an F2. From this F2,plants having both the characteristics of the com-mercial cultivar and also disease resistance areselected. The process continues with backcrossingof these selected plants with the original com-mercial parent to produce an F1, from which an F2is produced. More commercial characteristics maybe incorporated by further backcrossing and selec-tion over a number of generations, until all thecharacteristics of the commercial cultivar arerestored, but with the additional disease resistance.

POLYPLOIDS

Polyploids are plants with cells containing morethan the diploid number of chromosomes; e.g. atriploid has three times the haploid number, atetraploid four times and the polyploid series con-tinues in many species up to octaploid (eight timeshaploid). An increase in size of cells, with a result-ant increase in roots, fruit and flower size of manyspecies of chrysanthemums, fuchsias, strawberries,turnips and grasses, is the result of polyploidy.There is a limit to the number of chromosomesthat a species can contain within its nucleus.

Polyploidy occurs when duplication of chromo-somes (see mitosis) fails to result in mitotic cell divi-sion.The multiplication of a polyploid cell within ameristem may form a complete polyploid shootthat, after flowering, may produce polyploid seed.The crossing of a tetraploid and a diploid gives riseto a triploid. Polyploid plants are often infertile,especially triploids, with an odd number of chromo-somes which are unable to pair up during meiosis.A number of apple cultivars, such as Bramley’sseedling and Crispin, are triploid and require

pollinator cultivars for the supply of viable pollen.Polyploidy can occur spontaneously, and has led tomany variant types in wild plant populations. Itcan be artificially induced by the use of a mitosisinhibitor, colchicine.

MUTATIONS

Changes in the content or arrangement of the chromosomes cause changes in the characteristicsof the individual. Very drastic alterations result inmalformed and useless plants,while slight rearrange-ments may provide a horticulturally desirablechange in flower colour or plant habit, as shown inchrysanthemums, dahlias or Streptocarpus. Muta-tion breeding has produced these variations usingirradiation treatments with X-rays, gamma rays ormutagenic chemicals. Spontaneous mutations regu-larly occur in cells but, as in polyploids, the muta-tion only becomes significant in the plant when themutated cell is part of a meristem.

A shoot with a different colour flower or leafmay arise in a group of plants, and is termed asport.When a plant consists of two or more genet-ically distinct tissues, it is called a chimaera, oftenresulting in variegation of the leaves, e.g. Acer andPelargonium. These conditions are maintainedonly by vegetative propagation, which encouragescell division in both tissue types. All mutations are inherited by a succeeding generation in theways previously described, which enables new char-acteristics and potential new cultivars to be produced in just one generation.

RECOMBINANT DNA TECHNOLOGY

For the plant breeder it has historically been diffi-cult to predict whether the progeny from a breedingprogramme would show the desired characteristics.The term recombinant DNA technology refers tothe modern methods of breeding that enable novelsources of DNA to be integrated with greater cer-tainty into a plant’s existing genotype. Two newtechniques have appeared in the last few years thathave enabled this major shift in breeding practice.

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The first technique is marker-assisted breeding.Breeders are now able to analyse chromosomematerial and establish what DNA sequence is pre-sent on the chromosome. Some plant characterssuch as disease resistance are hard to evaluate innewly bred plants, as it may be difficult to achieveinfection under test conditions. Since the breedersare now able to recognize the chromosome DNAsequence for plant resistance, they can apply thisknowledge by analysing newly bred plants for thisdesirable character. Whilst resistance to a diseasemay be complex, involving several genes actingtogether, the marker-assisted technique has proveda powerful form of assistance in this area.

The second technique is genetic modification(now known as GM), or genetic engineering. Bythis method, genes derived from other plant speciescan be incorporated into the species in question.The commonest technique involves the bacteriumAgrobacterium tumifasciens. This organism (seepage 130) causes crown gall disease on plants suchas apple. The bacterium contains a circular piece ofDNA (plasmid) that on entering plant cells canintegrate its DNA into that of the infected plantcell. Breeders are able to develop strains of A. tumi-fasciens in large numbers. The new strains contain,in their plastids, a desirable gene taken from othervariants of the same plant species, or taken fromother sources. Wounded test plants can then beinfected and the chosen gene begins to multiply byintegrating itself into the cells of the plant. Tissuesdeveloping around the point of infection can thenbe used for propagation of the new geneticallymodified cultivar. Confirmation of successfulgenetic change can be achieved most easily whenthe newly introduced gene is linked in the bacterialplasmid with a marker gene. Two common kinds ofmarker have been used; resistance to an antibioticand resistance to a herbicide. In this way, thebreeder is able to test whether incorporation of adesirable new character has been successful byexposing it to the antibiotic or herbicide concerned.

There seems little doubt that major advances inthe quantity and quality of horticultural cropscould follow GM methods of breeding. However,the breeders have not been able to allay fears

from the public that such methods may result in aninsidious deterioration of food quality, or pose athreat to the environment.

THE PLANT VARIETIES AND SEEDS ACT, 1964

This Act protects the rights of producers of newcultivars. The registration of a new cultivar isacceptable only when its characteristics are shownto be significantly different from any existing type.Successful registration enables the plant breederto control the licence for the cultivar’s propagation,whether by seed or vegetative methods. Separateschemes operate for the individual genera of horti-cultural and agricultural crops, but all breedingactivities may benefit from the 1964 Act.

GENE BANKS

As new specialized cultivars are produced andgrown, using highly controlled cultivation methods,old cultivars and wild sources of variation, whichcould be useful in future breeding programmes, arebeing lost.There is much interest in gene conserva-tion. A gene bank provides a means of storing theseed of many cultivars and species at very low tem-peratures, while some plant material is maintainedin tissue culture conditions (see page 11).

FURTHER READING

George, R.A.T. Vegetable Seed Production (Longman,1999).

Have, D.J. van der. Plant Breeding Perspectives (Centrefor Agricultural Publishing and Documentation,Wageningen, 1979).

Ingram, D.S. et al. (Editors). Science and the Garden(Blackwell Science Ltd, 2002).

North, C. Plant Breeding and Genetics in Horticulture(Macmillan, 1979).

Simmonds, N.W. Principles of Crop Improvement(Longman, 1979).

Watts, L. Flower and Vegetable Breeding (Grower Books,1980).


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DAMAGE

A weed is a plant of any kind that is growing in thewrong place: groundsel smothering lettuce, mosscovering a lawn, last year’s potatoes emerging in aplot of cabbage, rose suckers spoiling a herbaceousborder. Damage caused by weeds may be categor-ized into seven main areas:

1 Competition between the weed and the plantfor water, nutrients and light may provefavourable to the weed if it is able to establishitself quickly. The plants are therefore deprivedof their major requirement and poor growthresults. The extent of this competition is largelyunpredictable, varying with climatic factors suchas temperature and rainfall, soil factors such as

soil type and cultural factors such as cultivationmethod, plant spacing and quality of weed con-trol in previous seasons. Large numbers of weedseed may be introduced into a plot in poor qual-ity composts or farmyard manure. The uncon-trolled proliferation of weeds will inevitablyproduce serious plant losses.

2 Seed quality is lowered by weed seed, e.g. fat hencontamination in batches of seed, e.g. carrots.

3 Machinery, e.g. mowing machines and harvest-ing equipment may be fouled by weeds, such asknotgrass, that have stringy stems.

4 Poisonous plants: Ragwort, sorrel and buttercupsare eaten by farm animals when more desirablefood is scarce. Poisonous fruits, e.g. black night-shade, may contaminate mechanically harvestedcrops such as peas for freezing.

5 Pests and diseases are commonly harboured onweeds. Chickweed supports whitefly, red spidermite and cucumber mosaic virus in greenhouses.Sowthistles are commonly attacked by chrysan-themum leaf miner. Groundsel is everywhereinfected by a rust which attacks cinerarias. Char-lock may support high levels of club root, a seri-ous disease of brassica crops. Fat hen and docksallow early infestations of black bean aphid tobuild up. Speedwells may be infested with stemand bulb nematodes.

6 Drainage (see Chapter 14) depends on a freeflow of water along ditches. Dense growth ofchickweed may seriously reduce this flow andincrease waterlogging of horticultural land.

9

Weeds

Most plants, whether wild or cultivated, growin competition with other organisms such aspests, diseases and other plants. Any competi-tive unwanted plant is termed as a weed. Thischapter indicates the growing situations whereweeds become problems, the means of weedidentification, the specific biological featuresof weeds that make them important and therelevant control measures used against them.Detailed principles of control are described inChapter 12.

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7 Tidiness is essential in a well-maintained garden.The amenity horticulturist may consider thatany plant spoiling the appearance of plants inpots, borders, paths or lawns should be removed,even though the garden plants themselves arenot affected.

IDENTIFICATION

As with any problem in horticulture, recognitionand identification are essential before any controlmeasures can be attempted. The weed seedlingcauses little damage to a crop and is relatively easyto control. Identification of this stage is thereforeimportant and, with a little practice, the gardeneror grower may learn to recognize the importantweeds using such features as shape, colour andhairiness of the cotyledons and first true leaves(see Figure 9.1).

Early recognition of a weed species may be cru-cial for control.The ivy-leaved speedwell is suscep-tible to the foliage-applied herbicides only at thecotyledon to three-leaf stages, and attempted con-trol against the more mature weed will give poorresults.

Within any crop or bedding display, a range ofdifferent weed species will be observed. Changesin the weed flora may occur because of environ-mental factors such as reduced pH; because ofnew crops that may encourage different weeds todevelop; or because repeated use of one herbicideselectively encourages certain weeds, e.g. ground-sel in lettuce crops or annual meadow grass in turf.Horticulturists must watch carefully for thesechanges so that their chemical control may beadjusted.

The mature weeds may be identified using anillustrated flora, which shows details of leaf andflower characters.

Weed biology

The range of weed species includes algae, mosses,liverworts, ferns (bracken) and flowering plants.

These species display one or more special featuresof their life cycle which enable them to compete assuccessful weeds against the crop and cause prob-lems for the horticulturist.

Many weeds, e.g. groundsel, produce seeds thatgerminate more quickly than crop seeds and thusemerge from the soil to crowd out the developingplants. Seeds of species such as charlock, annualmeadow grass and groundsel germinate through-out the year, while others such as orache appear inspring and cleavers in autumn. Carefully timedcultivations or herbicidal control is effective againstthis growth stage.

The soil conditions may favour certain weeds.Sheep’s sorrel prefers acid conditions, while mossesare found in badly drained soils. Knapweed com-petes well in dry soils, while common sorrel sur-vives well on phosphate-deficient land. Yorkshirefog grass invades poorly fertilized turf, while net-tle and chickweed favour highly fertile soils.

The growth habit of the weed may influence itssuccess. Chickweed and slender speedwell produceprostrate stems bearing numerous leaves that pre-vent light reaching emergent plant seedlings, whilegroundsel and fat hen have an upright habit thatcompetes less for light in the early period. Weedssuch as bindweed, cleavers and nightshades areable to grow alongside woody plants, such as canefruit and border shrubs, making control difficult.

Different weeds propagate in different ways.Chickweed completing several life cycles per year(see ephemeral, Table 3.3), and black nightshadecompleting one (annual), both produce seed inorder to continue the species. Annual seed pro-duction may be high in certain species. A scentlessmayweed plant may produce 300 000 seeds, fat hen70 000 and groundsel 1000. A dormancy period isseen in many weed species, groundsel being anexception. Seed germination commonly continuesover a period of 4 or 5 years after seed dispersal,presenting the grower with a continual problem.

Some of the most difficult perennial weeds tocontrol rely on vegetative propagation for theirlong-term survival in soils. Bracken and couch grasssurvive and spread by means of underground stems(rhizomes). Field bindweed and creeping thistle,


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Weeds 89

on the other hand, produce creeping roots that, inthe latter species, may grow 6 m in 1 year. Docks,dandelions and plantains develop swollen taproots,while horsetails survive the winter by means ofunderground tubers. The large quantities of foodstored in vegetative organs enable these species to

emerge quickly from the soil in spring, often fromconsiderable depths if the soil has been ploughedin.The fragmentation of underground rhizomes andcreeping roots by cultivation machinery enablesthese species to increase in disturbed soils. Weedswith swollen roots provide the greatest problems

Chickweed (�1.5)Bright green. Cotyledons have a light colouredtip and a prominent mid-vein. True leaveshave long hairs on their petioles

Creeping Thistle (�1.5)Cotyledons large and fleshy. True leaves haveprickly margins

Yarrow (�1.5)Small broad cotyledons. True leaves hairyand with pointed lateral lobes

Groundsel (�1.5)Cotyledons are narrow and purple under-neath. True leaves have step-like teeth

Broad-leaved Dock (�1.5)Cotyledons narrow. First leaves often crimson,rounded with small lobes at the bottom

Large Field Speedwell (�1.5)Cotyledons like the ‘spade’ on playing cards.True leaves hairy, notched and opposite

Figure 9.1 Seedlings of common weeds. Notice the difference between cotyledons and true leaves. (Reproduced by permission of Blackwell Scientific Publications).

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to the horticulturist in long-term crops such as softfruit and turf because foliage-acting and residualherbicides may have little effect.

Fragmentation of above-ground parts may beimportant. A lawnmower used on turf containingthe slender speedwell weed cuts and spreads thedelicate stems that, under damp conditions, estab-lish on other parts of the lawn.

Control measures are regularly necessary in mostcrop and amenity situations. Greenhouse produc-tion suffers much less from weed problems becausecomposts and border soils are regularly sterilized.

Below are described important annual andperennial weeds. Specific descriptions of identifi-cation, damage, biology and control measures aregiven for each weed species.

Detailed discussion of weed control measures(legislative, cultural and chemical) is presented inChapter 12.

ANNUAL WEEDS

While there are at least 50 successful annual weedspecies in horticulture, this book can cover only afew examples that illustrate the main points of lifecycles and control. Three species, chickweed,groundsel and speedwell, are described below todemonstrate some features of their biology thatmake them successful weeds.

Chickweed (Stellaria media)

This species is found in many horticultural situ-ations as a weed of flowerbeds, vegetables, soft fruitand greenhouse plantings. It has a wide distributionthroughout Britain, grows on land up to altitudesof 700 m, and is most often seen on rich, heavy soils.

The seedling cotyledons are pointed with a lightcoloured tip, while its true leaves have hairy peti-oles (see Figure 9.1). The adult plant has a charac-teristic lush appearance (see Figure 9.2) and growsin a prostrate manner over the surface of the soil;in some cases it covers an area of 0.1 m2, its leafystems crowding out young plants as it increases in

size. Small white, five-petalled flowers are producedthroughout the year, the flowering response beingindifferent to daylength. The flowers are self-fertile. An average of 2500 disc-like seeds (1 mm in diameter) may result from the oblong fruit capsules produced by one plant.

Since the first seed may be dispersed within 6 weeks of the plant germinating, and the plantcontinues to produce seed for several months, itcan be seen just how prolific the species is. Theseeds are normally released as the fruit capsuleopens during dry weather; they survive digestionby animals and birds and may thus be dispersedover large distances. Irrigation water may carrythem into channels and ditches.The large numbersof seed (up to 14 million/ha) are most commonly


Figure 9.2 Chickweed: note the opposite leaves andsmall white flowers.

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found in the top 7 cm of the soil where, under con-ditions of light, fluctuating temperatures andnitrate ions, they may overcome the dormancymechanism and germinate to form the seedling.Many seeds, however, survive up to the second,third and occasionally fourth years. Figure 9.3shows that germination can occur at any time ofthe year, with April and September as peak periods.

Chickweed is an alternate host for many aphidtransmitted viruses (e.g. cucumber mosaic), andthe stem and bulb nematode.

Control of this weed is best achieved by a com-bination of methods. Partial sterilization of soil ingreenhouses is effective, while hoeing in the springand autumn periods prevents the developingseedling from flowering. The weed may be killedby pre-emergent contact sprays, e.g. paraquatapplied before a crop emerges; by soil-applied root-acting herbicides, e.g. propachlor; or by foliage-acting herbicides applied in specific crop situations,e.g. linuron in potatoes. The weed in greenhouses

may, with care, be controlled by carefully directedsprays of contact herbicides, e.g. pentanochlor.

Groundsel (Senecio vulgaris)

This is a very common and important weed, foundin many countries, particularly on heavy soil. Itgrows on both rich and poor soils up to almost 600 min altitude.

The seedling cotyledons are narrow, purpleunderneath, and the true leaves have step-liketeeth (Figure 9.1). The adult plant has an uprighthabit, and produces as many as 26 yellow, small-petalled flower heads; flowering occurs in all sea-sons of the year. About 45 column-shaped seeds,2 mm in length, and densely packed in the fruithead, bear a mass of fine hairs which, when releasedin dry weather, carry the individual seeds along onair currents for many metres, while in wet weatherthe seeds become sticky and may be carried on thefeet of animals, including humans.The seeds survivedigestion by birds, and thus can be transported inthis way.As can be seen in Figure 9.3, the seeds maygerminate at any time of the year, with early Mayand September as peak periods. Since there may bemore than three generations of groundsel per year(the autumn generation surviving the winter), andeach generation may give rise to a thousand seeds,it is clear why groundsel is one of the most suc-cessful colonizers of cultivated ground. Its role as asymptomless carrier of the wilt fungus Verticilliumincreases its importance in certain crops, e.g. hops.

A combination of control methods may be neces-sary for successful control. Hoeing or alternativecultivation, particularly in spring and autumn, pre-vents developing seedlings from flowering, butuprooted flowering groundsel plants do produceviable seed. Contact herbicides, e.g. paraquat, maybe sprayed to control the weed on paths or in fallowsoil. Soil-acting chemicals, e.g. propachlor on bras-sicas, kill off the germinating seedling. An estab-lished groundsel population, especially in a cropsuch as lettuce, a fellow member of the Compositaefamily, requires careful choice of herbicide to avoiddamage to the crop.

Weeds 91

Chickweed

Groundsel

Fieldspeedwell

Ivy-leavedspeedwell

Fat hen

Greaterplantain

Blackbindweed

F M M AA J J S O N D J

SPRING SUMMER AUTUMN WINTER

Figure 9.3 Annual and perennial weeds: periods of seedgermination. Note that chickweed, groundsel and fieldspeedwell seeds germinate throughout the year. Manyother species are more limited. (Reproduced by permis-sion of Blackwell Scientific Publications.)

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Speedwells (Veronica persica and V. filiformis)

The first species, the large field speedwell, is animportant weed in vegetable production, while thesecond species, the slender or round-leaved speed-well, is no longer considered a pleasant rock gardenplant but has become a serious turf problem.

The seedling cotyledons are spade shaped, whilethe true leaves are opposite, notched and hairy(see Figure 9.1) in both species. The adult plantshave erect, hairy stems, and rather similar broad-toothed leaves.

V. persica produces up to 300 bright blue flowers,1 cm wide, per plant.The flowers are self-fertile andoccur throughout the year, but mainly betweenFebruary and November. The adult plant pro-duces an average of 2000 light brown boat-shapedseeds (2 mm across), which fall to the ground andmay be dispersed by ants; the rather large seedstravel as contaminants of crop seeds. The seeds ofthis species germinate below soil level all yearround, but most commonly from March to May(see Figure 9.3), the winter period being necessaryto break dormancy. Seeds may remain viable formore than 2 years.

V. filiformis produces self-sterile purplish-blueflowers between March and May, and spreads bymeans of prostrate stems which root at their nodesto invade fine and coarse turf, especially in dampareas. Segments of this weed cut by lawnmowerseasily root and further disperse the species. Seedsare not important in its spread.

Control of V. persica is best achieved by a com-bination of methods. Hoeing or alternative culti-vation, particularly in spring, prevents developingseedlings from growing to mature plants and pro-ducing their many seeds. Contact herbicides, e.g.paraquat, may be sprayed to control the weed onpaths or in fallow soils. Soil-acting chemicals,e.g. chlorpropham on onions, kill off the germinat-ing weed seedling, while contact chemicals, e.g.clopyralid, may be sprayed onto a young onion cropto control emerging seedlings.

The slender speedwell (V. filiformis) representsa different problem for control.While preventativecontrol on turf seedbed with a contact chemical,

e.g. paraquat, allows the turf to establish undis-turbed, use of a more selective contact chemical,e.g. chlorthal dimethyl, is necessary to preventchemical damage to established grass. Regular close mowing and spiking of turf removes the highhumidity necessary for this weed’s establishmentand development.

PERENNIAL WEEDS

Four species, creeping thistle, couch, yarrow andbroad-leaved dock are described below to demon-strate the major features of their biology (particu-larly perennating organs) that make them successfulweeds. The flowering period of these weeds ismainly between June and October (see Figure 9.4),but the main problem for growers is the ability ofthese plants to survive and reproduce vegetatively.

Creeping thistle (Cirsium arvense)

This species is a common weed in grassland andperennial crops, e.g. apples, where it forms denseclumps of foliage.

The seedling cotyledons are broad and smooth,the true leaves spiky (see Figure 9.1). It is readilyrecognizable by its dark green spiny foliage growing


Creepingthistle

Couch grass

Yarrow

Broad-leaveddock

SPRING SUMMER AUTUMN WINTER

F M A M J J A S O N D J

Figure 9.4 Perennial weeds: periods of flowering. Mostflowers and seeds are produced between June and Octo-ber. Annual weeds commonly flower throughout the year.However the slender speedwell flowers only betweenMarch and May.

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up to a metre in height. It is found in all areas, evenat altitudes of 750 m, and on saline soil.The speciesis dioecious, the male producing spherical and thefemale slightly elongated purple flower headsfrom July to September. Only when the two sexesof plants are within about 100 m of each other doesfertilization occurs in sufficient quantities to pro-duce large numbers of brown, shiny fruit, 4 mmlong. These are wind-borne by a parachute of longhairs. The seeds may germinate beneath the soilsurface in the same year as their production, or inthe following spring, particularly when soil tem-peratures reach 20°C. The resulting seedlingsdevelop a taproot which commonly reaches 3 mdown into the soil. Lateral roots, growing out hori-zontally about 0.3 m below the soil surface, maypenetrate 6 m in one season, and along their lengthare produced adventitious buds that, each spring,grow up as stems. Under permanent grassland, theroots may remain dormant for many years. Soildisturbance, such as ploughing, breaks up the rootsand may result in a worse thistle problem unlessdrought or frost coincides with this activity.

Control of the seedling stage of this weed is notnormally considered worthwhile.The main strategymust be to prevent the development of the peren-nial root system. Cutting down plants at the flowerbud stage when sugars are being transferred fromthe roots upwards is said to achieve this objective.Translocated chemicals, e.g. MCPA, are commonlyused on grassland, the spray chemical movingfrom the leaves to the perennial roots, particularlywhen applied in autumn at a time when sugars aresimilarly moving down the plant. In broad-leavedperennial crops, e.g. raspberries, soil-acting herbi-cides, e.g. bromacil, may be applied in late winterbefore crop bud burst, to suppress the emergenceof established plants. Deep ploughing is often successful in newly cultivated land.

Couch grass (Agropyron repens)

This grass, sometimes called ‘twitch’, is a widelydistributed and important weed found at altitudesup to 500 m.

The dull-green plant is often confused, in thevegetative stage, with the creeping bent (Agrostisstolonifera). However, the small ‘ears’ at the leafbase characterize couch. The plant may reach ametre in height and often grows in tufts. Floweringheads produced from May to October resembleperennial ryegrass, but the flat flower spikelets arepositioned at right angles to the main stem incouch. Seeds (9 mm long) are produced only aftercross-fertilization between different strains of thespecies, and the importance of the seed stage, there-fore, varies from field to field. Couch seeds may becarried in cereal seed stocks over long distances,and may survive deep in the soil for up to 10 years.From May to October, stimulated by high lightintensity, the overwintered plants produce hori-zontal rhizomes (see Figure 7.3) just under the soil;these white rhizomes may grow 16 cm per year inheavy soils, 32 cm in sandy soils. They bear scaleleaves on nodes that, under apical dominance,remain suppressed until the rhizome is cut byploughing. In the autumn, rhizomes attached to themother plant grow above ground to produce newplants that survive the winter.The rapid growth andextensive root system of this species provide severecompetition for light, water and nutrients in anyinfested crop.

Control is achieved by a combination of methods.In fallow soil deep ploughing, especially in heavyland, exposes the rhizomes to drying. Further con-trol by rotavating the weed when it reaches theone or two leaf stage disturbs the plant at its weak-est point, and repeated rotavating will eventuallycut up couch rhizomes to such an extent that nofurther nodes are available for its propagation.

Translocated herbicides, e.g. glyphosate sprayedonto couch in fallow soils during active vegetativegrowth, kill most of the underground rhizomes. Inestablished fruit and conifers, control is achievedin the dormant winter or early spring seasons by atranslocated chemical, e.g. dalapon, which, if appliedto a full weed cover, has a maximum effect withoutcausing root damage to the trees. Soil intended forvegetable production, e.g. carrots and potatoes,may be sprayed with a chemical such as dalaponto control couch so long as a 7 weeks period is

Weeds 93

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observed between application and planting. Controlby contact herbicides, e.g. paraquat, may be suc-cessful in exhausting couch rhizome reserves ifapplied repeatedly (Figure 9.5).

Yarrow (Achillea millifolium)

This strongly scented perennial, with its spreadingflowering head (see Figure 9.6), is a commonhedgerow plant found on most soils at altitudes upto 1200 m. Its persistence, together with its resist-ance to herbicides and drought in grassland, makesit a serious turf weed.

The seedling leaves are hairy, and elongated withsharp teeth (see Figure 9.1). The mature plant hasdissected pinnate leaves produced throughout the

year on wiry, woolly stems, which commonly reach45 cm in height, and which from May to Septemberproduce flat-topped white to pink flower heads.An annual production of about 3000 small, flatfruits per plant is dispersed by birds. The seedsgerminate on arrival at the soil surface. When notin flower, this species produces stolons along theground (up to 20 cm long per year), and in autumnrooting from the nodes occurs.

Control of this weed may prove difficult. Routinescarification of turf does not easily remove theroots. Repeated sprays of translocated chemicalmixtures, e.g. 2,4-D plus mecoprop, reach the rootsto give some control.


Figure 9.5 Couch grass: note the underground rhizome.Figure 9.6 Yarrow: note the divided leaves and branchinginflorescence.

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Broad-leaved dock (Rumex obtusifolius)

The seedling cotyledons are narrow, the true leaveslarge, broad and crimson coloured (see Figure 9.1).This is a common weed of arable land, grasslandand fallow soil. The mature plant is readily identi-fied by its long (up to 25 cm) shiny green leaves(see Figure 9.7), known to many as an antidote to‘nettle rash’. The plant may grow 1 m tall, pro-ducing a conspicuous branched inflorescence ofsmall green flowers from June to October. Thenumerous plate-like fruits (3 mm long) may fall tothe ground or be dispersed by seed-eating birdssuch as finches, by cattle, and in batches of seedstocks. The seed represents an important stage inthis perennial weed’s life cycle, surviving many

years in the soil, and most commonly germinatingin spring. Like most Rumex spp., the seedlingdevelops a stout, branched taproot, which maypenetrate the soil down to 1 m in the mature plant,but most commonly reaches 25 cm. Segments ofthe taproot, chopped by cultivation implements,are capable of producing new plants.

High levels of seed production, a tough taproot,and a resistance to most herbicides, thus present aproblem in the control of this weed. Attempts toexhaust the root system by repeated ploughingand rotavating have proved useful.Young seedlingsare easily controlled by translocated chemicals,e.g. 2,4-D, but the mature plant is resistant to all buta few translocated chemicals, e.g. asulam, whichmay be used on grassland, soft fruit, top fruit andamenity areas, during periods of active vegetativeweed growth when the chemical is moved mostrapidly towards the roots.

Mixed weed populations

In the field, a wide variety of both annual andperennial weeds may occur together. The growersmust recognize the most important weeds in theirholding or garden, so that a decision on the preciseuse of chemical control with the correct herbicideis achieved. Particular care is required to matchthe concentration of the herbicide or herbicidemixtures to the weed species present. Also, thegrower must be aware that continued use of onechemical may induce a change in weed species,some of which may be tolerant to that chemical.

MOSSES AND LIVERWORTS

These primitive plants may become weeds in wetgrowing conditions. The small cushion-formingmoss (Bryum spp.) grows on sand capillary benches,and thin, acid turf that has been closely mown.Feathery moss (Hypnum spp.) is common on lessclosely mown, unscarified turf. A third type (Poly-trichum spp.), erect and with a rosette of leaves, isfound in dry acid conditions around golf greens.

Weeds 95

Figure 9.7 Broad-leaved dock. Note the large shinyleaves.

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Liverworts (Pellia spp.) are recognized by theirflat (thallus) leaves growing on the surface of potplant compost.

These organisms increase only when the soil andcompost surface is excessively wet, or when nutri-ents are so low as to limit plant growth. Culturalmethods such as improved drainage, aeration, lim-ing, application of fertilizer and removal of shadeusually achieve good results in turf. Control withcontact scorching chemicals, e.g. alkaline ferroussulphate, may give temporary results. Moss onsand benches becomes less of a problem if thesand is regularly washed.

FURTHER READING

ADAS. Colour Atlas of Weed Seedlings (Wolfe Science,1987).

Hance, R.J. and Holly, K. Weed Control Handbook,Vol. 1 Principles (Blackwell Scientific Publications,1990).

Hill,A.H. The Biology of Weeds (Edward Arnold, 1977).Hope, F. Turf Culture: A Manual for Groundsmen

(Cassell, 1990).The UK Pesticide Guide (British Crop Protection

Council, 2003).Weed Guide (Hoechst Schering, 1997).


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MAMMAL AND BIRD PESTS

The rabbit (Oryctolagus cuniculus)

The rabbit is common in most countries of centraland southern Europe. It came to Britain aroundthe eleventh century with the Normans,and becamean established pest in the nineteenth century.

The rabbit may consume 0.5 kg of plant foodper day. Young turf and cereal crops are the worstaffected, particularly winter varieties that, in theseedling stage,may be almost completely destroyed.Rabbits may move from cereal crops to horticul-tural holdings.Stems of top fruit may be ring barkedby rabbits, particularly in early spring when otherfood is scarce. Vegetables and recently planted

garden-border plants are a common target for thepest, and fine turf on golf courses may be damaged,thus allowing lawn weeds, e.g. yarrow, to becomeestablished.

The rabbit’s high reproductive ability enables itto maintain high populations even when continuedcontrol methods are in operation.The doe, weighingabout 1 kg, can reproduce within a year of its birth,and may have three to five litters of three to sixyoung ones in 1 year, commonly in the months ofFebruary–July. The young are blind and naked atbirth, but emerge from the underground ‘maternal’nest after only a few weeks to find their own food.Large burrow systems (warrens), penetrating asdeep as 3 m in sandy soils, may contain as many asa 100 rabbits. Escape or bolt holes running off fromthe main burrow system allow the rabbit to escapefrom predators.

Control of rabbits is, by law, the responsibility ofthe land owner. Preventative measures are themost effective. Wire fencing, with the base 30 cmunderground and facing outwards, represents aneffective barrier to the pest, while thick plasticsheet guards are commonly coiled round the baseof exposed young trees. Repellant chemicals, e.g.aluminium ammonium sulphate, may be sprayedon bedding displays and young trees.

Shotguns, small spring traps placed in the rabbithole, winter ferreting or long nets placed at thecorner of a field to catch herded rabbits aremethods used as curative control. Gassing, however,is the most effective method. Crystals of powdered

10

Horticultural Pests

This chapter looks at the pests that reducegrowth in horticultural crops. In the case ofthe mammal and birds, which are relativelylarge and mobile, emphasis is placed on thecontrol directed against individuals. Withsmaller invertebrate pests, such as slugs,insects, mites and nematodes, the range of lifecycles is described to explain the kinds ofdamage caused and the particular strategy forcontrol against the target population. A moregeneral description of control measures isgiven in Chapter 12.

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cyanide are introduced (only by trained operators)into the holes of warrens by means of long-handledspoons or by power operated machines. On contactwith moisture hydrocyanic acid is released as a gasand, in well-blocked warrens, the rabbits are quicklykilled. Care is required in the storage and use ofpowdered cyanide, where an antidote, amyl nitrite,should be readily available. Myxamatosis, a flea-borne virus disease of the rabbit, causing a swollenhead and eyes, was introduced into Britain in1953, and within a few years greatly reduced therabbit population. The development of weakervirus strains, and the increase in rabbit resistance,has combined to reduce this disease’s effective-ness in control, although its importance in any onearea is constantly fluctuating.

The brown rat (Rattus norvegicus)

The brown rat, also called the common rat, is wellknown by its dark-brown colour, blunt nose, shortears and long, scaly tail. Its diet is varied; it will eatseeds, succulent stems, bulbs and tubers, and maygrind its teeth down to size by the unlikely act ofgnawing at plastic piping and electric cables. Arat’s average annual food intake may reach 50 kg,a large amount for an animal weighing only about300 g. This species has considerable reproductivepowers.The female may begin to breed at 8 weeksof age, producing an average of six litters of sixyoung ones per year. Its unpopular image is fur-ther increased by its habit of fouling the food iteats, and by the lethal human bacterium causingWeil’s disease, which it transmits through its urine.

Control is best achieved by a preliminary surveyof rat numbers in buildings and fields of the horti-cultural holding, and by the identification of the‘rat-runs’ along which the animals travel. Baitscontaining a mixture of anticoagulant poison andfood material such as oatmeal are placed near theruns, inside a container that, while attracting therat, prevents access by children and pets. Drainagetiles or oil drums drilled with a small rat-sized holeoften serve this purpose. The poison, e.g. difena-coum, takes about 3 days to kill the rat and, since

the other individuals do not associate their com-rade’s death with the chemical, the whole familymay be controlled. The bait should be placed wherever there are signs of rat activity,and repeatedapplications every 3 days for a period of 3 weeksshould be effective.

Strains of rat resistant to some anticoagulantsare commonly found in some areas, and a range ofchemicals may need to be tried before successfulcontrol is achieved. The poison, when not in use,should be safely stored away from children andpets. Dead rats should be burnt to avoid poisoningof other animals.

Sonic devices are sometimes used to disturb theanimal and provide a round-the-clock deterrent.

The grey squirrel (Sciurus carolinensis)

This attractive-looking, 45 cm long creature wasintroduced into Britain in the late nineteenth cen-tury, at a time when the red squirrel populationwas suffering from disease. The grey squirrelbecame dominant in most areas, with the redsquirrel in pockets such as the Isle of Wight.

The horticultural damage caused by grey squir-rels varies with each season. In spring, germinatingbulbs may be eaten, and the bark of many treespecies stripped off (see ring barking, page 44). Insummer, pears, plums and peas may suffer.Autumnprovides a large wild food source, although applesand potatoes may be damaged. In winter, littledamage is done. Fields next to wooded areas areclearly prone to squirrel damage.

Squirrels most commonly produce two litters ofthree young ones from March to June, in twig plat-forms high in the trees; the female may becomepregnant at an early age (6 months). As the squir-rels have few natural enemies, and this specieslives high above ground, control is difficult.

During the months of April–July,when most dam-age is seen, cage traps containing desirable food, e.g.maize seed, reduce the squirrel population to lessdamaging levels. Spring traps placed in naturalor artificial tunnels achieve rapid results at thistime of year if placed where the squirrel moves.


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Horticultural Pests 99

Poisoned bait containing a formulation of antico-agulant chemical, e.g. warfarin, when placed in awell-designed ground-level hopper (one hopperper 3 ha) may achieve successful squirrel controlwithout seriously lowering other small wild mam-mal numbers. In winter and early spring, thedestruction of squirrel nests (or dreys) by meansof long poles may achieve some success.

The mole (Talpa europea)

The mole is found in all parts of the British Islesexcept Ireland. This dark-grey, 15 cm long mam-mal, weighing about 90 g, uses its shovel-shapedfeet to create an underground system 5–20 cmdeep and up to 0.25 ha in extent. The tunnel con-tents are excavated into mole hills. The resultingroot disturbance to grassland and other cropscauses wilting, and may result in serious losses.

In its dark environment, the solitary mole moves,actively searching for earthworms, slugs, millipedesand insects.About 5 h of activity is followed by about3 h of rest. Only in spring do males and femalesmeet. In June, one litter of two to seven youngones are born in a grass-lined underground nest,often located underneath a dense thicket. Youngmoles often move above ground, reach maturity atabout 4 months, and live for about 4 years.

Natural predators of the mole include tawnyowls, weasels and foxes.

The main control methods are trapping andpoison baiting, usually carried out between Octoberand April, when tunnelling is closer to the surface.Pincer or half barrel traps are placed in fresh tun-nels and sprung without greatly changing the tun-nel diameter.The soil must be replaced so that themole sees no light from its position in the tunnel.The mole enters the trap, is caught and starves todeath. In serious mole infestations, strychnine saltsare mixed with earthworms at the rate of 2 g per100 worms, and single worms carefully insertedinto inhabited tunnels at the rate of 25 worms perhectare. Ministry of Agriculture authority isrequired before purchasing strychnine, a highlydangerous chemical, which must be stored with care.

Deer

Deer may become pests in land adjoining wood-land where they hide. Muntjac and roe deer ring-bark (see page 44) trees and eat succulent crops.High fences and regular shooting may be used intheir control.

The wood-pigeon (Columba palumbus)

This attractive, 40 cm long, blue-grey pigeon withwhite underwing bars is known to horticulturistsas a serious pest on most outdoor edible crops.In spring, seeds and seedlings of crops such as bras-sicas, beans and germinating turf may be systemat-ically eaten. In summer, cereals and clover receiveits attention; in autumn, tree fruits may be taken inlarge quantities, while in winter, cereals and bras-sicas are often seriously attacked, the latter whensnowfall prevents the consumption of other food.The wood-pigeon is invariably attracted to highprotein foods such as seeds when they are available.

Wood-pigeons lay several clutches of two eggsper year from March to September. The August/September clutches show highest survival.The eggs,laid on a nest of twigs situated deep inside thetree, hatch after about 18 days, and the young onesremain in the nest for 20–30 days. Predators suchas jays and magpies eat many eggs, but the mainpopulation control factor is the availability of foodin winter. Numbers in the British Isles are boosteda little by migrating Scandinavian pigeons in April,but the large majority of this species is residentand non-migratory.

The wood-pigeon spends much of its time feed-ing on wild plants, and only a small proportion ofits time on crops. Control of the whole population,therefore, seems both costly and impracticable.Protection of particular fields is achieved byscaring devices which include scarecrows, bangers(firecrackers or gas guns) or rotating orange andblack vanes, which disturb the pigeon. Changingthe type and location of the device every few dayshelps prevent the pigeon from becoming indiffer-ent. The use of the shotgun from hidden positions

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such as hides and ditches, particularly when artifi-cial pigeons (decoys) are placed in the field, is animportant additional method of scaring birds andthus protecting the crop.

The bullfinch (Pyrrhula pyrrhula)

This delightful, 14 cm long bird is characterized byits sturdy appearance and broad bill. The male hasa rose-red breast, blue-grey back and black head-cap. The female has a less striking pink breast andyellowish-brown back.

From April to September the bird progressivelyfeeds on seeds of wild plants, e.g. chickweed, but-tercup, dock, fat hen and blackberry. From Sep-tember to April, the species forms small flocksthat, in addition to feeding on buds and seeds ofwild species, e.g. docks, willow, oak and hawthorn,turn their attention to buds of soft and top fruit.Gooseberries are attacked from November toJanuary, apples from February to April and black-currants from March to April. The birds are shy,preferring to forage on the edges of orchards but,as winter advances, become bolder, moving towardsthe more central trees and bushes. The birds nipbuds out at the rate of about 30 per minute, eatingthe central meristem tissues. Leaf, flower and fruitdevelopment may thus be seriously reduced, andsince in some plums and gooseberries there is noregeneration of fruiting points, damage may beseen several years after attack.

The bullfinch produces a platform nest of twigsin birch or hazel trees, and between May andSeptember lays two to three clutches of four to fivepale blue eggs, with purple-brown streaks, and canthus quickly re-establish numbers reduced by lackof food or human attempts at control.

Fine mesh netting, cotton or synthetic threaddraped over trees, or bitter chemicals, e.g. ziramsprayed at the time of expected attack, are used tosome extent to prevent bullfinch attack. Most suc-cess is achieved by catching birds (usually imma-ture individuals) in specially designed traps, whichare closed when the bird lands on a perch to eatseeds. Trapping may be started as early as

September. Large-scale re-invasion by the birds inthe same season is unlikely, as they are territorial,rarely moving more than 2 miles throughout theirlives. Bullfinch control is permitted only in sched-uled areas of wide-scale fruit production, e.g. Kent.

INVERTEBRATE PESTS

Slugs

These animals belong to the phylum Mollusca, agroup including the octopus and whelk, and theslug’s close relatives, the snails, which cause a littledamage to plants in greenhouses and private gar-dens. Slugs move slowly by means of an undulat-ing foot, the slime trails from which may indicatethe slug’s presence. Unlike the snail, their lack ofshell permits movement through the soil in searchof their food source, seedlings, roots, tubers andbulbs.The slug feeds by means of a file-like tongue(radula), which cuts through plant tissue held bythe soft mouth, and scoops out cavities in theaffected plants (see Figure 10.1). In moist, warmweather it may cause above-ground damage toleaves of plants such as border plants, establishingturf, lettuce and brussels sprouts. Slugs are herm-aphrodite (bearing in their bodies both maleand female organs), mate in spring and summer,and lay clusters of up to 50 round, white eggsin rotting vegetation, the warmth from whichprotects this sensitive stage during cold periods.

Slugs range in size from the black keeled slug(Milax), 3 cm long, to the garden slug (Arion) whichreaches 10 cm in length. The mottled-carnivorousslug (Testacella) is occasionally found feedingon earthworms. Horticultural areas commonlysupport populations of 50 000 slugs per hectare.

Many non-chemical forms of control have beenused, ranging from baits of grapefruit skins andstale beer to soot sprinkled around larger plants.The most effective methods, however, involve thetwo chemicals metaldehyde, which dries the slugout, and methiocarb, which acts as a stomach poi-son. The chemicals are most commonly used assmall-coloured pellets (which include attractants


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such as bran and sugar), but metaldehyde mayalso be applied as a drench. Some growers estimatethe slug population using small heaps of pelletscovered with a tile or flat stone (to prevent birdpoisoning) before deciding on general control.Overuse of slug pellets in gardens has recentlybeen claimed as a major contribution to the declineof the mistle- and song-thrush numbers.

INSECTS

Belonging to the large group Arthropoda, whichinclude also the woodlice, mites, millipedes

(see Table 10.1) and symphilids, the insects are horti-culturally the most important arthropod group,both as pests, and also as beneficial soil animals.

Structure and biology

The body of the adult insect is made up of seg-ments, and is divided into three main parts: thehead, thorax and abdomen (see Figure 10.2).

The head bears three pairs of moving mouth-parts. The first, the mandibles in insects (such as incaterpillars and beetles) have a biting action (seeFigure 10.3). The second and third pairs, the max-illae and labia in these insects help in pushing foodinto the mouth. In the aphids, the mandibles andmaxillae are fused to form a delicate tubular stylet,which sucks up liquids from the plant phloem tis-sues. Insects remain aware of their environmentby means of compound eyes which are sensitive tomovement (of predators) and to colour (of flowers).Their antennae may have a touching- and smellingfunction.

The thorax bears three pairs of legs, and in mostinsects two pairs of wings.

The abdomen bears breathing holes (spiracles)along its length, which lead to a respiratory systemof tracheae. The blood is colourless, circulatesdigested food and has no respiratory function.Thedigestive system, in addition to its food absorbingrole, removes waste cell products from the bodyby means of fine, hair-like growths (malpighiantubules) located near the end of the gut.

Since the animal has an external skeleton madeof tough chitin, it must shed and replace its ‘skin’(cuticle, see Figure 10.2) periodically by a processcalled ecdysis, in order to increase in size. Aninsect may develop from egg to adult in one of twoways. In the first group, typified by the aphids,thrips and earwigs, the egg hatches to form a firststage, or instar, called a nymph, which resemblesthe adult in all but size, wing development andpossession of sexual organs. Successive nymphinstars more closely resemble the adult. Two toseven instars (growth stages) occur before the adultemerges (see Figure 10.4).


Figure 10.1 Slug damage on pot plant.

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In contrast, other groups of insects including themoths, butterflies, flies and beetles undergo a com-plete metamorphosis (complete changes of form);the egg hatches to form a first instar, called a larvawhich usually differs greatly in shape from the adult,e.g. the larva (caterpillar) of the cabbage white bearslittle resemblance to the adult butterfly. Some otherdamaging larval stages are shown in Figure 10.5 andthese can be compared with the often more famil-iar adult stage. The great change (metamorphosis)necessary to achieve this transformation occursinside the pupa stage (see Figure 10.4).

The method of overwintering differs betweeninsect groups.The aphids survive mainly as the eggs,while most moths, butterflies and flies survive asthe pupa (chrysalis).

The speed of increase of insects varies greatlybetween groups.Aphids may take as little as 20 daysto complete a life cycle in summer, often resulting invast numbers in the period June–September.On theother hand, the wireworm, the larva of the click bee-tle, usually takes 4 years to complete its life cycle.

Insect groups are classified into their appropri-ate order (Table 10.1) according to their general


abdomen thorax head

spiracle

waxylayerstructurallayerlivingcells

Cross-section of cuticle (�80)

maxillalabiumfrontleg

middle legspiracle

hindleg

hind wing

fore wing

compoundeye

mandible

labrum

antenna

Figure 10.2 External appearance of an insect. Note the mouthparts, spiracles and cuticle, the three main entry points forinsecticides.

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frontleg

labium

CATERPILLAR

APHID

leafphloem

rostrum

intestine

antenna

stylet

maxilla

mandible

antenna

ocelli(primitive eyes)

Figure 10.3 Mouthparts of the caterpillar and aphid. Note the different methods of obtaining nutrients. The aphidselectively sucks up dilute sugar solution from the phloem tissue.

larva or caterpillar (�1.5) true

legs

pupa or chrysalis (�2)

adult (female) (�1)

egg (�10)

falselegs

LARGE CABBAGE WHITE BUTTERFLY

egg (�15)

nymph (�20)

adult (�20)PEACH-POTATO APHID

Figure 10.4 Life cycle stages of a butterfly and an aphid pest. Note that all four stages of the butterfly’s life cycle are verydifferent in appearance. The nymph and adult of the aphid are similar.

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appearance and life cycle stages. There follows aselection of insect pests in which each species hasparticular features of its life cycle that warrantdescription. Whilst comments on control are men-tioned, the reader should refer to Chapter 12 fordetails of specific types of control (cultivations,chemicals, etc.) and for explanations of terms used.

Aphids and their relatives (order Hemiptera)

This important group of insects has the egg–nymph–adult life cycle, and sucking mouthparts.

Peach-potato aphid (Myzus persicae). This isoften referred to by the name ‘greenfly’, is commonin market gardens and greenhouses. It varies incolour from light green to orange, measures 3 mm

in length (Figure 10.6), and has a complex lifecycle, shown in Figure 10.7, alternating betweenthe winter host (peach) and the summer hosts, e.g.potato and bedding plants. In spring and summer,the females produce nymphs directly without anyegg stage (a process called vivipary), and withoutfertilization by a male (a process called partheno-genesis). Only in autumn, in response to decreas-ing daylight length and outdoor temperatures, areboth sexes produced, which, having wings, fly tothe winter host, the peach. After the female is fer-tilized, she lays thick-walled eggs. In glasshouses,the aphid may survive the winter as the nymphand adult female on plants such as begonias andchrysanthemums, or on weeds such as fat hen.

The nymph and adult of this aphid may causethree types of damage. Using its sucking stylet,


Chafer larva (�1.5)large with long legs

Sawfly larva (�4)more than four pairs of false legs

Wireworm larva (�3)long, shiny brown, small legs

Carrot fly larva (�10)white, no legs or mouthparts

Vine weevil larva (�8)dark head, white body, no legs

Moth or Butterflylarva (caterpillar)four pairs of false legs

Figure 10.5 Insect larva that damage crops. Identification into the groups above can be achieved by observing the fea-tures of colour, shape, legs and mouthparts.

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it may inject a digestive juice into the plant phloem,which in young organs may cause severe distor-tion. Having sucked up sugary phloem contents,the aphid excretes a sticky substance called honey-dew, which may block up leaf stomata and reducephotosynthesis, particularly when dark-coloured

fungi (sooty moulds) grow over the honeydew.Thirdly, the aphid stylets may transmit viruses suchas virus Y on potatoes and tomato aspermy viruson chrysanthemums.

The peach-potato aphid is controlled in threemain ways. In outdoor crops, several organisms,


Table 10.1 Arthropod groups found in horticulture

Group Key features of group Habitat Damage

Woodlice Grey, seven pairs of legs, up to Damp organic Eat roots and lower (Crustacea) 12 mm in length soils leaves

Millipedes Brown, many pairs of legs, Most soils Occasionally eat under-(Diplopoda) slow moving ground tubers and seed

Centipedes Brown, many pairs of legs, very active, Most soils Beneficial(Chilopoda) with strong jaws

Symphilids White, 12 pairs of legs, up to Glasshouse soils Eat fine roots(Symphyla) 8 mm in length

Mites Variable colour, usually have four pairs Soils and plant Mottle or distort leaves,(Acarina) of legs (e.g. red spider mites) tissues buds, flowers and bulbs;

soil species are beneficial

Insects Usually six pairs of legs, two pairs(Insecta) of wings

Springtails White to brown, 3–10 mm in length Soils and decaying Eat fine roots; some(Collembola) humus beneficial

Aphid group Variable colour, sucking mouthparts, All habitats Discolour leaves and (Hemiptera) produce honeydew (e.g. greenfly) stems; prevent flower

pollination; transmit viruses

Moths and Large wings; larva with three pairs of Mainly leaves and Defoliate leaves (stems butterflies legs, and four pairs of false legs and flowers and roots)(Lepidoptera) biting mouthparts (e.g. cabbage-white

butterfly)

Flies One pair of wings, larvae legless All habitats Leaf mining, eat roots(Diptera) (e.g. leatherjacket)

Beetles Horny front pair of wings which meet Mainly in the soil Eat roots and succulent (Coleoptera) down centre; well-developed tubers (and fruit)

mouthparts in adult and larva (e.g. wireworm)

Sawflies Adult like a queen ant; larvae have three Mainly leaves and Defoliation(Hymenoptera) parts of legs, and more than four pairs of flowers

false legs (e.g. rose-leaf curling sawfly)

Thrips Yellow and brown, very small, wriggle Leaves and flowers Cause spotting of leaves (Thysanoptera) their bodies (e.g. onion thrips) and petals

Earwigs (Dermaptera) Brown, with pincers at rear of body Flowers and soil Eat flowers

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e.g. ladybirds, lacewings, hoverflies and parasiticfungi (see biological control, Chapter 12), nat-urally found in the environment, may reduce thepest’s importance in favourable seasons. In thegreenhouse, a parasitic wasp, Aphidius matricariae,commonly controls the aphid. Contact chemicals,e.g. malathion, are able to penetrate the thin cuticleof the insect, while other chemicals, e.g. dimethoate,moving through the internal tissues of the plant(called systemic chemicals) are sucked up by theaphid stylet and reach the insect’s digestive system.Fumigant chemicals, e.g. nicotine, enter the insectthrough the spiracles.

There are many other horticulturally importantaphid species.The black bean aphid (Aphis fabae),which overwinters on Euonymus bushes, may ser-iously damage broad beans, runner beans and redbeet. The rose aphid (Macrosiphum rosae) attacksyoung shoots of rose.


Figure 10.6 Peach-potato aphid: note the sucking stylet(actual size of this aphid is about 3 mm long).

autumn and winter PEACH host

September–March March–May

June–September

September

September

June–September

eggs

femalesfertilized

winged parthenogenicfemale

wingedsexualfemale

nymph

summer hoste.g. POTATO,LETTUCE

spring PEACHhost

nymph

nymph

winged parthenogenicfemale

June

wingless parthenogenicfemale

Figure 10.7 Peach-potato aphid life cycle throughout the year. Female aphids produce nymphs on both the peach andsummer host. Winged females develop from June to September. Males are produced only in autumn. Eggs survive thewinter. In greenhouses the life cycle may continue throughout the year.

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Spruce-larch adelgid (Adelges viridis). Thisrelative of the aphid may cause serious damage onspruce grown for Christmas trees.

The green adult develops from overwinteringnymphs on spruce, and in May (year 1) lays about50 eggs on the dwarf shoots.The emerging nymphs,injecting poisons into the shoots, cause abnormalgrowth into green pineapple galls, which spoil the tree’s appearance. Although nursery trees ofless than 4 years of age are rarely badly damaged,early infestation in the young plant may result in serious damage as it gets older. In June–September the adults move to larch, acquire woollywhite hairs and may cause defoliation of the leaves.After a further year (year 2) on this host, theadelges returns to the spruce, where it lives foranother year (year 3) before the gall-inducing stagesare produced.

In Christmas trees, the adelges may be con-trolled by sprays of deltamethrin in May, when thegall-inducing nymphs are developing.

Glasshouse whitefly (Trialeurodes vaporario-rum). This small, moth-like pest was originallyintroduced from the tropics, and now causes ser-ious problems on a range of glasshouse food andflower crops. It should not be confused with theslightly larger cabbage whitefly, a less importantpest on brassicas.

The adult glasshouse whitefly (Figure 10.8) isabout 1 mm long. The fertilized female lays about200 minute, white, elongated oval eggs in a circularpattern on the lower leaf surface. After turningblack, the eggs hatch to produce nymphs (crawlers),which soon become flat immobile scales, the lastinstar being a thick-walled ‘pupa’ from which theadult emerges, and 3 days later lays eggs again.All stages after the egg have sucking stylets,which may cause large amounts of honeydew andsooty moulds on the leaf surface. The whole lifecycle takes 32 days in spring, and 23 days in thesummer.

Plants that are seriously attacked include fuch-sias, cucumbers, chrysanthemums and pelargo-niums. Chickweed, a common greenhouse weed,may harbour the pest over winter in all stages ofthe life cycle.

Control of glasshouse whitefly is achieved intwo main ways.A minute wasp (Encarsia formosa)lays an egg inside the scale of the whitefly which,being eaten away internally, turns black to releasea wasp. Chemical control methods include soil-applied systemic granular compounds, e.g. oxamyl,where the chemical enters the whitefly stylet viathe plant phloem. Fumigant chemicals, e.g.deltamethrin, quickly kill the scale and adultstages, but the eggs and ‘pupae’ are little affected.It is therefore suggested that serious infestationsof this pest receive a regular chemical spray or fogat 3 days intervals for a month to control emergentcrawlers and adults. An especially pure fatty aciddetergent (surfactant) is also used to controlwhitefly, and its relatively low toxicity to humansand most other organisms justify its use by organicgardeners and growers.

Greenhouse mealy bug (Planococcus citri).This pest, a distant relative of the aphid, spoils theappearance of some glasshouse crops, particularlyorchids, Coleus species, cacti and Solanum species.Being a tropical species, it develops most quicklyin high temperatures and humidities, and at 30°C


Figure 10.8 Adult whitefly – actual size is about 1 mmlong (courtesy: Shell Chemicals).

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completes a life cycle within about 22 days. Theadult measures about 3 mm in length and producesfine waxy threads. All of the stages except the eggsuck phloem juices by means of a tubular mouth-part (stylet), and when this pest is present in densemasses it produces honeydew and causes leaf dropon most plant species.

Mealy bugs are difficult pests to control, as thethick cuticle resists chemical sprays,and the dropletsfall off the waxy threads. A contact chemical, e.g.malathion, may be sprayed if the plant species isnot damaged by the chemical. Systemic chemicals,e.g. aldicarb, may be applied to the compost andreach the pest through the plant vascular tissuesand stylet of the pest.An introduced tropical lady-bird, Cryptolaemus montrouzieri, is effective incontrolling the pest above 20°C.

Brown scale (Parthenolecanium corni). Thefemale scale, measuring up to 6 mm, is tortoiseshaped (see Figure 10.9), and has a very thick cut-icle. On fruit trees, e.g. apple, it is rarely seen nowbecause of regular insecticide sprays, but in pri-vate gardens it may be a serious pest on vines, cur-rants and cotoneasters, and in greenhouses attackspeaches and Amaryllis, causing stunted growthand leaf defoliation. As with mealy-bug, control isdifficult because of the thick cuticle.

Leaf hoppers (Graphocephala fennahi). Theseslender, light green insects, about 3 mm long, wellknown in their nymph stage as ‘cuckoo spit’, arefound on a wide variety of crops, e.g. potato, rose,Primula and Calceolaria. They live on the under-surface of leaves, causing a mottling of the uppersurface. In strawberries, they are vectors of thegreen-petal disease, while in rhododendron theycarry the serious bud blast disease that kills off theflower buds.

August and September sprays of nicotine, preventegg laying inside buds of rhododendron, and thusreduce the entry points for the fungus disease.

Common green capsid (Lygocoris pabulinus).This very active, light green pest, measuring 5 mmin length, and resembling a large aphid, occurs onfruit trees and flower crops, most commonly out-doors. Owing to the poisonous nature of its sali-vary juices, young foliage shows distorted growth

with small holes, even when relatively low insectnumbers are present and fruit is scarred.

The chemicals used against aphids may controlthis pest.

Thrips (order Thysanoptera)

Onion thrips (Thrips tabaci). This 1 mm long,narrow-bodied insect has feather-like wings. Dueto its great activity during warm, humid weather, itis sometimes called ‘thunder fly’. Its mouthpartsare modified for piercing and sucking, and thetoxic salivary juices cause silvering in onion leaves,straw-brown spots on cucumber leaves and white


Figure 10.9 Brown scale on a pot plant – actual size isabout 5 mm long.

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streaks in carnation blooms. The last instar of thelife cycle, called the pupa, occurs in the soil, and itis this stage which overwinters. In greenhousesthere may be seven generations per year, while atoutdoors one life cycle is common.The occurrencein Britain of Western flower thrip (Frankliniellaoccidentalis) on both greenhouse and outdoorflower and vegetable crops has created seriousproblems for the industry, particularly because itcarries the serious tomato spotted wilt virus.

Thrips are sensitive to a wide range of insecti-cides, e.g. abamectin applied as a spray or fog. Ingreenhouse crops employing biological control, adrench of a residual insecticide, e.g. permethrinincorporated into a sticky solution, reduces insec-ticide contact with the predator or parasite.

Western flower thrip, however, has showngreater resistance and a careful rotation of chemi-cal groups are usually necessary.

Earwigs (Forficula auricularia)

These pests belong to the order Dermaptera, andbear characteristic pincers (cerci) at the rear of the15 mm long body (see Figure 10.10). They gnawaway at leaves and petals of crops such as beans,beet, chrysanthemums and dahlias, usually fromJuly to September, when the nymphs emerge fromthe parental underground nest. Upturned flowerpots containing straw are sometimes used inglasshouses for trapping these shy nocturnalinsects. Chemicals, e.g. pirimiphos methyl, areapplied in the form of sprays or smokes, but flowerscorch must be avoided.

Moths and butterflies

This order (Lepidoptera) characteristically containsadults with four large wings and curled feedingtubes. The larva (caterpillar), with six small legsand eight false legs, is modified for a leaf-eatinghabit (see Figures 10.3–10.5). Some species, how-ever, are specialized for feeding inside fruit (codlingmoth on apple), underground (cutworms), inside

leaves (oak leaf miner) or inside stems (leopardmoth). The gardener may find large caterpillarcolonies of the lackey moth (Malacosoma neustria)on fruit trees and hawthorns. The larva is the onlydamaging stage of this insect group.

Large cabbage-white butterfly (Pieris brassi-cae). This well-known pest on cruciferous plantsemerges from the overwintering pupa (chrysalis)in April and May and, after mating, the females laybatches of 20 to a 100 yellow eggs on the under-side of leaves. Within a fortnight, groups of firstinstar larvae emerge and soon moult to producethe later instars, which are 25 mm long, yellow orgreen in colour, with clear black markings, and havewell-developed mandibles. Pupation occurs usu-ally in June, in a crevice or woody stem, the pupa(chrysalis) being held to its host by silk threads. Asecond generation of the adult commences in July,causing more damage than the first. The secondpupa stage overwinters.The defoliating damage of


Figure 10.10 Larger earwig is male, smaller is female(courtesy: Shell Chemicals).

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the larva may result in skeletonized leaves of cab-bage, cauliflower, brussels sprouts and other hostssuch as wallflowers and the shepherd’s purse weed.

Care should be taken not to confuse thecabbage-white larva with the large smooth greenor brown larva of the cabbage moth, or the smallerlight green larva of the diamond-backed moth, bothof which may enter the hearts of cabbages and cauli-flowers, presenting greater problems for control.

There are several forms of control against thecabbage-white butterfly. A small wasp (Apantalesglomeratus) lays its eggs inside the pest larva (seeparasites). A virus disease may infect the pest,causing the larva to go grey and die. Birds such asstarlings eat the plump larvae. When damagebecomes severe, the larvae may be controlled withsprays of insecticides, e.g. diflubenzuron, contain-ing extra wetter/spreader, to maintain the spraydroplets on the waxy brassica leaves.

Winter moths (Operophthera brumata). Theseare pests, which may be serious on top fruit andornamental members of the Rosaceae family.Theyemerge as the adult form from their soil-bornepupa in November and December. The male is agreyish-brown moth, 2.5 cm across its wings, while

the female is wingless. The female crawls up thetree to lay 100–200 light green eggs in the budclusters, which hatch at bud burst to producegreen larvae with faint white stripes. These larvae,which move in a looping fashion, eat the leaf andform other leaves into loose webs, reducing theplant’s photosynthesis.They occasionally scar youngapple fruit before descending on a silk thread atthe end of May to pupate in the soil until winter.

Grease bands wound around the main trunk ofthe tree in October prove very effective in pre-venting the female moth’s progress. In largeorchards, springtime sprays of an insecticide, e.g.deltamethrin, kill the young instars of the larva.

Cutworms (e.g. Agrotis segetum). The larvaeof the turnip moth, unlike most other moth larvae,live in the soil, nipping off the stems of youngplants and eating holes in succulent crops, e.g. bed-ding plants, lawns, potatoes, celery, turnips andconifer seedlings. The damage resembles thatcaused by slugs. The adult moth, 2 cm across, withbrown forewings and white hindwings, emergesfrom the shiny soil-borne chestnut brown pupafrom June to July, and lays about 1000 eggs on thestems of a wide variety of weeds.

The first instar larvae, having fed on the weeds,descend to the soil and eventually reach 3.5 cm inlength They are grey to grey-brown in colour, withblack spots along the sides (see Figure 10.11).Several other cutworm species, e.g. heart and dartmoth (Agrotis exclamationis) and yellow under-wing moth (Noctua pronuba), may cause damagesimilar to that of the turnip moth. This typicallycaterpillar-like larva should not be confused withthe legless leatherjacket (see Figure 10.14). Thereare normally two life cycles per year, but in hotsummers this may increase to three.

Good weed control reduces cutworm damage.Soil drenches of residual insecticides, e.g. chlor-pyrifos, have proved successful against the larvastage of this pest.

Leopard moth (Zeuzera pyrina). It has anunusual life cycle. The adult female moth, 5–6 cmacross, is white with black spots, and in early sum-mer lays dark-yellow eggs on the bark of apples,ash, birch, lilac and many other tree species. The


Figure 10.11 Cutworm larva of the turnip moth – actualsize is about 30 mm long (courtesy of Ministry ofAgriculture, Fisheries and Food, MAFF).

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emerging larva enters the stem by a bud, and thentunnels for 2–3 years in the heartwood beforereaching 5 cm in length (see Figure 10.12), pupatingin the tunnel, and completing the life cycle as theadult. The tunnelling may weaken the branches of trees which, in high winds, commonly break.Where tunnels are observed, a piece of wire may bepushed along the tunnel to kill the larva, or a fumi-gant chemical, e.g. paradichlor-benzene crystals,may be placed in the tunnel before sealing it offwith moist soil.

Flies

This order (Diptera) is characterized by the singlepair of clear forewings, the hindwings being adaptedas balancing organs (halteres). The larvae are leg-less, elongated, and their mouthparts, where present,are simple hooks. The larvae cause crop damage.

Carrot fly (Psila rosae). This is a widespreadand serious pest on umbelliferous crops (carrots,celery and parsnips).The adult fly, shiny black witha red head, and 8 mm long, emerges from the soil-overwintering pupa from late May to early June.The small eggs laid on the soil near the host soonhatch to give white larvae (see Figure 10.5) thateat fine roots and then enter the mature root usingfine hooks in their mouths. When a month old, themature larva leaves the host to turn into the cylin-drical pale yellow pupa. A second generation of

adults emerges in late July, while in October athird emergence is seen in some areas.

Damage is similar in all crops. In carrots, seedlingsmay be killed, while in older plants the foliagemay become red, and wilt in dry weather. Stuntingis often seen, and affected roots, when lifted, areriddled with small tunnels that make the carrotsunsaleable. Damage should not be confused withcavity spot, a condition associated with Pythiumspecies of fungi, which produces elongated sunkenspots around the root.

Damaging levels of carrot fly can be avoided bykeeping hedges and nettle beds trimmed to reducesheltering sites for the flies; by planting carrots afterthe May emergence has occurred. Control has beenachieved by a seed treatment containing tefluthrin.

Chrysanthemum leaf miner (Phytomyza synge-nesiae). The leaf miners are a group of smallflies, the larvae of which can do serious damage tohorticultural crops. This species is found on mem-bers of the plant family Asteraceae, and the hostsattacked include chrysanthemum, cineraria andlettuce. Weed hosts, e.g. groundsel and sowthistle,may harbour the pest.

The flies emerge at any time of the year ingreenhouses, but normally only between July andOctober outdoors. These adults, which measureabout 2 mm in length and are grey-black with yellowunderparts, fly around with short hopping move-ments.The female lays about 75 minute eggs singlyinside the leaves, causing white spot symptoms toappear on the upper leaf surface.The larva stage isgreenish white in colour, and tunnels into the pal-lisade mesophyll of the leaf, leaving behind thecharacteristic mines seen in Figure 10.13. On reach-ing its final instar, the 3.5 mm long larva developswithin the mine into a brown pupa, from which theadult emerges. The total life cycle period takesabout 3 weeks during the summer months.

Certain chrysanthemum cultivars, e.g. Tuneful,may be badly attacked; others, e.g.Yellow Iceberg,often resist the attack by the pest. While chemicalsprays used against other pests, e.g. aphids andwhitefly, may help in control of the adult, thelarva stage is commonly controlled by systemicinsecticide, e.g. aldicarb, which, after application as


Figure 10.12 Leopard moth larva emerging from anapple stem – actual size is about 50 mm long.

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granules to the soil, then moves up to the leaf.A spray containing abamectin is also effective.

The occurrence of South American leaf miner(Liriomyza huidobrensis) and American serpen-tine leaf miner (Liriomyza trifolii) on a wide var-iety of greenhouse plants has created problems forthe industry.

Leatherjacket (Tipula paludosa). This is anunderground pest, which is a natural inhabitant of grassland and causes problems on fine turf.The ploughing up of grassland may, however,result in the pest damaging following crops such aspotatoes, cabbages, lettuce and strawberries.

The adult of this species is the crane fly, or‘daddy-long-legs’, commonly seen in late August.The females lay up to 300 small eggs on the surfaceof the soil at this period, and the emerging larvaefeeding on plant roots during the autumn, winterand spring months reach lengths of 4 cm by June.They are cylindrical, grey-brown in colour, leglessand possess hooks in their mouths for feeding (seeFigure 10.14). During the summer months, theysurvive as a thick-walled pupa.

This pest is particularly damaging in prolongedwet periods when the roots of young or succulentcrops may be killed off. Occasionally lower leavesmay be eaten.

Groundsmen sometimes control this pest byplacing tarpaulins over water-soaked ground. Thefollowing morning the emerging leatherjacketsmay be brushed up. Residual chemicals, e.g. chlor-pyrifos, may be drenched into soil to reduce thelarval numbers. Crops sown in autumn are rarelyaffected, as the larva is very small at this time.

Sciarid fly (Bradysia sp.). The larvae of this pest(sometimes called fungus gnat) feed on fine rootsof greenhouse pot plants, e.g. cyclamen, orchid andfreesia, causing the plants to wilt. Fungus strandsof mushrooms may be attacked in the compost.The slender black females, which are about 3 mmlong, fly to a suitable site (freshly steamed compost,moss on sand benches and well-fertilized compostcontaining growing plants), where 100 minute eggsare laid.The emerging legless larvae are white witha black head, and during the next month grow to alength of 3 mm before briefly pupating and start-ing the next life cycle.

The larva may be controlled by diflubenzuronincorporated into the compost. Biological controlby the tiny nematode Steinernema feltiae and miteHypoaspis miles is now available. The adults areexcluded from mushroom houses by means of finemesh screens placed next to ventilator fans.

Beetles

This order of insects (Coleoptera) is characterizedin the adult by hard, horny forewings which, whenfolded, cover the delicate hindwings used forflight. The meeting point of these hard wing cases


Figure 10.13 Chrysanthemum leaf miner damage: notethe light-coloured tunnels caused by the larva of thisspecies (courtesy: Glasshouse Crops Research Institute).

Figure 10.14 Leatherjacket larva – actual size is about40 mm (courtesy: Plant Pathology Laboratory, Ministryof Agriculture, Fisheries and Food, MAFF).

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produces the characteristic straight line down thebeetles back over its abdomen. Most beetles arebeneficial, helping in breakdown of humus, e.g.dung beetles, or feeding on pest species (see groundbeetle).A few, e.g. wireworm, raspberry beetle, andvine weevil, may cause crop damage.

Wireworm (Agriotes lineatus). This beetlespecies is commonly found in grassland, but willattack most crops. The 1 cm long adult (click bee-tle) is brown-black and has the unusual habit offlicking itself in the air when placed on its back.The female lays eggs in weedy ground in May andJune and these, after hatching, develop over a4 years period into slender 2.5 cm long, wirewormlarvae (Figure 10.5), shiny golden-brown in colour,and possessing short legs. After a 3 weeks pupa-tion period in the soil, usually in summer, the adultemerges, and in this stage survives the winter.

Turf grass may be eaten away by wireworms toshow dry areas of grass.The pest also bores throughpotatoes to produce a characteristic tunnel, whilein onions, brassicas and strawberries the roots areeaten away. In tomatoes, the larvae bore into thehollow stem.

Serious damage to young crops may be pre-vented by seed dressings containing tefluthrin.

Raspberry beetle (Byturus tomentosus). Thedeveloping fruit of raspberry, loganberry and black-berry may be eaten away by the 8 mm long, golden-brown larvae of this pest. Only one life cycle peryear occurs, the larva descending to the soil in Julyand August, pupating in a cell in which the golden-brown adult emerges and spends the winter beforelaying eggs in the host flower the next June.

Since the destructive larval stage may enter thehost fruit and thus escape insecticidal control, thetiming of the spray is vital. In raspberries, a contactchemical, e.g. malathion, applied when the fruit ispink, will achieve good control.

Vine weevils (Otiorhyncus sulcatus) belong tothe beetle group, but possess a longer snout ontheir heads than other beetles.This species is 9 mmlong, black in colour, with a rough textured cuticle.The forewings are fused together, making the pestincapable of flight. No males are known. Thefemales lay eggs in August and September in the

soil or compost. The emerging larvae are white,legless, and with a characteristic chestnut-brownhead (Figure 10.5). They reach 1 cm in length inDecember when they pupate in the soil beforedeveloping into the adult (Figure 10.15).

The larva stage is the most damaging, eatingaway roots of crops such as cyclamen and bego-nias in greenhouses, primulas, strawberries, youngconifers and vines outdoors. The adults may eatout neat holes in the foliage of hosts, e.g. rhodo-dendron and raspberry. Several related species, e.g.the clay-coloured weevil (Otiorhyncus singularis)cause similar damage to that of the vine weevil.

Traps of corrugated paper placed near infestedcrops have achieved successful control in outdoorcrops, while a residual chemical, e.g. chlorpyrifos,may be drenched into the soil or incorporated inpotting compost for control in greenhouse crops.Commercial growers now quite commonly usebiological control by the nematode Steinemenacarpocapsae incorporated into compost.

Flea beetles on leaves of cruciferous plants (e.g.stocks and cabbages) and chafer larvae on roots ofturf in sandy areas are two other important beetlepests.

Sawflies

This group that, together with bees, wasps and antsclassified in the order Hymenoptera, is characterized


Figure 10.15 Vine weevil adult – actual size is 9 mm long.Note the characteristic straight line between the wingcases down the back of this order of insects (Colepterathe beetles).

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by adults with two pairs of translucent wings andwith the fore- and hindwings being lockedtogether by fine hooks. The slender first segmentsof the abdomen give these insects a characteristicappearance.

Bees are described alongside details of theprocess of pollination in Chapter 7.

Rose leaf-rolling sawfly (Blennocampa pusilla).The black shiny adults, resembling winged queenants, emerge from the soil-borne pupa in May andearly June. Eggs are inserted into the leaf lamina,which, in responding to the pest, rolls up tightly(see Figure 10.16).

The emerging larva (see Figure 10.5), which ispale green with a white or brown head, feeds onthe rolled foliage and reaches a length of 1 cm byAugust, when it descends to the ground and formsan underground cocoon to survive the winter andpupate in March. All types of roses are affected,although climbing roses are preferred. Damagecaused by leaf-rolling tortrix caterpillars, e.g.Cacoecia oparana, may be confused with the sawfly,although the leaves are less curled.

Control is achieved, where necessary, by anapplication of nicotine dust which, on hot days inMay and June, has a good fumigant action.

Springtails (order Collembola)

This group of primitive wingless insects, about2 mm in length (Figure 10.17), has a spring-likeappendage at the base of the abdomen. They arevery common in soils, and normally aid in thebreakdown of soil organic matter. Two genera,Bourletiella and Collembola, however, may doserious damage to conifer seedlings and cucumberroots respectively.

MITES

The mites (Acarina) are classified with spiders andscorpions in the Arachnida. Although similar toinsects in many respects they are distinguishedfrom them by the possession of four pairs of legs,a fused body structure and by the absence of wings(Figure 10.18). Many of the tiny soil-inhabitingmites serve a useful purpose in breaking downplant debris. Several above-ground species areserious pests on plants. The life cycle is composedof egg, larva, nymph and adult stages.

Glasshouse red spider mite (Tetranychus urticaeand T. cinnabarinus). These pests are of tropicalorigin, and thrive best in high greenhouse tempera-tures. The first species is 1 mm long, yellowish incolour, with two black spots (see Figure 10.18).Thefemale lays about 100 tiny spherical eggs on theunderside of the leaf, and after a period of 3 daysthe tiny six-legged larva moults to produce thenymph stage that resembles the adult.The life cyclelength varies markedly from 62 days at 10°C, to6 days at 35°C.The pest’s multiplication potential is


Figure 10.16 Rose leaf-rolling sawfly damage (courtesy:Plant Pathology Laboratory, Ministry of Agriculture,Fisheries and Food, MAFF).

spring

Figure 10.17 The springtail – about 2 mm in size – canjump by means of spring at the end of its body.

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extremely high. In autumn, when the daylightperiod decreases to 14 h and temperatures fall, eggproduction ceases and the fertilized females, whichare now red in colour, move into the greenhousestructures to hibernate (diapause), representingfoci for next spring’s infestation.The second species,which is dark reddish-brown, has a similar lifecycle to the first, but does not hibernate. The firstspecies is common on annual crops such as toma-toes, cucumbers and chrysanthemums, while thesecond species is more common on the perennialcrops such as carnations, arums and hothouse pot plants. The two species often occur together onsummer hosts.

As the piercing mouthparts inject poisonoussecretions, the mites cause localized death of leafmesophyll cells. This results in a fine mottling onthe leaf, not to be confused with the larger spotscaused by thrips. In large numbers the mites cankill off leaves and eventually whole plants. Finesilk strands are produced in severe infestations,

appearing as ‘ropes’ on which the mites movedown the plant. On chrysanthemums, these ropesmake the plant unsaleable.

Control may be achieved in three ways. Apredatory mite, Phytoseiulus persimilis, is com-monly introduced into cucumber, chrysanthemumand tomato crops in spring. Winter fumigation ofgreenhouse structures with chemicals such as formalin or burning sulphur kills off some of thehibernating females. A chemical control usesabamectin. Care must be taken not to cause chem-ical scorch in plants, not to use chemicals to whichthe pest has become resistant, and not to kill pred-ators by unthinking use of these chemicals (seeintegrated control).

Gall mite of blackcurrant (Cecidophyopsis ribis).Unlike red spider mite, this species, sometimescalled big-bud mite, is elongated in shape and isminute (0.25 mm) in size. It spends most of theyear living inside the buds of blackcurrants and, toa lesser extent, other Ribes species. Breeding takesplace inside the buds from June to September, andJanuary to April. In May the mites emerge and dis-perse on silk threads and on the bodies of aphids.The damaged bud meristem produces many scaleleaves, which gives the bud its unusual appearance(see Figure10.19).These buds often fail to open, orproduce distorted leaves.

The mite carries the virus-like agent responsiblefor the damaging reversion disease, which stuntsthe plant and reduces fruit production.

The mite is controlled in three ways. Cleanplanting material is essential for the establishmentof a healthy crop. Pruning out of stems with bigbud and destruction of reversion-infected plantsslow down the progress of the pest. Since no chem-icals can control the mite in the bud, sprays of sul-phur are applied in the May–June period when themites are migrating.

Tarsonemid mite (Tarsonemus pallidus). Thisspherical mite, only 0.25 mm in length, lives in theunexpanded buds of a wide variety of pot plants,e.g. Amaranthus, Fuchsia, pelargonium and cycla-men, and is often called the ‘cyclamen mite’. Aclosely related but distinct strain is found onstrawberries. In greenhouses, the adults may lay


piercingandsuckingmouthparts

blackspots

fusedbodysegment

fourpairs oflegs

GLASSHOUSE RED SPIDER MITE (�100)

Figure 10.18 Glasshouse red spider mite. Note that thered spider mite may be light green or red in colour. Itsextremely small size (0.8 mm) enables it to escape agrower’s attention.

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eggs all the year round, and the 2 weeks life cycleperiod can cause a rapid increase in its numbers.

The small feeding holes and injected poisons fromthe mite mouthparts combine to distort the develop-ing leaf and flower buds of the affected crop tosuch an extent that leaves and petals are stuntedand misshapen, and flowers may not open properly.

Care should be taken to prevent introduction ofinfested plants and propagative material into green-houses. The contact acaricide, e.g. abamectin, iseffective against the mite. Addition of a wetter/spreader may help the spray penetrate the tight-knit scale leaves of the buds.

Four other horticulturally important mitesrequire a mention. The fruit tree red spider mite(Panonychus ulmi) causes serious leaf mottling ofthe ornamental Malus and apple. Conifer spinningmite (Oligonychus ununguis) causes spruce to yel-low, and spins a web of silk threads. Bulb-scalemite (Steneotarsonemus laticeps) causes internal

discoloration of forced narcissus bulbs. Bryobiamite (e.g. Bryobia rubrioculus) attacks fruit trees,and may cause damage to greenhouse crops, e.g.cucumbers, if blown in from neighbouring trees.

OTHER ARTHROPODS

In addition to insects and mites, the phylum Arthro-poda contains three horticulturally relevant classes,the Crustacea (woodlice), Symphyla (symphilids)and Diplopoda (millipedes).

Woodlouse (Armadillidium nasutum). A rela-tive of the marine crabs and lobsters, has adaptedfor terrestrial life, but still requires damp conditionsto survive. In damp soils it may number over a mil-lion per hectare, and greatly helps the breakdownof plant debris, as do earthworms. In greenhouses,where plants are grown in hot, humid conditions,this species may multiply rapidly, producing twobatches of 50 eggs per year. The adults roll into aball when disturbed.

The damage is confined mainly to stems andlower leaves of cucumbers, but occasionally youngtransplants may be nipped. Partial soil sterilizationby steam effectively controls woodlice. Rottingbrickwork provides refuge for them and should bereplaced.

Symphilids (Scutigerella immaculata). Thesedelicate white creatures, with 12 pairs of legs,resemble small millipedes. The adult female, 6 mmlong, lays eggs in the soil all the year round, andthe development through larvae to the adult takesabout 3 months. Symphilids may migrate 2 mdown into soil during hot, dry weather.

In greenhouse crops the root hairs are removed,and may cause lettuce to mature without a heart.Infectious fungi, e.g. Botrytis, may enter the rootsafter symphilid damage. The recognition of thispest is made easier by dipping a suspect root andsurrounding soil into a bucket of water and search-ing for organisms floating on the water surface.

Millipedes. These elongated, slow-movingcreatures are characterized by a thick cuticle andthe possession of many legs, two pairs to eachbody segment (Figure 10.20). Many species are


Figure 10.19 Blackcurrant gall mite damage: note the‘big-bud’ symptoms on the left compared with the normalbuds on the right (courtesy: Plant Pathology Laboratory,Ministry of Agriculture, Fisheries and Food, MAFF).

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useful in breaking down soil organic matter, buttwo pest species, the flat millipede (Brachydesmussuperus) and a tropical species (Oxidus gracilus),cause damage to roots of strawberries and cucum-bers respectively.

Centipedes (Figure 10.21). These resemblemillipedes, but are much more active. They helpcontrol soil pests by searching for insects andworms in the soil.

NEMATODES

This group of organisms, also called eelworms, isfound in almost every part of the terrestrial envir-onment, ranging in size from the large animal

parasites, e.g. Ascaris (about 20 cm long) in live-stock, to the tiny soil-inhabiting species (about0.5 mm long). Non-parasitic species may be bene-ficial, feeding on plant remains and soil bacteria,and helping in the formation of humus. The gen-eral structure of the nematode body is shown inFigure 10.22. A feature of the plant parasiticspecies is the spear in the mouth region, which isthrust into plant cells. Salivary enzymes are theninjected into the plant and the plant juices suckedinto the nematode (see Figure 10.23). Nematodesare very active animals, moving in a wrigglingfashion in soil moisture films, most actively whenthe soil is at field capacity, and more slowly as thesoil either waterlogs or dries out. Five horticultur-ally important types are described below.

Potato cyst nematode (Globodera rostochiensisand G. pallida). This serious pest is found inmost soils that have grown potatoes. A proportionof the eggs in the soil hatch in spring, stimulatedby chemicals produced in potato roots. The larvaeinvade the roots, disturbing translocation in xylemand phloem tissues, and sucking up plant cell con-tents. When the adult male and female nematodeshave developed, they migrate to the outside of theroot, and the now swollen female leaves only herhead inserted in the plant tissues. After fertiliza-tion, the white female becomes almost spherical,about 0.5 mm in size, and contains 200–600 eggs(see Figure 10.22).As the potato crop reaches har-vest, the female changes colour. In G. rostochiensis(the golden nematode), the change is from whiteto yellow, and then to dark brown,while in the otherspecies, G. pallida, no yellow phase is seen.The sig-nificance of the species’ differences is seen later.Eventually the dark-brown female dies and fallsinto the soil. This stage, which looks like a minuteonion, is called the cyst, and the eggs inside thisprotective shell may survive for 10 years or more.

The pest may be diagnosed in the field by themature white or yellow females seen on the potatoroots. Leaves show a yellowing symptom, plantsare often severely stunted, and occasionally killed.The distribution of damage in a field is character-istically in patches.Tomatoes grown in greenhousesand outdoors may be similarly affected.


Figure 10.20 Millipedes: this group of animals has twopairs of legs to each segment.

Figure 10.21 Centipedes: two dissimilar species areshown.

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Several forms of control are available againstthis pest. Since it attacks only potatoes and toma-toes, rotation is a reliable if sometimes inconveni-ent way of overcoming the problem. Since it isknown that an average soil population of 10 cystsper 100 gm of soil results in a 3 t/ha decline inyield, a soil count for cysts can indicate to a growerwhether a field should be used for a potato crop.Early potatoes are lifted before most nematodeshave reached the cyst stage, and thus escape seriousdamage.

Some potato cultivars, e.g. ‘Pentland Javelin’ and‘Maris Piper’, are resistant to golden nematodestrains found in Great Britain, but not to G. pallida.

Since the golden nematode is dominant in the southof England, use of resistant cultivars has provedeffective in this region.

Residual chemicals (e.g. aldicarb), incorporatedinto the soil at planting time, provide economicalcontrol when the nematode levels are moderate tofairly high, but are not recommended at low levelsbecause they are not economic, or at high levelsbecause the chemical kills insufficient nematodes.

Stem and bulb eelworm (Ditylenchus dipsaci).This attacks many plants, e.g. narcissus, onions, beansand strawberries. Several strains are known, buttheir host ranges are not fully defined. The 1 mmlong nematodes enter plant material and breed


intestine

egg containingnymph

egg

releasedegg

mouth

spear

oesophagealbulb

summer

Early summerSpring

Spring

FEMALENEMATODE(�150)

(�15)

AutumnandWinter

cyst containing eggsin the soil survivesmany years

male leaves rootand fertilises female.Female eventually turnsinto a cyst and dropsfrom the root. (Magnified)

male and femalenematodes inducegiant cells (stippled)in vascular tissues(shaded) of root.(Magnified)female nematode begins

to swell. (Magnified)

LIFE CYCLE OFPOTATO CYST NEMATODE

nematode larvae hatchand enter potato roots

Figure 10.22 The generalized structure of a nematode, and life cycle of potato cyst nematode.

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continuously, often with thousands of individualsin one plant. When an infected plant matures, thenematodes dry out in large numbers, appearing aswhite fluffy eelworm wool that may survive forseveral years in the soil. Weeds, such as bindweed,chickweed and speedwells, act as alternate hosts tothe pest.

The damage caused by this species varies with thecrop attacked. Onions show a loose puffy appear-ance (called bloat); carrots have a dry mealy rot;the stems of beans are swollen and distorted.Narcissus bulb scales show brown rings when cutacross.Their leaves show raised yellow streaks andthe crop flowers late.

Control is achieved in several ways. Control ofweeds (see chickweed);rotation with resistant crops,e.g. lettuce and brassicas; use of clean, nematode-free seed in onions; hot water treatment of onionsand narcissus at precisely controlled temperatures;all these methods help reduce this serious pest.

Chrysanthemum eelworm (Aphelenchoidesritzemabosi). It has been seen that some nema-todes live in soil (e.g. cyst eelworms), others moveinto stems (e.g. stem eelworms). This 1 mm longnematode spends most of its life cycle in youngleaves of crops, e.g. chrysanthemum, Saintpauliaand strawberries. The adults move along films ofwater on the surface of the plant, and enter the leafthrough the stomata. They breed rapidly, thefemales laying about 30 eggs, which complete a lifecycle in 14 days.During the winter they live as adultsin stem tissues, but very few overwinter in the soil.

The first symptom is blotching and purpling ofthe leaves, which spreads to become a dead brownarea between the veins.The lower leaves are worstaffected. When buds are infested, the resultingleaves may be misshapen. Greenhouse grownchrysanthemums are rarely affected, as they areraised from pest-free cuttings. Warm-water treat-ment of dormant chrysanthemum stools, e.g. at 46°Cfor 5 min, is very effective for outdoor grown plants.

Root knot eelworm (Meloidogyne spp.). Foundmainly in greenhouses, is of tropical origin, andthrives in high temperature conditions, causingtypical galls, up to 4 cm in size (see Figure 10.24),on the roots of plants such as chrysanthemum,Begonia, cucumber and tomato. The swollenfemale lays 300–1000 eggs inside the root and onthe root surface. These eggs can survive in rootdebris for over a year, and are an important sourceof subsequent infestations. The larvae hatch fromthe eggs and search for roots, reaching soil depthsof 40 cm, surviving in damp soil for severalmonths. On entering the plant, the nematode larvastimulates the adjoining root cells to enlarge.These cells block movement of water to the rootstele (see root structure), which results in the wilt-ing symptoms so commonly seen with this pest.

Care should be taken not to transfer infested soilwith transplants from one greenhouse to another.


Figure 10.23 Nematode feeding: note the spear insidethe mouth, used to penetrate plant tissues (courtesy:J. Bridge).

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Steam sterilization effectively controls the nema-tode only if the soil temperature reaches 99°C to adepth of 45 cm. Less stringent sterilization oftenresults in a severe infestation in the next crop.

Chemical sterilization of soil with fumigantchemicals, e.g. dichloropropene and methyl brom-ide, is effective if a damp seedbed tilth is firstprepared. Resistant tomato rootstocks, e.g. KVNF,have been used on grafted plants.A 2.5 cm layer ofpeat placed around roots of infested plants allowsnew root growth.

Nutrient film and soil-less methods of growingreduce the pest’s likely importance in a crop.

Migratory plant nematodes. The four speciesof nematodes previously described spend most oftheir life cycle inside plant tissues (endoparasites).Some species, however, feed from the outside ofthe root (ectoparasites). The dagger nematodes(e.g. Xiphinema diversicaudatum) and needlenematodes (e.g.Longidorus elongatus),which reachlengths of 0.4 and 1.0 cm respectively, attack theyoung roots of crops such as rose, raspberry andstrawberry, and cause stunted growth. In addition,

these species transmit the important viruses, ara-bis mosaic on strawberry and tomato black ring onornamental cherries. The nematodes may surviveon the roots of a wide variety of weeds.

Control is achieved in fallow soils by the injec-tion of a fumigant chemical, e.g. dichloropropeneor incorporation of dazomet.

FURTHER READING

Alford, D.V. A Colour Atlas of Fruit Pests (Wolfe Science,1984).

Alford, D.V. Pests of Ornamental trees, Shrubs andFlowers (Wolfe Publishing, 1991).

British Crop Protection Council. The UK PesticideGuide (2003).

Brown, L.V. Applied Principles of Horticulture, 2nd edition (Butterworth–Heinemann, 2002).

Buckle, A.P. and Smith, R.H. Rodent Pests and TheirControl (C.A.B. International, 1984).

Buczacki, S. and Harris, K. Guide to Pests, Diseases andDisorders of Garden Plants (Collins, 1992).

Dent, P. Integrated Pest Management (Chapman & Hall,1995).

Gratwick, M. Crop Pests in the UK (Chapman & Hall,1992).

Greenwood, P. and Halstead, A. Pests and Diseases(Dorling Kindersley, 1997).

Hope, F. Turf Culture: A Manual for Groundsmen(Cassell, 1990).

Ingram, D.S. et al. (Editors). Science and the Garden(Blackwell Science Ltd, 2002).

Lloyd,J.Plant Health Care for Woody Ornamentals (1997).MAAF Leaflets: No. 165 The Woodpigeon (1978);

No. 234 The Bullfinch (1973); No. 318 The Mole(1975); No. 534 The Rabbit (1988); No. 608 Control ofRats on Farms (1987).

Malais, M. and Ravensburg, W.J. Biology of GlasshousePests and Their Natural Enemies (Koppert, 1992).

Morgan, W.M. and Ledieu, M.S. Pests and DiseaseControl in Glasshouse Crops (British Crop ProtectionCouncil, 1979).

Murton, R.K. The Wood-pigeon (Collins, 1982).Pirone, P.P. Diseases and Pests of Ornamental Plants

(Wiley, 1985).Savigear, E. Garden Pests and Predators (Blandford,

1992).Scopes, N. and Stables, L. Pests and Disease Control

Handbook (British Crop Protection Council, 1992).


Figure 10.24 Root knot nematode damage on cucum-ber. Note the enlarged roots (courtesy: C.C. Doncaster).

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FUNGI


These organisms, commonly called moulds, causeserious losses in all areas of horticulture. They are

thought to have common ancestors with the filamentous algae, a group including the present-day green slime in ponds. Some details of theirclassification are given in Chapter 3.

The fungus is composed, in most species, of micro-scopic strands (hyphae), which may occur togetherin a loose structure (mycelium), form dense rest-ing bodies (sclerotia, see Figure 11.1) or producecomplex underground strands (see rhizomorphs).

The hyphae in most fungi are capable of produ-cing spores. Wind-borne spores are generally verysmall (about 0.01 mm), not sticky and often borneby hyphae protruding above the leaf surface, e.g.grey mould, so that they catch turbulent wind cur-rents. Water or rain-borne spores are often sticky,e.g. damping off. Asexual spores produced with-out fusion of two hyphae commonly occur in sea-sons favourable for disease increase, e.g. humidweather for downy mildews and dry, hot weatherfor powdery mildews. Sexual spores, produced afterhyphal fusion, commonly develop in unfavourableconditions, e.g. a cold, damp autumn, and they maybe produced singly as in the downy mildews, or ingroups within a protective hyphal spore case, oftenobservable to the naked eye, as in the powderymildews. Different genera and species are identifiedby microscopic measurement of the shape and sizeof the spores.

Horticulturists without microscopes must usethe symptoms as a guide to the cause of the dis-ease. While disease-causing or parasitic fungi arethe main concern of this chapter, in many parts ofthe environment there are saprophytic fungi that

11

Fungi, Bacteria and Viruses

In this chapter the three groups of organismsare described, and emphasis is placed on thedamage they cause, the important aspects oftheir life cycles and the control measures thatare available to reduce their infection andspread. Within each group, the diseases areclassified according to the part of the plantattacked. Owing to the microscopic size of thefungal spore, bacterial cell or virus particle, itmay be difficult for the grower to relate thecausitive organism to the disease as he sees it.

For example, Phytophthora infestans, afungal organism, travels through the air as aspore to infect potato leaves. When the leavesstart to turn black, the plant is said to show signsor symptoms of the potato blight disease. Onlywhen the plant’s growth and tuber productionare decreased by the organism is yield losssaid to occur.This chapter emphasizes the factthat symptoms of a disease are the horticul-turist’s main guide to the presence of fungi,bacteria and viruses. A more general descrip-tion of control measures is given in Chapter 12.

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break down organic matter (see Chapter 15) andsymbiotic fungi that may live in close associationwith the plant, e.g. mycorrhiza fungi in fine rootsof conifers (see Chapter 1).

The spore of a leaf-infecting fungal parasite, afterlanding on the leaf, produces a germination tube,which being delicate and easily dried out, mustenter through the cuticle or stomata within a fewhours before dry, unfavourable conditions recur.Within the leaf, the hyphae grow, absorbing fooduntil, within a period of a few weeks, they producea further crop of spores (see Figure 11.2). Leaf dis-eases such as potato blight often increase very rap-idly when conditions are favourable. Roots may beinfected by spores, e.g. in damping off; hyphae, e.g.

wilt diseases; sclerotia, e.g. Sclerotinia rot; or rhi-zomorphs, e.g. honey fungus. Root diseases are gen-erally less affected by short periods of unfavourableconditions and often increase at a constant rate.

Flower and leaf diseases

Downy mildew of cabbage and related plants(Perenospora brassicae)This serious disease causes a white bloom mainly onthe under surface of leaves of ornamental crucifer-ous plants such as stocks and wallflowers, on bras-sicas, and occasionally on weeds such as shepherd’spurse.The disease is most damaging when seedlings


cell wall

protein protectivelayer nucleic acid

core

flagella

cytoplasm

mycelium

mycelium

BACTERIUM (Pseudomonas species)(�25,000)

Fusarium species(�1000)

spore-bearing hyphae(sporangium) of Botrytis species(�400)

VIRUS (tomato mosaic virus particle)(�100,000)

A FUNGUS SCLEROTIUM CUT THROUGHTO SHOW DENSE MYCELIUM (�25)

mucilage

spores

THE SPORE-PRODUCING STRUCTURE OF A FUNGUS

Figure 11.1 Microscopic details of a virus, bacterium and three fungi. Note the relative sizes of the organisms.

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Fungi, Bacteria and Viruses 123

are germinating, particularly in spring when theyoung infectable tissues of the host plant andfavourable damp conditions may combine to killoff a large proportion of the developing plants.Other crops such as lettuce and onions are attackedby different downy mildews, Bremia lactucae and Perenospora destructor respectively, and nocross-infection is seen between different unrelatedcrops.

Asexual spores (zoospores) are produced mainlyin spring and summer, and a spray of a protectivechemical (e.g. dichlofluanid) is commonly used atthe seedling stage to kill off spores on the leaf.

Thick-walled sexual spores (oospores) producedwithin the leaf tissues fall to the ground with thedeath of the leaf, survive the winter and initiate thespring infections. It is thus not advisable to growsuccessive brassicas in the same field, and particu-larly not to sow in spring next to overwinteredcrops.

Potato blight (Phytophthora infestans)This important disease is a constant threat to potatoproduction, and caused the Irish potato famine inthe nineteenth century.The first symptoms seen in

HYPHAE

B – Hyphae obtaining food from theleaf for spore production(magnified � 500)

disease reaches the leaf (2), movesfrom leaf to leaf (3), and eventuallyinfects the tubers (4)

(1)

(2)

(3)

(4)

the infected shoot is the source ofdisease for the subsequent crop

the infected tuber (1) containingmycelium and sexual spores whichsurvive in the store or field

SPORES being produced for wind and water dispersal

spore germinating

spore landing onthe leaf surface

A – infection by the sporethrough the leaf epidermis(magnified � 500)

Figure 11.2 Infection and life cycle of potato blight fungus. The left side illustrates microscopic infection of the leaf.The right side shows how the disease survives and spreads.

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the field are yellowing of the foliage, which quicklygoes black and then produces a white bloom on the under surface of the leaf in damp weather.The stems may then go black, killing off the wholeplant.The tubers may show dark surface spots that,internally, appear as a deep dry red-brown rot.Thisfungus may attack tomatoes, the most notable symp-tom being the dark brown blisters on the fruit. Thefungus survives the winter as mycelium and sexualspores in the tubers (see Figure 11.2). The springemergence of infected shoots results in the produc-tion of asexual spores which, when carried by wind,land on potato leaves or stems and can, after infec-tion, result in a further crop of spores within a fewdays under warm, wet weather conditions.Thus thedisease can spread very quickly. Later in the crop,badly infected plants cause tuber infection as rain-fall may wash down spores into the soil.

Several preventative control measures are used.Clean ‘seed’ lowers infection within neighbouringcrops. Removal and herbicidal destruction (e.g.dichlobenil) of diseased tubers from stores or‘clamps’ similarly prevent disease spread. Know-ledge of the disease’s moisture requirement leadsto better control. By measuring both the durationof atmospheric relative humidity greater than 92per cent, and the mean daily temperatures greaterthan 10°C (together known as critical periods),forecasts of potato blight outbreaks may be madeand protectant sprays of chemicals such as man-cozeb applied before infection take place. Resistantpotato cultivars prevent rapid buildup of disease,although resistance may be overcome by newlyoccurring fungal strains. Early potato cultivarsusually complete tuber production before seriousblight attacks, while maincrop top growth maydeliberately be killed off with foliage-acting herbicides such as diquat to prevent disease spreadto tubers.

A curative control measure employs a systemicfungicide, e.g. ofurace, which penetrates the leafand kills the infecting mycelium. Such chemicalsmust be mixed with a protectant ingredient by themanufacturer to reduce the development of fungusresistance to fungicides.

Powdery mildew of ornamental Malus and apple(Podosphaera leucotricha)Powdery mildews should not be confused withdowny mildews.This disease is distinguished by itsdry powdery appearance, most commonly foundon the upper surface of the leaf, and by its prefer-ence for hot, dry weather conditions (see examplesof powdery mildew in Plates 11 and 12).

The disease survives the winter as myceliumwithin the buds, which often appear small, and the infected twigs have a dried, silvery appearance.The emergence of the mycelium with the germin-ating buds in spring results in a white bloom overthe young leaves (primary mildew), which may be reduced by a winter application of a selectivedetergent-like fungicide sprayed before bud burstor by winter pruning. As the spring progresses,chains of asexual spores produced on the outside ofthe leaf are carried by wind and cause the destruc-tive secondary mildew which, growing externallyover the leaf surface, sucks out the leaf’s moistureand may cause premature leaf drop. Floweringnormally occurs before the secondary infectionstage, but infection in young fruit may produce a rough skin (russeting). This organism may affectother species of fruit such as pears, quinces, med-lars and ornamental Malus. Other species of powdery mildew commonly occur in horticulture:Sphaerotheca pannosa on rose (Plate 12), S. fulig-inea on cucumber and chrysanthemum powderymildew (Plate 11). Cross-infection between thesecrops does not occur.

Sexual spores may be produced inside a darkcoloured spore case (cleistothecia), about 1 mm insize in autumn. Although not important in horti-culture as an overwintering stage, it may assume avital role in powdery mildew of cereals.

Control is achieved by preventative measuresmentioned above, and by curative spring and summer sprays, using a wide choice of activeingredients which may act in a contact manner onthe external mycelium, e.g. dinocap (see protec-tant), or in a systemic manner on the internal feeding hyphae (haustoria), e.g. carbendazim (seesystemic).


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Black spot of roses (Diplocarpon rosae)This common disease in gardens and greenhouse-produced roses is first seen as dark leaf spots,which may be followed by general leaf yellowingand then leaf drop (see Plate 6). The infection ofyoung shoots has a slow weakening effect on thewhole plant.

Asexual spores produced within the leaves arereleased in wet and mainly warm weather con-ditions, and are then carried a limited distance byrain drops or irrigation water before beginning thecycle of infection again. No overwintering sexualstage is seen in Britain, and it is probable thatasexual spores surviving in autumn-produced woodor in fallen leaves begin the infection process thefollowing spring. Control is difficult, as resistancein rose cultivars is not common, and vigorousgrowth in late spring and summer prevents con-tinuous protective control by chemicals, e.g. captan.The addition of a wetter/spreader may improvecontrol by spreading the active ingredient moreeffectively over the leaf surface. In industrial areasthe sulphur dioxide in the air may be at sufficientconcentration to reduce black spot.

Carnation rust (Uromyces dianthi)The rust fungi are a distinct group, which may produce five spore forms within the same species.When the spore forms occur on more than onehost, e.g. blackcurrant rust (Cronartium ribicola),which attacks both blackcurrants and five-needlepines, the close association of the two crops may give rise to high rust levels. Most horticul-tural rust species are found on only one hostspecies.

Carnation rust first appears as an indistinct yellowing of the leaf and stem, soon turning to an elongated raised brown spot which yieldsbrown dust (spores) when rubbed (see Figure 11.3).The more common thin-walled spores (ure-dospores) are spread by wind currents, and infectthe leaf by way of the stomata in damp conditions.The less common thick-walled spores (teleu-tospores) may survive and overwinter in the soil.The resistance of carnation cultivars varies, while

related species, e.g. pinks or sweet williams, arerarely affected.

Preventative control includes the use of rust-free cuttings, sterilization of border soils and careful maintenance of greenhouse ventilators toprevent damp patches occurring in the crop.Chemical control is achieved by a protectant (e.g.mancozeb) spray. The occurrence of white rust(Puccinia horiana) on chrysanthemums has cre-ated serious problems for the industry and for gardeners, because of the ease with which the disease is carried in cuttings, and because of itsspeed of increase and spread.


Figure 11.3 Carnation rust: note the dark rusty lesionson both leaf and stem (courtesy: Glasshouse CropsResearch Institute).

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Stem diseases

Grey mould (Botrytis cinerea)This disease is most commonly recognized by thedense, light grey fungal mass which follows its infec-tion. In lettuce, the whole plant rots off at the baseand the plant goes yellow and dies. In tomatoes,infection in damaged side shoots, and yellow spots(ghost spots) on the unripe and ripe fruit arefound. In many flower crops, e.g. chrysanthemums,infected petals show purple spots which, in verydamp conditions, lead to a mummified flower head.This disease may affect many crops.

Grey mould requires wounded tissue for infec-tion, which explains its importance in crops whichare de-leafed, e.g. tomatoes, or disbudded, e.g.chrysanthemums. Damp conditions are essentialfor its infection and spore production.The millionsof spores are carried by wind to the next woundedsurface. Black sclerotia, about 2 mm across, pro-duced in badly infected plants, often act as theoverwintering stage of the disease after falling tothe ground, and are particularly infective inunsterilized soils on young seedlings and delicateplants, e.g. lettuce.

Preventative control may involve soil steriliza-tion, or a fumigant, e.g. dicloran used against thesclerotia. Strict attention to greenhouse humiditycontrol, particularly overnight, limits the dew for-mation so important in the organism’s infection.Removal of infected tissue is possible in sturdyplants, e.g. tomatoes, and a protective fungicidepaste, e.g. iprodione, is often then applied to the cutsurface. Protective sprays, e.g. iprodione, are oftenapplied to greenhouse grown crops to prevent thespores germinating,and to reduce spore production.

Apple canker (Nectria galligena)This fungus causes sunken areas in bark of bothyoung and old branches of ornamental Malus,apples or pears (see Plate 13h), and occasionallyon fruit. Poor shoot growth is seen, and the woodmay fracture in high winds. The cankers may bearred spore cases (perithecia), resembling red spidermite eggs in autumn.

The fungus enters through leaf scars in autumnor through pruning wounds during winter. Care is therefore necessary to prevent infection, par-ticularly in susceptible apple cultivars, e.g. ‘Cox’sOrange Pippin’, by avoiding pruning in damp conditions.

Removal of cankered shoots may be necessary toprevent further infection, while in cankers of largebranches, cutting out of brown infected tissue mayallow the branch to be used. Removed tissue shouldbe burnt. Some growers apply a spray of copper(Bordeaux) at bud burst (spring) and leaf fall(autumn) to prevent entry of germinating spores.

Dutch elm disease (Ceratocystis ulmi)The first symptom of this disease is a yellowing offoliage in one part of the tree in early summer.Thefoliage then dies off progressively from this areaof the tree, often resulting in death within 3 months.Trees that survive 1 year’s infection may fullyrecover in the following year. All common speciesand hybrids of elm growing in Great Britain aresusceptible to the disease.

The causative fungus lives in the xylem tissuesof the stem, and produces a poison that results in ablockage of the water-conducting vessels, causingthe wilt that is observed. Two black and red wood-boring species of beetle, Scolytus scolytus (5 mmlong) and Scolytus multistriatus (3 mm long), enterthe stems, leaving characteristic ‘shot holes’. Eggsare laid, and a fan-shaped pattern of galleries isproduced under the bark by the larvae that later,as adults, emerge from the wood, carrying stickyasexual and sexual spores to continue the spreadof the disease to other uninfected elms. Grafttransmission from tree to tree by roots commonlyoccurs in hedge grown elms.

The cost of preventative control on a large num-ber of uninfected trees is uneconomic, althoughhigh pressure injection of a systemic fungicide (e.g.benomyl salts), which travels upwards through thexylem tissues, has proved successful in some cases.Selections of disease-tolerant hybrids, e.g. Ulmus �vegeta ‘Groenveld’, may replace the common speciesand hybrids.


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Root diseases

Club root (Plasmodiophora brassicae)This disease causes serious damage to most mem-bers of the Cruciferae family, which includes cab-bage, cauliflowers, brussels sprouts, stocks andAlyssum. Infected plants show signs of wilting andyellowing of older leaves, and often severe stunt-ing. On examination, the roots appear stubby andswollen (see Figure 11.4), and may show a wet rot.The club root organism survives in the soil formore than 5 years as minute spores which germin-ate to infect the root hairs of susceptible plants.The fungus is unusual in forming a jelly-like mass(plasmodium), not hyphae, within the tissue. Theplasmodium stimulates root cell division and causes

cell enlargement, which produces swollen roots.The flow of food and nutrients in phloem andxylem is disturbed, with consequent poor growthof the plant. With plant maturity, the spores produced by the plasmodium within the root arereleased as the root rots.

The disease is favoured by high soil moisture,high soil temperatures and acid soils. Several pre-ventative control measures may be used. Rotationhelps by keeping cruciferous crops away fromhigh spore levels.Autumn-sown plants establish insoil temperatures unfavourable to the disease, andare normally less infected. In transplanted crops,the use of a seed bed previously treated with asterilant, e.g. dazomet, ensures healthy transplants.Liming of soil inhibits spore activity (see soil fertility). Compost made from infected brassicaplants should be avoided.

Damping off (Pythium and Phytophthora species)These two fungi cause considerable losses to thedelicate seedling stage. The infection may occurbelow the soil surface, but most commonly theemerging seedling plumule is infected at the soilsurface, causing it to topple. Occasionally the rootsof mature plants, e.g. cucumbers, are infected, turnbrown and soggy, and the plants die. Both Pythiumand Phytophthora occur naturally in soils as sapro-phytes, and under damp conditions produce asex-ual spores that cause infection. Sexual spores(oospores) are produced in infected roots andmay survive several months of dry or cold soil conditions.

Prevention control is best achieved against thesediseases by sterilization of soil by heat, by chem-icals, e.g. methyl bromide which kills off all stagesof these fungi, or by coating the crop seed with aprotective chemical (see seed dressing), e.g. captan,to prevent early infection. Water tanks with opentops, harbouring rotting leaves, are a commonsource of infected water and should be cleanedout regularly.Sand and capillary matting on benchesin greenhouses should be regularly washed in hotwater, and the use of door mats soaked in a fungi-cide, e.g. formalin, may prevent foot spread of the


Figure 11.4 Club root on cabbage: note the swollen tapand side roots.

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organisms from one greenhouse to another.Water-logged soils should be avoided, as the fungi increasemost rapidly under these conditions.

Chemical prevention is achieved in some cropsby the incorporation of a fungicide, e.g. etridiazoleinto compost, or by drenching with a fungicide nottoxic to roots, e.g. copper sulphate and ammoniumcarbonate (Cheshunt) mixture.

Conifer root rot (Phytophthora cinnamomi)This soil-inhibiting fungus causes the foliage ofplants to turn grey green, then brown, and eventu-ally to die off completely. Sliced roots show a chest-nut brown rot, with a clear line between infectedand non-infected tissues.Two hundred plant species,including Chaemaecyparis, Erica and Rhododen-dron species may be badly attacked.The disease iscommonly introduced on infected stock plants, orcontaminated footwear. It multiplies most rapidlyunder wet conditions, and within a temperaturerange of 20°C and 30°C, infecting the root tissuesand producing numerous asexual spores, which maybe carried by water currents to adjacent plants.Sexual oospores produced further inside the rootare released on decay and allow the fungus to survive in the soil several months without a host.

Preventative control (see hygienic growing) isimportant. Reliable stock plants should be used.Water supply should be checked to avoid contam-ination. The stock plant area should be slightlyhigher than the production area to prevent infectionby drainage water. Rooting trays, compost andequipment, e.g. knives and spades, should be ster-ilized (e.g. with formalin) before use. Placing con-tainer plants on gravel reduces infection throughthe base of the pot. Some chemicals, e.g. etridiazole,incorporated in compost protect the roots, but donot kill the fungus. Some species, e.g. Juniperushorizontalis, have some tolerance to this disease.

Honey fungus (Armillaria mellea)This fungus primarily attacks trees and shrubs, e.g.apple, lilac and privet. In spring the foliage wiltsand turns yellow. Death of the plant may take a

few weeks or several years in large trees.The rootsare infected by rhizomorphs, sometimes referredto as ‘bootlaces’, which radiate out undergroundfrom infected trees or stumps (see Plate 13i) for adistance of 7 m, and to a depth of 0.7 m. The nutri-ents they are able to conduct provide the consid-erable energy required for the infection of thetough, woody roots. Mycelium, moving up the stemto a height of several metres, is visible under thebark as white sheets, smelling of mushrooms. Inautumn, clumps of light brown toadstools may beproduced, often at the base of the stem.The millionsof spores produced by the toadstools are not con-sidered to be important in the infection process.

Honey fungus often establishes itself in newlyplanted trees and shrubs that have been plantedtoo deeply.A less vigorous plant that is more vulner-able to infection results from this because most of itsfeeding roots should be very close to the surface(see Figure 5.4).

Control is difficult. Removal of the diseasesource, the infected stump, is recommended. Inlarge stumps, a surrounding trench is sometimesdug to a depth of 0.7 m to prevent the progress ofrhizomorphs. Loosening with a fork and then apply-ing a diluted sterilant, e.g. formalin, may sterilizeinfected soil containing no crops.

Fusarium patch on turf (Fusarium nivale)This disease appears as irregular circular patchesof yellow then dead brown grass up to 30 cm indiameter on fine turf. Under extreme damp condi-tions, dead leaves become slimy and then are cov-ered with a light pink bloom, most evident betweenMay and September.

Infection of the leaves by spores and hyphaeoccurs most seriously between 0°C and 8°C,conditions that are present under a layer of snow(hence the name snow mould). However, condi-tions of high humidity at temperatures up to 18°Cmay result in typical patch symptoms. Spread is bymeans of wind-borne asexual spores, while thefungus survives in frosty or dry summer conditionsas dormant mycelium in dead leaf matter or newlyinfected leaves.


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Preventative control measures are most import-ant. Avoid high soil nitrogen levels in autumn, asthis promotes lush, susceptible growth in autumnand winter. Avoid thatchy growth of the turf, asthis encourages high humidity and thus favoursthe disease organism. Drenches of preventativefungicides, e.g. iprodione, applied in autumn may slow down infection of the fungus, while a summer-applied systemic fungicide, e.g. thio-phanate methyl, moves within the plants to achievecurative control.

Vascular wilt diseases (Fusarium oxysporum andVerticillium dahliae)These two organisms infect the xylem tissues ofhorticultural plants, causing the leaves to ‘flag’ orwilt in hot conditions, a symptom which can alsobe caused by other factors, e.g. lack of soil mois-ture (see wilt), and nematode infestation (see rootknot nematode page 119).The wilt diseases can berecognized by yellowing and eventual browning ofthe lower leaves, and by brown staining of thexylem tissue when it is exposed with a knife. Bothorganisms may live as saprophytes in the soil.Fusarium survives unfavourable conditions as thick-walled asexual spores, while Verticillium formssmall sclerotia. Infection by both genera occursthrough young roots or after nematode attack inolder roots. The fungal hyphae enter the rootxylem tissue and then move up the stem, some-times reaching the flowers and seeds. The diseasesspread by water-borne asexual spores.

Verticillium may attack a wide range of plants,e.g. dahlia, strawberry, lilac, tomato and potato,so that rotation is not a feasible control measure.Fusarium oxysporum, however, exists in many dis-tinct forms, each specializing in a different crop,e.g. tomato, broad bean or carnation. Verticilliummore commonly attacks in springtime, having an optimum infection temperature of 20°C, whileFusarium is more common in summer, with anoptimum temperature of 28°C.

Control is often necessary in greenhouse crops.Infected crop residues should be removed fromthe soil at the end of the growing season, as soil

sterilization by steam or chemicals, e.g. methamsodium, may not penetrate to the centre of stemsand roots. Peat bags may be used as a disease-freegrowing medium. In unsterilized soils, growers may use resistant rootstocks, e.g. in tomatoes, whichare grafted onto scions of commercial cultivars.Rotation may be employed against Fusariumoxysporum, as different forms attack differentcrops. Careful removal of infected and surround-ing plants, e.g. in carnations, may slow down theprogress of the diseases, especially if the soil areais drenched with a systemic chemical, e.g. thiaben-dazole, which reduces the infection in adjacentplants.

BACTERIA

These minute organisms (see Figure 11.1) measureabout 0.001 mm and occur as single cells thatdivide rapidly to build up their numbers. They areimportant in the conversion of soil organic matter(see Chapter 15), but may, in a few parasitic species,cause serious losses to horticultural plants. Somedetails of their classification are given in Chapter 3.

Fireblight (Erwinia amylovora)This disease, which first appeared in the BritishIsles in 1957, can cause serious damage on mem-bers of the Rosaceae family. Individual brancheswilt, the leaves rapidly turning a ‘burnt’ chestnutbrown, and when the disease reaches the maintrunk, it spreads to other branches and may causedeath of the tree within 6 weeks of first infection,the general appearance resembling a burnt tree,hence the name of the disease. On slicing throughan infected stem, a brown stain will often be seen.Pears, hawthorn and Cotoneaster are commonlyattacked, while apples and Pyracantha suffer lesscommonly.

The bacterium is carried by pollinating andharmful insects (e.g. aphids), and by small dropletsof rain. Humid conditions and temperatures inexcess of 18°C, which occur from June to Septem-ber, favour the spread. Natural plant openings


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such as stomata and lenticels are common sites forinfection. Flowers are the main entry point inpears. Badly infected plants produce a bacterialslime on the outside of the branches in humidweather. This slime is a major source of furtherinfections. Fireblight, once notifiable, must now bereported only in fruit-growing areas. The compul-sory removal of the susceptible ‘Laxton’s Superb’pear cultivar has eliminated a serious source ofinfection. Preventative measures such as removalof badly infected plants to prevent further infec-tion, and replacement of hawthorn hedges close to pear orchards, help in control. Careful pruning,60 cm below the stained wood of early infection,may save a tree from the disease. Wounds shouldbe sealed with protective paint, and pruning imple-ments should be sterilized with 3 per cent lysol.

Bacterial canker (Pseudomonas morsprunorum)This disease affects the plant genus Prunus thatincludes ornamental species, plum, cherry andapricot. Symptoms typically appear on the stem(see Figure 11.5) as a swollen area exuding a lightbrown gum. The angle between branches is themost common site for the disease. Severe infectionsgirdling the stems cause death of tissues above theinfection, and the resulting brown foliage resem-bles the damage caused by fireblight. In May andJune, leaves may become infected; dark brown leafspots 2 mm across develop and may be blown out,giving a ‘shot-hole’ effect. The bacteria present inthe cankers are mainly carried by wind-blown raindroplets, infecting leaf scars and pruning woundsin autumn, and young developing leaves in summer.

Preventative control involves the use of resist-ant rootstocks and scions, e.g. in plums.The carefulcutting out of infected tissue followed by applica-tion of a paint, and the use of autumn sprays ofBordeaux mixture, help reduce this disease.

Soft rot (Erwinia carotovora)This bacterium affects stored potatoes, carrots,bulbs and iris, where the bacterium’s ability to dis-solve the cell walls of the plant results in a mushy

soft rot. High temperatures and humidity causedby poor ventilation promote infection through lenti-cels, and major losses may occur. A related strainof this bacterium causes black leg on potatoes inthe field.

Preventative control measures are important.Crops should be damaged as little as possiblewhen harvesting, and diseased or damaged speci-mens should be removed before storage. Hot,humid conditions should be avoided in store. Nocurative measures are available.

Crown gall (Agrobacterium tumifasciens)This bacterium affects apples, grapes, peaches, roses,and many other herbaceous plants. The disease is


Figure 11.5 Bacterial canker on flowering cherry: notethe swollen and cracked stem (courtesy: Plant PathologyLaboratory, Ministry of Agriculture, Fisheries and Food,MAFF).

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first seen just above ground level as a swollen,cancer-like structure growing out of the stem.It may occasionally cause serious damage, but usu-ally is not a very important problem.The bacteriumis able to survive well in soils, and infects the plantthrough small wounds in the roots. It is of specialscientific interest in the area of plant breeding,having the unusual ability to add its genetic infor-mation into that of the plant cell. It does this bymeans of a small unit of DNA called a ‘plasmid’.This plasmid ability of A. tumifasciens has beenharnessed by plant breeders to transfer geneticinformation between unrelated plant species.And so, it is the properties of this bacterium thathave led, in our language, to the new term ‘geneti-cally modified crops’ or more simply ‘GM’ (seeChapter 8).

Control of crown gall depends on cultural con-trol methods such as disease-free propagatingmaterial, avoiding wounds at planting time andbudding scions to rootstocks (rather than grafting)to avoid injuries near the soil level.

VIRUSES


Viruses are extremely small, very much smallereven than bacteria (see Figure 11.1). They appearas rods or spheres when seen under an electronmicroscope. The virus particle is composed of anucleic acid core surrounded by a protective pro-tein coat. The virus, on entering a plant cell, takesover the organization of the cell nucleus in orderto produce many more virus particles. Since thevirus itself lacks any cytoplasm cell contents, it isusually considered to be a non-living unit. Somedetails of its classification are given in Chapter 3.The virus’s close association with the plant cellnucleus presents difficulties in the production of acurative virus control chemical that does not alsokill the plant. No such commercial ‘viricide’ hasyet been produced against plant viruses.

In recent years the broad area called ‘virus diseases’ has been closely investigated. Virus

particles have, in most cases, been isolated as thecause of disease, e.g. cucumber mosaic. Otheragents of disease to be discovered are viroids (e.g.in chrysanthemum stunt disease), and these aremuch smaller than viruses. Mycoplasmas (e.g. asteryellows disease) are larger than and unrelated toviruses. In some diseases, e.g. ‘reversion disease’ onblackcurrant, a causitive agent has yet to be isol-ated. All agents are placed together in this section.

A number of organisms (vectors) transmit virusesfrom plant to plant. Peach-potato aphid is capableof transmitting over 200 strains of virus (e.g.cucumber mosaic) to different plant species; theaphid stylet injects salivary juices containing virusinto the parenchyma and phloem tissues, and, alongthe phloem, the virus may travel to other parts ofthe plant. Other vector/virus combinations includeblackcurrant big bud mite and reversion virus; beanweevils and broad bean stain virus; migratory soilnematodes, Xiphinema and arabis mosaic; andOlpidium soil fungus and big vein agent on lettuce.Other important methods of transmission involvevegetative material (e.g. chrysanthemum stuntviroid and plum pox), infected seed (e.g. beancommon mosaic virus), seed testa (e.g. tomatomosaic virus) and mechanical transmission by hand(e.g. tomato mosaic virus).

General symptoms

The presence of a damaging virus in a plant is recognized by horticulturists only by the symp-toms, although they may consult the virologist,whose identification techniques include electronmicroscopy, transmission tests on sensitive plants,e.g. Chenopodium spp., and serological reactionsagainst specific antisera. Leaf mosaic, a yellowmottling, is the most common symptom (e.g. tomatomosaic virus). Other symptoms include leaf dis-tortion into feathery shapes (cucumber mosaicvirus), flower colour streaks (e.g. tulip break virus),fruit blemishing (tomato mosaic and plum pox),internal discoloration of tubers (tobacco rattlevirus-causing ‘spraing’ in potatoes) and stunting ofplants (chrysanthemum stunt viroid). Symptoms


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similar to those described above may be caused bymisused herbicide sprays, genetic ‘sports’, poor soilfertility and structure (see deficiency symptoms)and mite damage. In the following descriptions ofmajor viruses, Latin names of genus and speciesare omitted, since no consistent classification is yetaccepted.

Cucumber mosaicSeveral strains of virus cause this disease, andmany families of plants are attacked. On cucum-bers, a mottling of young leaves occurs, followedby a twisting and curling of the whole foliage,and fruit may show yellow sunken areas. On theshrub Daphne oderata, a yellowing and slight mottle is commonly seen on infected foliage, whileEuonymus foliage produces bright yellow leafspots. Infected tomato leaves are reduced in size(fern-leaf symptom).

The virus may be transmitted by infected hands,but more commonly an aphid (e.g. peach-potatoaphid) is involved. Many crops (e.g. lettuce, maize,Pelargonium and privet) and weeds (e.g. fat henand teasel) may act as a reservoir for the virus.

Since there are no curative methods for control,care must be taken to carry out preventativemethods. Choice of uninfected stock is vital in vegetatively propagated plants, e.g. Pelargonium.Careful control of aphid vectors may be importantwhere susceptible crops (e.g. lettuce and cucumbers)are grown in succession. Removal of infectedweeds, particularly from greenhouses, may preventwidespread infection.

Tulip breakThe petals of infected tulips produce irregularcoloured streaks and may appear distorted. Leavesmay become light green, and the plants stuntedafter several years infection. The virus is transmit-ted mechanically by knives, while three aphid vec-tors are known: the bulb aphid in stores, the melonaphid in greenhouses, and the peach-potato aphidoutdoors and in greenhouses. Preventative controlmust be used against this disease. Removal of

infected plants in the field prevents a source ofvirus for aphid transmission. Aphid control infield, store and greenhouse further reduces thevirus’s spread.

Tomato mosaicThis disease may cause serious losses in tomatoes.Infected seedlings have a stunted, spiky appear-ance. On more mature plants leaves have a palegreen mottled appearance, or sometimes a brightyellow (aucuba) symptom. The stem may showbrown streaks in summer when growing condi-tions are poor, a condition often resulting in deathof the plant. Fruit yield and quality may be lowered, the green fruit appearing bronze, and the ripe fruit hard, making the crop unsaleable.

The virus may survive within the seed coat (testa)or endosperm. Heat treatment of dry seed at 70°Cfor 4 days by seed merchants helps remove initialinfection. Infected debris, particularly roots, in thesoil enables the virus to survive from crop to crop,and soil temperatures of 90°C for 10 min are nor-mally required to kill the organism. Peat-growingbag and nutrient-film methods enable the growerto avoid this source of infection. The virus isspread very easily by sap. Hands and tools should bewashed in soapy water after working with infectedplants. Clothing may harbour the virus.The periodfrom plant infection to symptom expression isabout 15 days.

Cultivars and rootstocks containing several fac-tors for resistance are commonly grown, but chan-ging virus strains may overcome this resistance. Amild strain spray inoculation method has been usedat the seedling stage to protect non-resistant culti-vars from infection with severe strains. Extremecare is required to avoid mosaic-contaminatedequipment when using this method.

Plum poxThis disease, sometimes called ‘Sharka’, hasincreased in importance in the British Isles since 1970, after its introduction from mainlandEurope. Plums, damsons, peaches, blackthorn andornamental plum are affected, while cherries and


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flowering cherries are immune. Leaf symptoms offeint interveinal yellow blotches can best be seenon leaves from the centre of the infected tree. Themost reliable symptoms, however, are found onfruit (see Figure 11.6), where sunken dark blotchesare seen. Ripening of infected fruit may be severalweeks premature, yield losses may reach 25 percent, and the fruit is often sour.

The virus enters a nursery or orchard throughinfected planting stock. Spread of the virus duringthe early summer and autumn by aphids (e.g.leaf curling plum aphid) is slow in widely spacedorchards,but more rapid in closely planted nurseries,e.g. nursery stock areas.

Preventative control is the only option open togrowers. Clean ministry-certified stock should beused. Routine aphid-controlling insecticides shouldbe applied in late spring, summer and autumn.Suspected infected trees must be reported and theinfected trees must be removed and burnt.

Chrysanthemum stunt viroidThis disease, found only on plants of the Asteraceaefamily and mainly on the Chrysanthemum genus,

produces a stunted plant, often only half the nor-mal size but without any distortion. Flowers oftenopen 1 week earlier than normal, and may besmall and lacking in colour. The virus enters gar-dens and nurseries through infected cuttings, andis readily transmitted by leaf contact and by hand-ling. Symptoms may take several months toappear, thus seriously reducing the chance of earlyremoval of the disease source.

The grower must use preventative control.Certified planting material derived from heat-treated meristem stock (see tissue culture) reducesthe risk of this disease.

Arabis mosaicThis virus infects a wide range of horticulturalcrops. On strawberries, yellow spots or mottlingare produced on the leaves, and certain cultivarsbecome severely stunted. On ornamental plants,e.g. Daphne odorata, yellow rings and lines areseen on infected leaves, and the plants may slowlydie back, particularly when this virus is associatedwith cucumber mosaic inside the plant. Severalweeds, e.g. chickweed and grass spp., may harbourthis disease, and in soft fruit severe attacks of thedisease may occur when planted into ploughed-upgrassland. The virus is transmitted by a commonsoil-inhabiting nematode, Xiphinema diversicau-datum, which may retain the virus in its body forseveral months.

Control of this disease may be achieved only bypreventative methods.Virus-free soft fruit plantingmaterial is available through the Nuclear StockScheme. Fumigation of soil with chemicals such as dichloropropene, applied well before plantingtime, eliminates many of the eelworm vectors.No curative chemical is available to eliminate thevirus inside the plant.

Reversion disease on blackcurrantsThis virus disease seriously reduces blackcurrantyields. Flower buds on infected bushes are almosthairless on close inspection, and appear brighter incolour than healthy buds. Infected leaves may have


Figure 11.6 Plum pox: note the patches on the fruit(courtesy: Plant Pathology Laboratory, Ministry ofAgriculture, Fisheries and Food, MAFF) (see notifiablediseases).

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fewer main veins than healthy ones. After severalyears of infection, the bush may cease to producefruit, the effect of different virus strains producingdifferent levels of sterility. The virus is transmittedby the blackcurrant gall mite, and reversion infectedplants are particularly susceptible to attack by thispest. Removal and burning of infected plants is animportant form of control.

Use of certified plant material, raised in areasaway from infection and vectors, is strongly rec-ommended. Control of the mite vector in springand early summer has already been described.

FURTHER READING

Agrios, E.N. Plant Pathology (Academic Press, 1997).Brown, L.V. Applied Principles of Horticulture, 2nd edi-

tion (Butterworth-Heinemann, 2002).Buczacki, S. and Harris, K. Guide to Pests, Diseases and

Disorders of Garden Plants (Collins, 1998).Cooper, J.I. Virus Diseases of Trees and Shrubs (Institute

of Terrestrial Ecology, 1979).

Fletcher, J.T. Diseases of Greenhouse Plants (Longman,1984).

Forsberg, J.L. Diseases of Ornamental Plants (Universityof Illinois, 1975).

Greenwood, P. and Halstead, A. Pests and Diseases(Dorling Kindersley, 1997).

Hope, F. Turf Culture: A Manual for Groundsmen(Cassell, 1990).

Ingram, D.S. et al. (editors). Science and the Garden(Blackwell Science Ltd, 2002).

Lloyd,J.Plant Health Care for Woody Ornamentals (1997).Morgan,W.M. and Ledieu, M.S. Pest and Disease Control

in Glasshouse Crops (British Crop Protection Council,1979).

Scopes, N. and Stables, L. Pest and Disease ControlHandbook (British Crop Protection Council, 1989).

Snowdon, A.L. Post-harvest Diseases and Disorders ofFruits and Vegetables (Wolfe Scientific, 1991).

Welcome to the World of Environmental Products.(Rhone-Poulenc, 1992).

Whitehead, R. (editor) UK Pesticide Guide (CABIPublishing, 2003).


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LEGISLATION AND CONTROL

Before 1877, no legal measures were available inthe UK to prevent importation of plants infestedwith pests such as Colorado beetle. Measures takenat ports from that year onwards were broughttogether in the 1927 Destructive Insects and PestsActs, empowering government officials to inspectand, if necessary, refuse plant imports. Within thisAct, the ‘Sale of Diseased Plants Order’ placed onthe grower the responsibility for recognition and

reporting of serious pests and diseases, e.g. black-currant gall mite and silver leaf of plums. Lack ofeducation and enforcement led to the need forspecific orders relevant to particular current prob-lems, e.g. in 1958 the fireblight-susceptible pearcultivar, ‘Laxton’s Superb’, was declared notifiableand prohibited under the Fireblight Disease Order.Recent orders have helped prevent outbreaks ofwhite rust on chrysanthemums, plum pox virus andtwo American leaf miner species; less success hasbeen achieved with the Western flower thrip organ-ism. Further importation legislation under the ‘1967Plant Health Act’ prohibits the landing of any non-indigenous pest or disease by aircraft or post, andthe ‘Importation of Plants, Produce and PotatoesOrder 1971’ specifically names prohibited crops,e.g. plum rootstocks from eastern Europe; cropssubject to a healthy plant (phytosanitary) certifi-cate, e.g. potato tubers from Europe; and crops tobe examined, e.g. Acacia shrubs and apricot seeds.The operation of these orders is supervised by the Plant Health Branch of the Department forEnvironment, Food and Rural Affairs (DEFRA).Complete success in preventing the introductionof damaging organisms may be limited by dis-honest importations and by the difficulty of detec-tion of some diseases, especially viruses. EuropeanCouncil directives,e.g.77/93/EEC,are moving mem-ber nations towards a unified approach in redu-cing the transfer of infected plant material acrossnational boundaries. The Weeds Act 1959 places a legal obligation on each grower to prevent the

12

Control Measures

In the three preceding chapters, importantweeds, pests and diseases have been describedwith an emphasis on symptoms and damage,life cycles, and brief comments on control rel-evant to the particular causative organism. Inthis chapter, the main types of control measuresare described in some detail to include legalaspects, crop management methods, use ofbeneficial organisms and the rather complexarea of chemical control. The horticulturistshould aim to use as many appropriate methods as possible within a crop cycle tobring about precise and efficient control. Dis-tinction is drawn between preventative andcurative control methods. The concept of eco-nomic damage is discussed in relation to super-vised control.

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spread of creeping thistle, spear thistle, curleddock, broad-leaved dock and ragwort.

CULTURAL CONTROL

Horticulturists, in their everyday activities, mayremove or reduce damaging organisms, and thusprotect the crop. Below are described some of themore important methods used.

Cultivation

Ploughing and rotavating of soils enable a physicalimprovement in soil structure as a preparation for the growing of crops. The improved drainageand tilth may reduce damping-off diseases, disturbannual and perennial weeds, e.g. chickweed andcouch grass, and expose soil pests, e.g. leather jackets and cutworms, to the eager beaks of birds.Repeated rotavation may be necessary to depletethe reserves of perennial weeds, e.g. couch grass.Hoeing annual weeds is an effective method, pro-vided the roots are fully exposed, and the soil dryenough to prevent their re-establishment.

Partial soil sterilization

Greenhouse soils are commonly sterilized by highpressure steam released to penetrate downwardsinto the soil, which is covered by heat resistantplastic sheeting (sheet steaming). The steam con-denses on contact with soil particles, and movesdeeper only when that layer of soil has reachedsteam temperature. Some active soil pests, e.g. sym-philids, may move downwards ahead of the steam‘front’. The temperatures required to kill mostnematodes, insects, weed seeds and fungi are 45°C, 55°C, 55°C and 60°C respectively. Beneficial bacteria are not killed below 82°C, and thereforegrowers attempt to reach but not exceed this soiltemperature. In this way, organisms difficult to ster-ilize, such as fungal sclerotia, and Meloidogyne andVerticillium in root debris may be killed. Sheet

steaming is effective only to depths of about 15 cm,and its effect is reduced when soil aggregates arelarge and hard to penetrate, or when soils are wetand hard to heat up. When soil pests and diseasesoccur deep in the soil, heating pipes may be placedbelow the soil surface, as grids or spikes, to achieve a more thorough effect.The ‘steam-plough’ achievesa similar result, as it is winched along the green-house. If soil is to be used in composts it should besterilized (see sterilizing equipment). The clearadvantage of soil sterilization may occasionally belost if a serious soil fungus (e.g. Pythium) is acci-dentally introduced into a crop where it may quicklyspread in the absence of fungal competition.

Chemical sterilization

This involves the use of substances toxic to mostliving organisms, thus necessitating application dur-ing the fallow, or intercrop, period. Soil appliedchemicals include methyl bromide (applied as agas), metham sodium (commonly applied througha nutrient diluter), dazomet (applied as a granule)and dichloropropene (applied by an injectionapparatus against nematodes). The fumigantaction of these substances is prolonged by rollingthe soil, or covering it with plastic sheeting (in thecase of the methyl bromide). Care should be takenthat no chemical residues are allowed to remain,and thus kill off early stages of the succeedingcrop. Greenhouse structures may be sterilized bytoxic gases and liquids such as formaldehyde,formic acid and burning sulphur. Common pestsand diseases such as whitefly, red spider mite andgrey mould may be greatly reduced by this inter-crop method of control.

European legislation is causing the withdrawalof methyl bromide use by 2005 on the grounds ofenvironmental safety. Whilst being very effectiveas a sterilant of growing media, methyl bromidehas three serious drawbacks. It is very poisonousto man and animals. It is a gas and therefore findsits way into the atmosphere. It is also chemicallyrelated to the chlorinated hydrocarbons, such asthe refrigerator coolants, that have been held


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Control Measures 137

responsible for much of the ‘global warming’ phe-nomenon. These three factors combine to rule outits continued use in horticulture.

Soil fertility

While the correct content and balance of majorand minor nutrients (see Chapter 16) in the soilare recognized as vitally important for optimumcrop yield and quality, excessive nitrogen levels mayencourage the increase of insects such as peach-potato aphid, fungi, e.g. grey mould, and bacteria,e.g. fireblight. Adequate levels of potassium helpcontrol fungal diseases, e.g. Fusarium wilt on carna-tion, and tomato mosaic virus. Club root disease ofbrassicas is less damaging in soil with a pH greaterthan 6, and lime may be incorporated before plant-ing these crops to achieve this aim.Amenity horti-culturists apply mulches, e.g. composted bark,grass cuttings or straw, to bare soil in order to con-trol annual weeds by excluding the source of light.Black polythene sheeting is used in soft fruit pro-duction to achieve a similar objective.

Crop rotation

Some important soil-borne pests and diseasesattack specific crops, e.g. potato cyst nematode onpotato and clubroot on cruciferous plants.As theyare soil-borne, they are slow in their dispersal andcan be difficult to control. By the simple methodof planting a given crop in a different plot eachseason, such pests or diseases are excluded fromtheir preferred host crop for several seasons. Thismethod does not work well against unspecific prob-lems, e.g. grey mould, or rapidly spreading organ-isms such as aphids.

Planting and harvesting times

Some pests emerge from their overwintering stageat about the same time each year, e.g. cabbageroot fly in late April. By planting early to establishtolerant brassica plants before the pest emerges, a

useful supplement to chemical control is achieved.The deliberate planting of early potato cultivarsenables harvesting before the maturation of potatocyst nematode cysts, so that damage to the cropand the release of the nematode eggs is avoided.Annual weeds may be induced to germinate in aprepared seedbed by irrigation. After they havebeen controlled with a contact herbicide, e.g.paraquat, a crop may then be sown into the undis-turbed bed or stale seedbed, with less chance offurther weed germination.

Clean seed and planting material

Seed producers take stringent precautions toexclude weed seed contaminants and pests anddiseases from their seed stocks. While weed seedsare, in the main, removed by mechanical separ-ators, and insects can be killed by seed dressings,systemic fungal seed infections, e.g. celery leaf spotdisease, are best controlled by immersion of dryseed in a 0.2 per cent thiram solution at 30°C for24 h (thiram soak treatment). The seed is then re-dried. Equal care is taken to monitor seed cropslikely to carry virus disease (e.g. tomato mosaic).Curative control by dry heat at 70°C for 4 days is usually effective, although it may reduce subse-quent seed germination rates.

Vegetative propagation material is used in allareas of horticulture, as bulbs (e.g. tulips andonions), tubers (e.g. dahlias and potatoes), runners(e.g. strawberries), cuttings (e.g. chrysanthemumsand many trees and shrubs) and graft scions intrees. The increase of nematodes, viruses, fungiand bacteria by vegetative propagation is a particu-lar problem, since the organisms are inside theplant tissues, and since the plant tissues are sensi-tive to any drastic control measures. Inspection ofintroduced material may greatly reduce the risk ofthis problem. Soft narcissus bulbs, chrysanthemumcuttings with an internal rot, whitefly or red spidermite on stock plants, virus on nursery stock, are all symptoms that would suggest either carefulsorting, or rejection of the stocks. Accurate andrapid methods of virus testing (using test plants,

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electron microscopy and staining by ELISA tech-nique) now enable growers quickly to learn thequality of their planting stocks. Fungal levels incuttings (e.g. Fusarium wilt of carnations) can beroutinely checked by placing plant segments insterile nutrient culture.

Warm water treatment is used for pests such as stem and bulb nematodes in narcissus bulbs.Immersion of bulbs for 2 h at 44°C controls the pest without seriously affecting bulb tissues.Chrysanthemum stools and strawberry runners maysimilarly be treated, using temperature/time com-binations favourable to each crop. Viruses (e.g.aspermy virus on chrysanthemum) are more diffi-cult to control, since they are more intimatelyassociated with the plant cells. Virus concentra-tions may be greatly reduced in meristems of stockplants grown at temperatures of 40°C for about amonth. This has enabled the production of tissue-cultured disease-free stock material of both edibleand non-edible crops.

Hygienic growing

During the crop, the grower should aim to provideoptimum conditions for growth. Water content ofsoil should be adequate for growth (see field cap-acity), but not be so excessive that root diseases (e.g.damping off in pot plants, club root of cabbage andbrown root rot of conifers) are actively encouraged.Water sources should be analysed for Pythiumand Phytophthora species if damping-off diseasesare a constant problem. Covering, and regularcleaning, of water tanks to prevent the breeding of these fungi in rotting organic matter may beimportant in their control. Conifer nursery stockgrown on raised gravel beds are less likely to suf-fer the water-borne spread of conifer brown rot.Carnations are grown in isolated beds or peat mod-ules to reduce spread of wilt-inducing organisms(e.g. Fusarium spp.).

High humidity encourages many diseases. Ingreenhouses, the careful timing of daily overheadirrigation, and of ventilation (to reduce overnightcondensation on leaves or flowers), may greatly

reduce levels of diseases such as grey mould on potplants or downy mildew on lettuce. Transmissionof pests and diseases from plant to plant or field tofield can be slowed down. Virus diseases such astomato mosaic virus may be confined by delayingtill the end of the de-leafing or harvesting of infectedplants.Washing knives and hands regularly in warm,soapy water will reduce subsequent viral spread.Soil-borne problems, e.g. club root, eelworms anddamping-off diseases, are easily carried by bootsand tractor wheels. Foot and wheel dips, contain-ing a general chemical sterilant, e.g. formaldehyde,have successfully been used, especially in prevent-ing damping-off problems in greenhouses.

Traps and repellants

Measures described in this section aim to preventthe pest or disease from arriving at the crop.A grease band wrapped around an apple trunk pre-vents the wingless female winter moth’s attemptedcrawl up the tree to lay eggs in winter.A fine meshscreen placed in front of the ventilator fan preventsthe entry of many mushrooms and glasshouse pests,e.g. sciarid flies.A trench dug around a large stumpinfected with honey fungus may prevent the rhi-zomorphs (‘bootlaces’) from radiating out to infectother plants. Interplanting of onions amongst car-rots may deter the carrot fly from attacking itshost crop. Pheremone traps containing a specificsynthetic attractant are used in apple orchards tolure male codling and tortrix moths onto a stickysurface, thus enabling an accurate assessment oftheir numbers, and a more effective control.

Alternate hosts

Alternate hosts harbouring pests and diseasesshould be removed where possible.Soil-borne prob-lems, e.g. club root of cabbage and free-living eel-worms on strawberries,are harboured by shepherd’spurse (Capsella bursapastoris) and chickweed(Stellaria media) respectively. Groundsel (Seneciovulgaris) is an alternate host of the air-borne


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powdery mildew of cucumber, while docks (Rumexspp.) act as a reservoir of dock sawfly, which dam-ages young apple trees.

Removal of infected plant material

With rapid-increase problems, e.g. peach-potatoaphid and white rust of chrysanthemum fungus ingreenhouses, removal is practicable in the earlystages of the crop, but becomes unmanageableafter the pest or disease has increased in numberand dispersed throughout the plants. Slow-increaseproblems, e.g. Fusarium wilt disease on carnationsand vine weevil larvae, may be removed through-out the crop cycle, but the infected roots and soilmust be carefully placed in a bag to prevent dis-persal of the problem. In outdoor production,labour costs often prevent removal during thegrowing season.

However, chemical destruction of blight-infectedpotato foliage with a herbicide, e.g. diquat, reducesinfection of the tubers. Burning of post-harvestleaf material and lifting of root debris after har-vest (against grey mould on strawberries and clubroot on brassicas respectively) may help preventproblems in the next crop. In tree species, routinepruning operations may remove serious pests, suchas fruit tree red spider mite, and diseases, such aspowdery mildew. Tree stumps harbouring seriousunderground diseases, e.g. honey fungus, shouldbe removed where practicable.

RESISTANCE

Wild plants show high levels of resistance to mostpests and diseases. In the search for high yieldsand extremes of flower shape and colour, plantbreeders have often failed to include this wildplant resistance. However, in crops such as antir-rhinum, lettuce and tomatoes, one or more resist-ance genes have been deliberately incorporated to give protection against rust, downy mildew andtomato mosaic virus respectively. The diseaseorganisms may overcome these genetic barriers,

and the crop thus again becomes infected. Growersmay sow a sequence of cultivars (e.g. lettuce), eachwith different resistance genes, in order that thedisease organism (e.g. downy mildew) is constantlyexposed to a new resistance barrier, and thus limitthe disease.

Vegetatively propagated species (e.g. potatoes)and tree crops (e.g. apple) which remain genet-ically unaltered for many years are now being bredwith high levels of ‘wild plant’ resistance (to blightand powdery mildew respectively on potato andapple) so that the crops may more permanentlyresist these serious problems. Crop resistance toinsects is now being more seriously considered byplant breeders. Some lettuce cultivars are resistantto lettuce root aphid (Pemphigus bursarius).

BIOLOGICAL CONTROL

Many pests of outdoor horticultural crops, e.g.peach-potato aphid, are indigenous (i.e. they origin-ally evolved, and are still present, in wild plantcommunities of this country). Pest populations areoften reduced in nature by other organisms which,as predators, eat the pest, or as parasites, lay eggswithin the pest. These beneficial organisms, foundalso on horticultural crops, are to be encouraged,and in some cases deliberately introduced.

The black-kneed capsid (Blapharidopterus angu-latus) found on fruit trees alongside its pestilentrelative, the common green capsid, eats more than1000 fruit tree red spider mites per year. Its eggsare laid in August and survive the winter. Winterwashes used against apple pests and diseases oftenkill off this useful insect.The closely related antho-corid bugs, e.g. Anthocoris nemorum, are preda-tors on a wide range of pests, e.g. aphids, thrips,caterpillars and mites, and have recently been usedfor biological control in greenhouses.

Some species of lacewings, e.g. Chrysopa carnea,lay several hundred eggs per year on the end offine stalks, located on leaves. The resulting hairylarvae predate on aphids and fruit tree red spidermite, reaching their prey in leaf folds which lady-birds cannot reach.


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The 40 species of ladybird beetle are a wel-come sight to the professional horticulturist andlay-man alike. Almost all are predatory. The redtwo-spot ladybird (Adalia bipunctata) emergesfrom the soil in spring, mates and lays about 1000 elongated yellow eggs on the leaves of arange of weeds and crops, e.g. nettles and beans,throughout the growing season. The emergingslate-grey and yellow larvae and the adults feedon a range of aphid species.The ground beetle (e.g.Bembidion lampros) actively predates on soil pests,e.g. cabbage root fly eggs, greatly reducing theirnumbers.

Hoverflies superficially resembling wasps arecommonly seen darting above flowers in summer.Several of the 250 British species, e.g. Syrphusribesii, lay eggs in the midst of aphid colonies, andthe legless larva consumes large numbers of aphids.

Predatory mites, e.g. Typhlodromus pyri on fruittree red spider mite, may contribute importantly topest control.The numerous species of web-forming

and hunting spiders help in a largely unspecific wayin the reduction of all forms of insects.

About 3000 wasp species of the families Ichneu-monidae, Braconidae and Chalcidae are parasiticon insects in Britain. The red ichneumon (Opionspp.) lays eggs in many moth caterpillars. The cab-bage white caterpillar may bear 150 parasitic lar-vae of the braconid wasp (Apanteles glomeratus)which pupate outside the pest’s body as yellowcocoon masses. The woolly aphid on apples maybe reduced by the chalcid Aphelinus mali.

The spiracles of insects provide access to spe-cialized parasitic fungi, particularly under dampconditions.Aphid numbers may be quickly reducedby the infection of the fungus Entomophthoraaphidis, while codling moth caterpillars on applemay be enveloped by Beauveria bassiana. Cabbagewhite caterpillar populations are occasionally muchreduced by a virus, which causes them to burst.

The predators and parasites previously describedmay be irregular in their distribution and time of


Table 12.1 Biological control organisms used in horticulture

Disease or pest Crop Name of controlling organism Type of controlling organism

Thrips C, F, P Amblyseius cucumeris MiteC, F, P Orius laevigatus Anthocorid bug

Aphids G Aphidoletes aphidimyza MidgeG Aphidius spp. WaspCh Verticillium lecanii Fungus

Glasshouse whitefly T Macrolophus caliginosus Anthocorid bugG Encarsia formosa Wasp

Red spider mite G Phytoseiulus persimilis MiteT, C, F Therodiplosis persicae Midge

Leaf miner T, C, F Dacnusa sibirica WaspT, C, F Diglyphus isaea Wasp

Caterpillar G Trichogramma spp. WaspG Bacillus thuringiensis Bacterium extract

Leaf hopper T Anagrus atomus WaspMealy bug G Cryptolaemus montrouzieri Ladybird

G Leptomastix dactylopii WaspVine weevil G Steinernema capsicarpae NematodeSciarid fly G Steinernema feltiae Nematode

G Hypoaspis miles MiteSilver leaf fungus Tf Trichoderma viride FungusTomato mosaic virus T – Mild strain of virus

T: tomato; C: cucumber; P: sweet peppers; F: flowers; Tf: top fruit; Ch: chrysanthemum; G: general use.

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emergence. For reliable results, deliberate intro-duction may be necessary.The culturing and meas-ured application of certain predators and parasitesare now common features of greenhouse plant pro-tection, particularly where pests have developedresistance to chemicals. The greenhouse representsa controlled environment where pest–predatorinteractions can be accurately predicted. Develop-ments in outdoor plant production may yield similarly successful results in future. Some of themore effective species being used are describedbelow and the wider selection of species is given inTable 12.1.

Phytoseiulus persimilis (see Figure 12.1)

This is a globular, deep orange, predatory tropicalmite used in greenhouse production to controlglasshouse red spider mite (see page 114). It israised on spider mite-infected beans, and thenevenly distributed throughout the crop, e.g. cucum-bers at the rate of one predator per plant. Somegrowers who suffer repeatedly from the pest firstintroduce the red spider mite throughout the cropat the rate of about five mites per plant a weekbefore predator application, thus maintaining evenlevels of pest–predator interaction. The predator’s

short egg–adult development period (7 days),laying potential (50 eggs per life cycle) and appe-tite (five pest adults eaten per day), explain itsextremely efficient action.

Encarsia formosa (see Figure 12.2)

This is a small (2 mm) wasp, which lays an egg intothe glasshouse whitefly (see page 107) scale, caus-ing it to turn black and eventually release anotherwasp. This parasite is raised on whitefly-infestedtobacco plants. It is introduced to the crop, e.g.tomato, at a rate of 100 blackened scales per 100plants. The parasite’s introduction to the crop issuccessful only when there is less than one white-fly per 10 plants, and its mobility (about 5 m) andsuccessful parasitism are effective only at tempera-tures greater than 22°C when its egg-laying abilityexceeds that of the whitefly. The wasp lays most of


Figure 12.1 Phytoseiulus predator: note the differencein appearance between the round, long-legged preda-tors (left) and the glasshouse red spider mite (right)(courtesy: Glasshouse Crops Research Institute).

Figure 12.2 Encarsia parasite of glasshouse whitefly(courtesy: Glasshouse Crops Research Institute).

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its 60 or more eggs within a few days of emergencefrom the black scale.Thus a series of weekly or fort-nightly applications from late February onwardsensures that parasite egg-laying covers the suscep-tible whitefly scale stage.

A combination of biological methods may beused on some crops, e.g. chrysanthemums, toma-toes and cucumbers, in order to control a range oforganisms (see integrated control). Biological con-trol programmes enable pollution-free reductionof some major pests without the development ofresistant pest organisms. An understanding of thepests’ and biological control organisms’ life cyclesis, however, necessary to ensure success. Severalspecialist firms now have contracts to apply bio-logical control organisms to greenhouse units.

The careful selection of pesticides (see pirimi-carb) enables simultaneous biological and chem-ical control, to the grower’s advantage.

CHEMICAL CONTROL

This method of control aims ideally to use a selectedchemical for reduction of weed, pest or diseasewithout harming man, crop or wildlife. This aim isnot always achieved, although increasingly strin-gent demands are placed on the chemical manu-facturer, both in terms of the chemical’s efficiencyand its safety. In past centuries, pests, e.g. applewoolly aphid, were sprayed with natural products,e.g. turpentine and soap, while weeds were removedby hand. In the nineteenth century, the chancedevelopment of Bordeaux mixture from inorganiccopper sulphate and slaked lime, and in the earlytwentieth century the expansion of the organicchemical industry, enabled a change of emphasis incrop protection from cultural to chemical control.The word ‘pesticide’ is used in this book to coverall crop protection chemicals, which include herbi-cides (for weeds), insecticides (for insects), acari-cides (for mites), nematicides (for nematodes) andfungicides (for fungi). About 3 million tonnes ofcrop protection chemicals are used worldwide eachyear, two fifths being herbicides, two fifths insecti-cides and one fifth fungicides.

Active ingredient. Each container of commer-cial pesticide contains several ingredients. Theactive ingredient’s role is to kill the weed, pest ordisease. More detailed lists of the range of activeingredients can be found in government literature.The other constituents of pesticides are describedunder formulation.

Herbicides

Herbicides that are applied to the seedbed orgrowing crop must have a selective action, i.e. killthe weed but leave the crop undamaged.

This selective action may succeed for one ofseveral reasons. Chemicals often affect differentplant families in different ways. The broad-leavedturf weed, daisy (Bellis perennis, a member of theAsteraceae) is controlled by 2,4-D, leaving the turfgrasses (Graminae) unaffected.

Sometimes plant species are affected by differ-ent concentrations of the chemical to a degree thatcan be exploited. The correct concentration ofselective chemicals may be vital if the crop is toremain unharmed. The following relative values(parts per million, ppm) for the amount of propy-zamide herbicide required to kill different plantspecies illustrate this point:

• Crops– carrot 0.8;– cabbage 1.0;– lettuce 78.0;

• Weeds– knotgrass 0.08;– black nightshade 0.2;– fat hen 0.2;– pennycress 0.6;– groundsel 78.0.

It can thus be seen that a concentration of25 ppm of propyzamide applied to lettuce wouldleave the crop unaffected, but control all the weedsexcept groundsel. A low concentration of simazineis used to control annual weeds around raspberryplants, while a high concentration gives total con-trol on paths and other uncultivated areas.


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A third form of selectivity operates by correcttiming of herbicide application. A seedbed withcrop seeds below and weed seeds germinating at the surface may receive a contact chemical,e.g. paraquat, which permits germination of thecrop without weed competition. A similar effect isachieved when a residual herbicide, e.g. propachlor,is sprayed onto the soil surface to await weed seedgermination. The situations for weed control aresummarized in Figure 12.3.

Herbicides may conveniently be divided intotwo main groups: the foliage-acting, and the soil-acting (residual) chemicals.

Foliage-acting herbicides. These enter the leafthrough fine pores in the cuticle, or the stomata.The herbicide may move through the vascular sys-tem (translocated chemicals) to all parts of theplant before killing plant cells, or it may kill on

contact with the leaf. Four active ingredients aredescribed, each belonging to a different chemicalgroup, and each having a different effect on weeds:

1 Paraquat is commonly used to scorch and killtop growth of a wide spectrum of weeds in staleseedbeds, after harvest, in perennial crops, or inwaste land.Although translocated when in diluteconcentrations, its rapid, light-induced unselec-tive contact action is most commonly utilized. Itis quickly absorbed, in damp soils, by clay parti-cles, thus allowing planting soon after its appli-cation. Its absorption prevents any problem ofresidual action except in extremely dry sum-mers. It should never be used on foliage of grow-ing plants, although carefully directed spraysbetween the rows of growing soft fruit are rec-ommended.

2 Amitrole is used in similar situations to paraquat,but is more residual, surviving in the soil for sev-eral weeks. It stops photosynthesis, scorchingboth grass and broad-leaved weeds (unselec-tive). It is especially useful on uncropped landand, when applied in autumn, it is translocatedto underground rhizomes of couch that are thenkilled. It should not be sprayed onto the foliageof growing plants.

3 2,4-D is an auxin and causes uncontrolled abnor-mal growth on leaves, stems and roots of broad-leaved weeds, which eventually die. It is a usefulselective herbicide on turf because the protectedmeristems of grasses can survive unaffected.It must be kept well away from nearby borderplants and from some crops, e.g. tomatoes, whichare extremely sensitive to minute quantities.

4 Glyphosate enters the foliage of actively grow-ing annual and perennial weeds (unselective)and is translocated (see page 58) to under-ground organs, subsequently killing them. It iscommonly used several weeks before drilling orplanting of crops, around perennial plants suchas apples or in established nursery stock trees.Glyphosate is inactivated in soils (particularlypeats), thus preventing damage to newly sowncrops. It may cause damage if spray-drift toadjoining plants or fields occurs.


SOIL TREATMENTRESIDUAL

FOLIAGE TREATMENTCONTACT OR

TRANSLOCATED

OverallPre-sowing

OverallPre-emergence

OverallPost-emergence

DirectedPost-emergence

BandPre-emergence

Weed

Crop

Key

Figure 12.3 Types of herbicide action (Reproduced bypermission of Blackwell Scientific Publications).

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Soil-acting herbicides. These are either sprayedonto the soil surface (see Figure 12.3) or soil incorp-orated. They must be persistent (residual) for sev-eral weeks or months to kill the seedling before orafter it emerges. Root hairs are the main point ofentry. Increased rates may be necessary for peatsoils that inactivate some herbicides.The chemicalmay be applied as a spray or granule before thecrop is sown (pre-sowing stage), before the cropemerges (pre-emergence) or, with more selectivechemicals,after the crop emerges (post-emergence).Three active ingredients are described, each belong-ing to a different chemical group, and each havinga different effect on weeds:

1 Chlorpropham, a relatively insoluble compound,is applied as a pre-emergent spray to controlmany germinating weeds species, e.g. chickweed,in crops such as bulbs, onions, carrots and let-tuce. It usually persists for less than 3 months inthe soil. In light, porous soils with low organicmatter, its rapid penetration to underlying seedsmake it an unsuitable chemical. Earthworm num-bers may be reduced by its presence.

2 Propachlor, a relatively insoluble compound, isapplied as a pre-sowing or pre-emergent spray tocontrol a wide variety of annual weeds in bras-sicas, strawberries, onions and leeks. For weeds inestablished herbaceous borders (e.g. rose), thegranular formulation gives a residual protectionagainst most germinating broad-leaved andgrass weeds.

3 Dichlobenil gives total control against ger-minating weeds, couch grass and some perennialweeds in waste ground, soft fruit, top fruit andestablished ornamental shrub and tree areas.The compound remains in the soil for more than6 months, and young crops should not be plantedwithin a year of its application.

Mixtures. The horticulturist must deal with awide range of annual and perennial weeds. Thesomewhat specialized action of some of the herbi-cide active ingredients previously described maybe inadequate for the control of a broad weed spec-trum. For example, in the case of chlorpropham, the

addition of diuron enables an improved control of charlock and groundsel, while a different for-mulation containing chlorpropham plus linuron isdesigned to have greater contact action and thuscontrol both established and germinating weeds inbulb crops. Careful selection of the most suitablemixture of active ingredients is therefore neces-sary for a particular crop/weed situation.

Insecticides and acaricides

The insects and mites have three main points ofweakness for attack by pesticides which are as follows: their waxy exoskeletons (see Figure 10.2)may be penetrated by wax-dissolving contact chem-icals; their abdominal spiracles allow fumigantchemicals to enter tracheae; and their digestivesystems, in coping with the large food quantitiesrequired for growth, may take in stomach poisons.Four main groups of insecticides are described(Details of associated hazards to spraying oper-ators and the general public are discussed later inthis chapter:

1 Dormant-season control of pests may be achievedon trees such as apple by the use of toxic contactinsecticides, e.g. tar oil. It kills eggs (ovicide) ofaphids, red spider mite and capsids, but usefulpredators are also eliminated (see integratedcontrol). Most other insecticides are active on amore limited number of pests.

2 Malathion belongs to the organophosphorusgroup. It enters through the cuticle and, onreaching the nervous system, interferes with‘messages’ crossing the nerve endings of pestssuch as aphids, mealy bug, flies and red spidermites.The related chemical dimethoate is slightlysoluble in water, and may systemically movethrough the xylem and phloem tissues of theplant before being sucked up by the aphid stylet.The long-lasting (residual) property of anotheringredient, chlorpyriphos, enables its use as asoil insecticide against emerging root fly larvae.

3 Pirimicarb belongs to the third (carbamate)group, is slightly systemic, and controls many


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aphid species without affecting beneficial lady-birds. Aldicarb combines a soil action againstnematodes with a systemic, broad-spectrumactivity against foliar pests, e.g. aphids, whitefly,leaf miners, mites and nematodes of ornamentalplants.This group of chemicals acts on the insectnervous system in a similar manner to theorganophosphorus group.

4 Chemicals derived from plants. Nicotine, a nat-ural extract from tobacco, is used as a spray orsmoke, and enters the spiracles of aphids, capsids,leaf miner adults and thrips to reach their ner-vous system. It persists only briefly on the plant.Pyrethrum, a natural extract from a chrysanthe-mum species, has been largely superseded by a range of related synthetic products (calledpyrethroids). Cypermethrin is one such chem-ical which has stomach and contact action againstaphids, caterpillars and thrips. Derris, an extractfrom a tropical legume (and originally a fish-pond poison), interferes with the respiration ofa wide range of insects and mites. The insecti-cide diflubenzuron acts selectively on cater-pillars and sciarid fly larvae by disturbing theinsect’s moulting.

Nematicides

No active ingredients are, at present, availableexclusively for nematode control. Soil-inhabitingstages of cyst nematodes, stem and bulb eelworm,and some ectoparasitic root eelworms are effect-ively reduced by soil incorporation of granularpesticides, e.g. aldicarb at planting time of cropssuch as potatoes and onions. This group of chem-icals acts systemically on leaf nematodes of plantssuch as chrysanthemum and dahlia.

Fungicides

Fungicides must act against the disease but notseriously interfere with plant activity. Protectantchemicals prevent the entry of hyphae into roots,and the germination of spores into leaves andother aerial organs (see Figure 11.2). Systemic

chemicals enter roots, stems and leaves, and aretranslocated to sites where they may affect hyphalgrowth and prevent spore production. Althoughthere are many fungicidal chemical groups, threeare chosen here as examples:

1 Inorganic chemicals (i.e. containing no carbon)have long been used to protect horticulturalcrops. Copper salts, when mixed with slakedlime (Bordeaux mixture) form a barrier on theleaf to fungi, e.g. potato blight. Fine-grained(colloidal) sulphur controls powdery mildews,and may be heated gently in greenhouse ‘sulphurlamps’ to control this disease by vapour action onplants such as roses.

2 Organic chemicals contain carbon. Mancozeb(dithiocarbamate group) and related syntheticcompounds act protectively on a wide range of quite different foliar diseases, e.g. downymildews, celery leaf spot and rusts, by prevent-ing spore germination.

3 Carbendazim (benzimidazole group) is an example of a systemic ingredient, which movesupwards through the plant’s xylem tissues, slow-ing hyphal growth and spore production of fun-gal wilts, powdery mildews and many leaf spotorganisms. Damping off, potato blight and downymildews are unaffected by this chemical group.Many different systemic groups are now used in horticulture.

About 200 active ingredients for weed, pest anddisease control are available. Some are withdrawnand some introduced every year. The DEFRA andcommercial literature are constantly updated tokeep growers informed.

Resistance to pesticides

The development of resistant individuals from the millions of susceptible weeds, pests and dis-eases occurs most rapidly when exposure to a par-ticular chemical is continuous, or when a pesticideacts against only one body process of the organ-ism. Resistance, e.g. in aphids, to one member,


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e.g.malathion,of a chemical group confers resistanceto other chemicals in the same (e.g. organophos-phorus) group. Horticulturists should thereforefollow the strategy of alternating between differ-ent groups and not simply changing active ingredi-ents. Particular care should be taken with systemicchemicals that present to the organism inside theplant a relatively weak concentration against whichthe resistant organism can develop. Increase indosage of the chemical will not, in general, providea better control against resistant strains. Biologicalcontrol, unlike chemical control, does not createresistant pests.

Formulations

Active ingredients are mixed with other ingredi-ents to increase the efficiency and ease of applica-tion, prolong the period of effectiveness or reducethe damaging effects on plants and man.The wholeproduct (formulation) in its bottle or packet is givena trade name, which often differs from the name ofthe active ingredient.The main formulations are asfollows:

• Liquids (emulsifiable concentrates) contain alight oil or paraffin base in which the active ingre-dient is dissolved. Detergent-like substances(emulsifiers) present in the concentrate enablea stable emulsion to be produced when the for-mulation is diluted with water. In this way, thecorrect concentration is achieved throughoutthe spraying operation. Long chain molecularcompounds (wetter/spreaders) in the formula-tion help to stick the active ingredients onto theleaf after spraying, particularly on smooth, waxyleaves such as cabbage.

• Wettable powders containing extremely smallparticles of active ingredient and wetting agentsform a stable suspension for only a short periodof time when diluted in the spray tank. Continu-ous stirring or shaking of the diluted formula-tion is thus required. An inert filler of clay-likematerial is usually present in the formulation toease the original grinding of particles, and also

to help increase the shelf life of the product. Itis suggested that this formulation is mixed intoa thin paste before pouring through the filter ofthe sprayer. This prevents the formation oflumps that may block the nozzles.

• Dusts are applied dry to leaves or soil, and thusrequire less precision in grinding of the con-stituent particles, and less wetting agent.

• Seed dressings protect the seed and seedlingagainst pests and diseases. A low percentage of active ingredient, e.g. iprodione applied in an inert clay-like filler or liquid reduces the risk of chemical damage to the delicate germin-ating seed.

• Baits contain attractant ingredients, e.g. branand sugar mixed with the active ingredient, e.g.methiocarb, both of which are eaten by thepest, e.g. slugs.

• Granules formulated to a size of about 1.0 mmcontain an inert filler, e.g. pumice or charcoal,onto which is coated the active ingredient. Gran-ules may act as soil sterilants (e.g. dazomet),residual soil herbicide (e.g. dichlobenil), residualinsecticide (e.g. chlorpyrifos), or broad-spectrumsoil nematicide and insecticide (e.g. aldicarb).Granular formulations normally present fewerhazards to the operator and fewer spray-driftproblems.

• Smokes containing sodium chlorate, a sugar andheat resistant active ingredient, e.g. dicloran,produce, on ignition, a vapour that reaches allparts of a greenhouse to fumigate against dis-eases, e.g. Botrytis.

Labels on commercial formulations give detailsof the active ingredient contained in the product.Application rates for different crops are included.The DEFRA approves pesticide products foreffectiveness..

Application of herbicides and pesticides

This subject is described in detail in machinerytexts. However, certain basic principles related tothe covering of the leaf and soil by sprays will be


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mentioned.The application of liquids and wettablepowders by means of sprayers may be adjusted interms of pressure and nozzle type to provide therequired spray rate. Cone nozzles produce a tur-bulent spray pattern suitable for fungicide andinsecticide use, while fan nozzles produce a flatspray pattern for herbicide application. In periodsof active plant growth, fortnightly sprays may benecessary to control pests and diseases on newlyexpanding foliage. High volume sprayers applythe diluted chemical at rates of 600–1000 l/ha inorder to cover the whole leaf surface with dropletsof 0.04–0.10 mm diameter. Cover of the under-leafsurface with pesticides may be poor if nozzles arenot directed horizontally or upwards. Soil appliedchemicals, e.g. herbicides or drenches, may besprayed at a larger droplet size, 0.25–0.5 mm indiameter, through a selected fan nozzle. The cor-rect height of the sprayer boom above the plant isessential for downward-directed nozzles if thespray pattern is to be evenly distributed. Savingscan be achieved by band spraying herbicides innarrow strips over the crop to leave the inter-rowfor mechanical cultivation.

Medium volume (200–600 l/ha) and low vol-ume (50–200 l/ha) equipment, such as knapsacksprayers, apply herbicides and pesticides onto theleaf at a lower droplet density, and in tree crops,mist blower equipment creates turbulence, andtherefore increased spray travel, by means of apower driven fan. Ultra-low volume sprays (up to50 l/ha) are dispersed on leaving the sprayer by arapidly rotating disc which then throws regular-sized droplets into the air. Larger droplets (about0.2 mm) are created by herbicide sprayers to pre-vent spray-drift problems, while smaller droplets(about 0.1 mm) allow good penetration and leafcover for insecticide and fungicide use.

Fogging machines used in greenhouses andstores produce very fine droplets (about 0.015 mmdiameter) by thermal and mechanical methods,and use small volumes of concentrated formula-tion (less than 1 l in 400 cm3) which act as fumi-gants in the air, and as contact pesticides whendeposited on the leaf surface. Dust and granuleapplicators spread the formulations evenly over

the foliage, or ground surface. When mounted on seed drills and/or fertilizer applicators, granulesmay be incorporated into the soil. Care must be taken to ensure good distribution to preventpesticide damage to germinating seeds or plantingmaterial.

Integrated control

Integrated control increasingly termed IntegratedPest Management requires the grower to under-stand all types of control measure, particularlybiological and chemical, in order that they com-plement each other. In greenhouse production ofcucumbers, the Encarsia formosa parasite andPhytoseiulus persimilis predator are used for white-fly and red spider mite control respectively. How-ever, the other harmful pest and disease speciesmust be controlled, often by chemical means,but without killing the parasites and predators.Drenches of systemic insecticide, e.g. pirimicarbagainst aphids, soil insecticides, e.g. deltamethrinagainst thrips pupae, and systemic fungicidedrenches, e.g. carbendazim against wilt diseasesand powdery mildew, are all applied away fromthe sensitive biological control organisms. Simi-larly, high volume sprays of selective chemicals,e.g. dichlofluanid against grey mould, Bacillusthuringiensis extracts against caterpillars, have lit-tle or no effect on the parasite and predator.Similar considerations may be given in control of apple pests and diseases. Reduced usage ofextremely toxic winter washes, and increased useof selective caterpillar and powdery mildew con-trol by chemicals, e.g. diflubenzuron and fenarimolrespectively, allow the almost unhindered build-up of beneficial organisms, e.g. predatory capsidsand mites.

The methods of organic growers emphasize thenon-chemical practices in plant protection (as wellas in soil fertility). Hedges are developed within100 m of production areas and are clipped only 1year in 4 to maintain natural predators and para-sites. Rotations are closely followed to enable soil-borne pest or disease decline, while encouraging


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soil fertility. Resistant cultivars of plant are chosen, and judicious use of mechanical cultivationsand flame weeding enables pests, diseases andweeds to be exposed or buried. A restricted choiceof pesticide products such as pyrethrins, derris,metaldehyde (with repellant), sulphur, copper saltsand soft soap are allowed to be applied, should theneed arise. Bacillus thuringiensis extract, and alsopheremone attractants, are similarly used. Table12.2 gives a list of permitted substances.

Supervised control

Most plants can tolerate low levels of pest and disease damage without yield reduction unless the damage is to parts of the plant that becomeunacceptable (e.g. fruits for the supermarkettrade). Thus, cucumbers require more than 30 percent leaf area affected by red spider mite before

economic damage occurs in terms of yield loss.This enables methods of control that depend onsome damage being done to ensure continued suc-cess such as the use of predators. Damage assess-ments are used in apple orchards to decide whethercontrol measures are necessary. Thus, at green-cluster stage (before flowers emerge) chemicalsprays are considered only when an average ofhalf the observed buds have five aphids/bud.Similarly, three winter moth larvae per bud-clustermerit control at late blossom time.

Pheremone traps enable the precise time of max-imum codling moth emergence to be determined in early June. Catches of less than 10 moths per trap per week do not warrant control.The DEFRAissue spray warning information to growers whenserious pests, e.g. carrot root fly, and diseases, e.g.potato blight, are likely to occur. Supervised controlmay greatly reduce pesticide costs.

Safety with herbicides and pesticides

The basic biochemical similarities between allgroups of plants and animals demand careful exam-ination of active ingredients before their com-mercial release; otherwise serious damage or deathto non-target organisms, particularly humans, mayresult.

Regulations under the Food and EnvironmentalProtection Act 1985 (FEPA) control the approvalof pesticide products in terms of their safety tohumans and wildlife.The lethal dose figure (LD50),expressing the amount of active ingredient in milli-grams which kills 1 kg of animal tissue, dictates toa large extent the precautions needed for a growerto safely mix and apply a product. The lower theLD50 figure for a chemical, the more toxic it is.Thelabel present on each packet or bottle of pesticidemust provide detailed instructions with regard toprecautions and protective clothing required for achemical’s safe usage. The amount of protectionstated on the label commonly reflects the LD50 sta-tus of the toxic ingredient. Highly toxic chemicals,e.g. the soil-sterilant methyl bromide (LD50 � 1),must be applied only by specially trained personnel.


Table 12.2 Permitted products for plant pest and diseasecontrol in organic crop production

Preparations on basis of metaldehyde containing a repellent to higher animal species and as far as possible applied within traps

Preparations on basis of pyrethrins extracted from Chrysanthemum cinerariaefolium, containing possibly a synergist

Preparations from Derris ellipticaPreparations from Quassia amaraPreparations from Ryania speciosaPropolisDiatomacious EarthStone mealSulphurBordeaux mixtureBurgundy mixtureSodium silicateSodium bicarbonatePotassium soap (soft soap)Pheremone preparationsBacillus thuringiensis preparationsGranulose virus preparationsPlant and animal oilsParaffin oil

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The Control of Substances Hazardous toHealth Regulations 1988 (COSHH) requires thehorticulturist to assess whether each pesticideapplication is necessary. They also require the useof the safest ingredients, equipment and protectiveclothing to reduce the risk to humans.

Health and Safety Regulations 1975, sum-marized in the government ‘Poisonous Chemicals on the Farm’ leaflet, specify the correct proced-ures for pesticide use. A detailed register must be kept of spraying operations and any dizzinessor illness reported. Correct washing facilities mustbe provided. A lockable dry store is necessary to keep chemicals safe.Warnings of spraying oper-ations should be prominently displayed. Suitablefirst aid measures should be available. Used pesti-cide containers should be burnt or buried in a safeplace.

Protective clothing appropriate to the pesticideshould be worn. A one-piece fabric coverall suitwith a hood protects most of the body from dilutedpesticide. Rubberized suits may be used in condi-tions of greater danger, e.g. in an enclosed green-house environment when dealing with ultra-lowvolume spray, or when applying upward-directedsprays into orchards. Rubber boots should be worninside the legs of the suit. Thick gauge gauntletsare worn outside the suit when dealing with con-centrates, but inside when spraying. Face shieldsshould be worn when mixing toxic concentrates.A face mask covering the mouth and/or nose iscapable of filtering out less toxic active ingredientsfrom sprays, but a respirator with its large filter isrequired for toxic products, particularly whenused in greenhouses where toxic fume levels buildup.

Wildlife. Pesticides should not be sprayednear ponds and streams unless designed foraquatic weed control. Crops frequented by bees,e.g. apples and beans, should be sprayed withinsecticides only in the evening when most of theinsects’ foraging has ceased. Beekeepers shouldbe informed of spraying operations. Very long-lasting insecticides, e.g. gamma-HCH and DDT,are no longer used because they were shown toaccumulate in concentration along the food chains

(see page 10). The levels of pesticide present inpredators such as hawks rose to such high levelsthat legislative action became a necessity. Publicand government awareness of pesticide dangershave resulted in more wide-ranging legislationunder Section III of the FEPA. This requires thatchemical manufacturers, distributors and profes-sional horticulturists show demonstrable skills inthe choice and application of pesticides. The Actalso seeks to make information about pesticidesavailable to the public.

Phytotoxicity

Phytotoxicity (or plant damage) may occur whenpesticides are unthinkingly applied to plants. Soilapplied insecticides, e.g. aldicarb, cause pot plants,e.g. begonias, to go yellow if used at more than therecommended rate. Plants growing in greenhousesare more susceptible because their leaf cuticle isthinner than plants growing at cooler tempera-tures. Careful examination of the pesticide (partic-ularly herbicide) packet labels often prevents thisform of damage.

FURTHER READING

Alford, D. (editor) Pest and Disease ManagementHandbook (Blackwell Science Ltd, 2000).


Cremlyn, R.J. Agrochemicals, Preparation and Mode ofAction (Wiley, 1991).

Debach, P. Biological Control by Natural Enemies(Cambridge University Press, 1991).

Dent, D. Integrated Pest Management (Chapman & Hall,1995).

Hance, R.J. and Holly, K. Weed Control Handbook,Vol. 1 (Blackwell Scientific Publications, 1990).

Helyer, N. et al. A Colour Handbook of BiologicalControl in Plant Protection (Manson Publishing, 2003).

Hussey, N.W. and Scopes, N. Biological Pest Control(Blandford Press, 1985).

Ingram, D.S. et al. (editors) Science and the Garden(Blackwell Science Ltd, 2002).


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Ivens, G.W. Plant Protection in the Garden (British CropProtection Council, 1989).

Lever, B.E. Crop Protection Chemicals (Ellis Horwood,1990).

Lloyd, J. Plant Health Care for Woody Ornamentals(1997).

Van Emden, H.F. and Peahall, D. Beyond Silent Spring –Integrated Pest Control and Chemical Safety (Chapman& Hall, 1996).

Whitehead, R. (editor) UK Pesticide Guide (CABIPublishing, 2003).


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ROOT REQUIREMENTS

The growing tip of the root wriggles through thegrowing medium following the line of least resist-ance. Roots are able to enter cracks that are, or canbe readily opened up to, about 0.2 mm in diameter,which is about the thickness of a pencil line. Com-pacted soils severely restrict root exploration butonce into these narrow channels the root is able toovercome great resistance to increase its diameter.Anything, which reduces root exploration andactivity, can limit plant growth.When this happensaction must be taken to remove the obstruction toroot growth or to supply adequate air, water andnutrients through the restricted root volume.

The root normally provides the anchorage neededto secure the plant in the soil. Plants, notably treeswith a full leaf canopy, become vulnerable if theirroots are in loose material, in soil made fluid by highwater content or are restricted, e.g. shallow rootsover rock strata close to the surface. Until their roots

13

Soil as a Growing Medium

The plant takes up water and nutrients fromthe growing medium through its roots, whichalso provide anchorage (see root structure).The root also requires a supply of oxygen andproduces carbon dioxide, which is harmful ifit builds up in the root zone (see respiration).

In order to secure water throughout the sea-son the roots penetrate deep into the soil. Theplant nutrients are extracted from a very dilutesoil solution and so the roots must explore thor-oughly in the top layers to maintain nutrientsupplies. Consequently, the roots are normallyvery extensive and with a shape that brings themaximum absorptive surface into contact withsoil particle surfaces around which is the waterand nutrients.Within one growing season a sin-gle plant growing in open ground developssome 700 km of root, which has a surface areaof 250m2; and 700m2 if the surface area includesthe root hairs. The vast root system that devel-ops is usually more than is required to supplythe plant in times of plenty but the extent of theroot system is indicative of what the plant needsto protect itself against unfavourable condi-tions. It also serves to remind horticulturists ofwhat has to be provided when growth isrestricted accidentally or deliberately. Whengrowing in containers, there is the opportunityto provide an ideal growing medium but thecontinued supply of water and nutrientsbecomes more critical (see Chapter 17).

In this chapter, the formation and develop-ment of different soils is explained and thecharacteristics of the main soil types found inthe British Isles are outlined. The nature ofsoil is detailed and its qualities as a root envi-ronment are explored. The basis of managingsoils as a growing medium for horticulturalplants is established.

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have penetrated extensively into the surroundingsoil, transplants are very susceptible to wind rocking:water uptake remains limited as roots becomedetached from the soil and delicate root growth isbroken off.The plant may be left less upright.

In order to grow and take up water and nutrients,the root must have an energy supply. A constantsupply of energy is only possible so long as oxygenis brought to the site of uptake (see respiration).Consequently the soil spaces around the root mustcontain air as well as water. There must be goodgaseous exchange between the atmosphere aroundthe root and the soil surface. This may sometimesbe achieved by the selection of plants that havemodifications of their structure that enables this tooccur throughout the plant tissues (see adapta-tions), but is normally a result of maintaining asuitable soil structure.A lack of oxygen or a build-up of carbon dioxide will reduce the root’s activity.Furthermore, in these conditions anaerobic bact-eria will proliferate and many produce toxins suchas ethylene. In warm summer conditions roots canbe killed back after 1 or 2 days in waterlogged soils.

COMPOSITION OF SOILS

Mineral soils form in layers of rock fragments overthe Earth’s surface. They are made up of mineralmatter comprising sand, silt and clay particles.Thereis also a small quantity of organic matter which isthe part derived from living organisms.This frame-work of solid material retains water and gases in thegaps or pore space. The water contains dissolvedmaterials including plant nutrients and oxygen,and is known as the soil solution. The soil atmos-phere normally comprises nitrogen, rather less oxy-gen and rather more carbon dioxide than in normalair and traces of other gases. Finally, a soil capableof sustaining plants is alive with micro-organisms(organisms, such as bacteria, fungi and nematodes,too small to be seen with the naked eye). Largerorganisms such as earthworms and insects are alsonormally present (see page 35).

The composition of a typical mineral soil is givenin Figure 13.1, which also illustrates the variation

that can occur.The content of the pore space variescontinually as the soil dries out and is re-wetted.Thespaces can be altered by the compaction or ‘openingup’ of the soil which in turn has a significant effecton the proportions of air and water being held.

Over a longer period the organic matter level canvary. The composition of the soil can be influencedby many factors and, under cultivation, these have tobe managed to provide a suitable root environment.Organic soils have a considerably higher organicmatter content and are dealt with in Chapter 15.

SOIL FORMATION

The Earth is formed from a ball of molten rockminerals. The least dense rocks floated on the topand as they cooled the surface layer of granite, withbasalt just below, solidified to form the Earth’scrust.The Earth’s surface has had a long and turbu-lent history during which it has frequently frac-tured, crumpled, lifted and fallen, with more moltenmaterial being pushed up from below through thebreaks in the crust and in volcanoes.

Weathering and erosion

From the moment rocks are formed and exposedto the elements they are subjected to weathering,


OMWATER

AIR

50–60%MINERALMATTER

PORE SPACE

SOLID FRACTION

Figure 13.1 Composition of a typical cultivated soil.The solid fraction of the soil is made up of mineral (50–60per cent) and organic matter (OM), (1–5 per cent). Thisleaves a total pore space of 35–50 per cent that is filled byair and water, the proportions of which vary constantly.

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Soil as a Growing Medium 153

the breakdown of rocks, and erosion, the move-ment of rock fragments and soil.

Chemical weathering is mainly brought aboutby the action of carbonic acid that is producedwherever carbon dioxide and water mix, as in rain-fall. Some rock minerals dissolve and are washedaway. Others are altered by various chemical reac-tions, most of which occur when the rock surface isexposed to the atmosphere. All but the inert partsof rock are eventually decomposed and the rockcrumbles as new minerals are formed and solublematerial is released. Oxidation is particularlyimportant in the formation of iron oxides, whichgive soils their red and yellow (when aerobic), or blue and grey colours (in anaerobicconditions).

Physical or mechanical weathering processesbreak the rock into smaller and smaller particleswithout any change in the chemical character ofthe minerals.This occurs on exposed rock surfacesalong with chemical weathering but, in contrast, haslittle effect on rocks protected by layers of soil.The main agents of physical weathering are frost,heat, water, wind and ice. In temperate zones, frostis a major weathering agent. Water percolates intocracks in the rock and expands on freezing. Thepressures created shatter the rock and, as the watermelts, a new surface is exposed to weathering. Inhot climates the rock surface can become very muchhotter than the underlying layers. The strains cre-ated by the different amounts of expansion and thealternate expansion and contraction cause frag-ments of rock to flake off the surface; this is some-times known as the ‘onion skin’ effect. Movingwater or wind carries fragments of rock that rubagainst other rocks and rock fragments, wearingthem down. Where there are glaciers the rock isworn away by the ‘scrubbing brush’ effect of ahuge mass of ice loaded with stones and bouldersbearing down on the underlying rock.

Biological weathering is attributable to organ-isms such as mosses, ferns and flowering plantswhich fragment rock by both chemical and physicalmeans, e.g. they produce carbon dioxide which, inconjunction with water, forms carbonic acid; rootspenetrate cracks in the rock and, as they grow

thicker, they exert pressure which further opensup the cracks.

Igneous rocks

Igneous rocks are those formed from the moltenmaterial of the Earth’s crust. All other rock types,as well as soil, are ultimately derived from them.When examined closely, most igneous rocks canbe seen to be a mixture of crystals. For example,granite contains crystals of quartz, white and shiny,felspars that are grey or pink, and micas, which areshiny black (see Plate 19). Many of these crys-talline materials have a limited use in landscapingas formal structures rather than in the construc-tion of rock gardens.

As granite is weathered (‘rotted’) the felsparsare converted to kaolinite (one of the many formsof clay) and soluble potassium, sodium and cal-cium, which are the basis of plant nutrition. Simi-larly, the mica present is chemically changed.Whilst the clays retain much of the potassium, theother soluble material is carried by water to thesea making the sea ‘salty’. The inert quartz grainsare released and form sand grains.

Sedimentary rocks

Sedimentary rock is derived from accumulatedfragments of rock. Most have been formed in thesea or lakes to which agents of erosion carry wea-thered rock. Organisms in the seas with shells dieand accumulate on the bottom of the sea. Layersof sediment build up and, under pressure and slowchemical change, eventually become rock stratasuch as shale, chalk or limestone. In subsequentearth movements much of it has been raised upabove sea level and weathered again. Similarly, thesand grains that accumulate to great depths indesert areas eventually become sandstones.

Moving water and winds are able to carry rockparticles and are thus important agents of erosion.As their velocity increases, the ‘load’ they are ableto carry increases substantially. The fast-moving

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water in streams is able to carry large particles butin the slower-moving rivers some of the load isdropped. The particles settle out in order of size(see settling velocities). This leads to the sorting ofrock fragments, i.e. material is moved and depositedaccording to particle size. By the time the rivershave reached the sea or lakes only the finest sands,silts and clays are in the water. As the river slowson meeting the sea or lake all but clay is dropped.The clay eventually settles slowly in the still waterof the sea or lake. Moving ice is also an agent oferosion but the load dropped on melting consistsof unsorted particles known as boulder clay or till.

The type of sedimentary rock formed dependson the nature of its ingredients. Sandstones, silt-stones and mudstones are examples of sedimentaryrocks derived from sorted particles in which char-acteristic layering is readily seen (see Plate 19).Limestones are formed from the accumulation ofshells or the precipitation of materials from solu-tion mixed with varying amounts of deposited mud.Chalk is a particularly pure form derived from thecalcium carbonate remains of minute organismsthat lived in seas in former times. Many of theseare attractive materials for use in hard landscap-ing, where care should be taken to align the strata(layers) for a natural effect.

Metamorphic rocks

Metamorphic rock is formed from igneous or sedi-mentary rocks.The extreme pressures and tempera-tures associated with movements and fracturing inthe Earth’s crust or the effect of huge depths ofrock on underlying strata over very long periodsof time has altered them. Slate is formed fromshale, quartzite from sandstone, and marble fromlimestone. Metamorphic rock tends to be moreresistant to weathering than the original rock.

SOIL DEVELOPMENT

Soil development occurs in the loose rock fragmentsoverlying the Earth’s crust. This is the parent

material that has an important effect on the natureof the soil formed. However, the soil formed is also influenced by climate, vegetation, topog-raphy, drainage as well as animals including manand time. Young soils (regosols) are those thathave just formed, but over time they take on char-acteristics that depend on the influence of theother factors to give rise to the main soil types.

Topography is the detailed description of land fea-tures that are the result of the interaction betweenthe underlying rocks, the agents of weathering anderosion, and time. The landscape may look asthough it is quite stable but even dramatic mountainranges are worn down to flat plains, a process thathas occurred several times in the history of theEarth following the numerous crustal disturbances.

On rocks and rock debris plant growth beginswith mosses and lichens that help to stabilize theloose material by binding the surface and reducingwind speed over the surface. The dead plantsbecome incorporated in the young soils and theirbreakdown products also help hold it together.The released nutrients are held in the top surfaceand recycled by a complex colony of soil organ-isms instead of being washed away (see nutrientcycles). As the young soil deepens, larger plantsare able to develop. Their roots help stabilize thesoil further and, along with the carbonic acid theyproduce, extend the depth of weathering. Theprocess of soil formation, which takes thousandsof years, speeds up as the particles become morefinely divided (see surface area) and the livingorganisms become established.

The physical characteristics of soil in the fieldare normally described with reference to thelayers, or horizons, revealed by digging a soil pit(see Figure 13.2). In the soil profile so exposed, theorganic litter on the surface that has not beenincorporated in the soil is usually referred to as the‘L’ layer. The upper layer of the soil from whichcomponents are normally washed downward isthe ‘A’ horizon, usually recognized by its darkercolouring, which is a result of the significant levelsof humus present.The lighter layer below it, wherefiner materials tend to accumulate, is the ‘B’ orilluvial horizon. Under cultivation, the ‘A’ horizon


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broadly aligns with the ‘topsoil’ and the ‘B’ with the‘subsoil’.The parent material below these is the ‘C’horizon and where there is an underlying unwea-thered rock layer it is often known as bedrock.

Sedentary soils

Sedentary soils develop in the material graduallyweathered from the underlying rock. True seden-tary soils are uncommon because most loose rockis eroded, but the same process can be seen wheregreat depths of transported material have formedthe parent material, as in the boulder clays leftbehind after the Ice Ages.A hole dug in such a soilshows the gradual transition from unweatheredrock to an organic matter rich topsoil (Figure13.2). Under cultivation a distinct topsoil developsin the plough zone.

Transported soils

Transported soils are those that form in erodedmaterial that has been carried from sites of wea-thering sometimes many hundreds of miles awayfrom where deposition has occurred, e.g. by gla-ciers. They can be recognized by the definiteboundary between the eroded material and theunderlying rock and its associated rock fragments.

Where more than one soil material has beentransported to the site, as in many river valleys,several distinct layers can be seen. The right-handpart of Figure 13.2 shows an example.

Raindrops striking soil dislodge loose particlesthat tend to move downhill. As a result, surfacesoil is slowly removed from higher ground andaccumulates at the bottom of slopes. This meansthat soils on slopes tend to be shallow, whereas atthe bottom of the slopes deep, transported soils


I II III

TRANSPORTED SOILSTAGES IN THE FORMATION OF SEDENTARY SOILS

B(subsoil)

A(topsoil)

C(parent material)

horizons:leaf litter

originalrock

e.g.alluvium

mosses

Figure 13.2 The development from a young soil consisting of a few fragments of rock particles to a deep sedentary soilis shown alongside a transported soil. A subsoil, topsoil and leaf litter layer can be identified in each soil.

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develop, known as colluvial soils. Material washedaway in running water eventually settles outaccording to particle size.The river valley bottomsbecome covered with material, called alluvium, inwhich alluvial soils develop. Wind removes drysands and silts that are not ‘bound in’ to the soiland large areas have become covered with wind-blown deposits known as ‘loess’ or ‘brick earth’.Many of these transported soils provide idealrooting conditions for horticultural crops becausethey tend to be deep, loose and open. Most areeasily cultivated. However, those that have a highsilt or fine sand content, notably the brick earths,may be prone to compaction.

SOIL TYPES

Soils continue to undergo changes under the influence of climate, vegetation, topography anddrainage which interact over time to give rise, inthe British Isles, to four main types of mineral soil:brown earths, gleys, rendzinas and podsols (seePlate 22). Peats develop in waterlogged conditions(see organic soils, page 190).

Brown earth soils develop in the well-drainedmedium to heavy soils in the lowlands of the BritishIsles. They are associated with a climax vegetationof broadleaved woodland especially oak, ash andsycamore, the roots of which have ensured thatnutrients moving down the soil profile are capturedand returned to the soil via the leaf fall. Surpluswater does not accumulate and the soil remainsaerobic for most of the year. The plentiful earth-worms incorporate the deep litter layers. Theresultant dark A horizon (‘topsoil’) rich in organicmatter (a ‘mull’) mergers gradually into a brightbrown and deep B horizon (‘subsoil’). The soilstructures that develop in the surface layers aregranular and rounded fine blocky in which there isan excellent balance of air and water and intowhich roots can readily penetrate.

These soils are usually mildly acid (pH 5.5–6.5),but acid brown earths (pH 5.5–4.5) can develop onlighter-textured soils in wetter areas (800–1000 mmper year) especially under beech or birch woodland.

There is less earthworm activity with a resultantreduced incorporation of organic matter down theprofile. The soil structure is usually less satisfac-tory and clay particles that work their way downcan form clay pans (see page 165) in the B horizon.These can be productive soils if ameliorated withlime and fertilizer (see Chapter 16).

Gleys occur in poorly drained soils (see page 176).Surface water gleys occur where the percolationof water is restricted by the poor structure in the A or B horizon to produce a perched water table(see page 173).This is typically where the subsoil isheavy and impervious especially in wetter regions.Oxygen in the waterlogged soil is depleted and, inthese anaerobic conditions below the water table,the iron oxides that colour the soil become dullgrey or bluey (in aerobic conditions the iron oxidesare rust coloured). The extent of the waterloggingthat the soil has been subjected to as the watertable fluctuates can be judged from the degree towhich it has become completely grey; usually thereis a rusty mottle present indicating that aerobicconditions exist in the soil for part of the year (seePlate 23). Plants growing in them are often shal-low rooted and suffer from drought in dry periods.These soils are only productive after they havebeen drained, limed and fertilized.

Ground water gleys develop where there is apermanent water table that is very near the surfaceof the soil so that to lower the water table thedrainage has to be undertaken on a regional basis,e.g. Romney Marsh. Drainage pipes can only beused when the water can run to a ditch with a waterlevel below that desired in the field; for some areasthis can only be achieved by maintaining an artifi-cially low level by the use of pumps (powered informer times by windmills).

Podsols (from the Russian meaning ‘ash like’)are strongly leached, very acid soils that developon freely draining soils such as coarse sands andgravels commonly under heather or pine or spruceforest in high rainfall areas. Because of the highacidity levels, earthworms are absent so there is abuild-up of the litter layer. Poorly decomposedorganic matter that is not incorporated (a ‘mor’humus) is characteristic of this soil type. Some of


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the organic matter combines with the iron in thetop layers to form soluble compounds, which areleached (‘podsolization’) to leave a grey (‘ash like’)A horizon (all that remains are bleached sandgrains). These compound become insoluble againin the conditions that prevail in the B horizonwhere organic matter accumulates to create a darkor black horizon below which is an iron-rich redlayer. The iron compounds that accumulate canform a strongly cemented ‘iron pan’.As a continu-ous pan that water (and roots) cannot penetrate isformed, a waterlogged area develops and peat canform at the surface.

Podsols are droughty and ‘hungry’ soils thatrequire considerable ongoing inputs of lime andmanures to make them productive. They are of little use in horticulture except for the growing of acid-loving trees and shrubs.

Rendzinas are very thin dark-brown, sometimesblack, soils with a strong granular structure sittingdirectly onto chalk or limestone. They are typicalof the soils on the steeper slopes of chalk or limestone hills under grass. Shallow soils developbecause the continued erosion on the slopes, whichalso keeps these soils heavily charged with lime.Where the soils become deeper on the less severeslopes it is common for the A horizons to becomeacid as the lime is leached downwards. They arewell drained because of the slope and because ofthe porous nature of the underlying rock. Rendzi-nas are not suitable for most horticultural purposesbecause the high lime content causes inducednutrient deficiencies (see page 205). Roots areseverely restricted by the shallow soils and vulner-able to drought.

SOIL COMPONENTS

The solid parts of soils consist of mineral matterderived from rocks and organic matter derived fromliving organisms. Levels of organic matter are dealtwith in Chapter 15. Most soils have predominantlymineral particles that vary enormously in size fromboulders, stones and gravels down to the soil par-ticles, sand, silt and clay.

Particle size classes

There is a continuous range of particle sizes but itis convenient to divide them into classes. Threemajor classification systems in use today are thoseof the International Society of Soil Science (ISSS),United States Department of Agriculture (USDA)and the Soil Survey of England and Wales (SSEW).These are illustrated in Figures 13.3 and 13.4. Inthe text the SSEW scale used by the AgriculturalDevelopment and Advisory Service of Englandand Wales (ADAS) is adopted. In each case, soil isconsidered to consist of those particles that areless than 2 mm in diameter. The silt and clay par-ticles are sometimes referred to as ‘fines’.

Sand

Sand grains are particles between 0.06 and 2.0 mmin diameter. They are gritty to the touch; even fine


InternationalSociety of

Soil Science(ISSS)

United StatesDept. of

Agriculture(USDA)

Soil Surveyof Englandand Wales

(SSEW)

gravel gravel stones

2.0

0.6

0.06

0.0020.002

0.02

0.2

2.0

0.2

clay clay clay

siltsilt silt

fine sand

coarse sandcoarse sand

v. coarse sand

medium sand

medium sand

coarse sand

fine sand

fine sand

Figure 13.3 Particle size classes (diameters in mm).

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sand has an abrasive feel. Sand is mainly com-posed of quartz. (Although any particle of this sizeis a sand grain, it is most often quartz because,unlike other minerals, it resists weathering.)

The shape of the particles varies from the roughand angular sand to more weathered, roundedgrains.They are frequently coated with iron oxides,giving sand colours from very pale yellow to rich,rusty brown. Silver sand has no iron oxide cover-ing. Chemically most of the sand grains are inert;

they neither release nor hold on to plant nutrientsand they are not cohesive.

The influence of sand on the soil is mainly phys-ical and as such the size of the particles is theimportant factor. As the particles become smallerand the volume of individual grains decreases, thesurface area of the same quantity of sand becomesgreater. Taking a cube and then cutting it up intosmaller cubes readily demonstrates this (Figure13.5). While the total volume is the same in all thesmall cubes compared with the original large cube,the sides of each are smaller but the total surfacearea is much greater because new surfaces havebeen exposed. Many soil particle characteristicsare directly related to particle size and in particu-lar to surface area. Sand grains are non-porous sotheir water-holding properties are directly relatedto their surface areas. It can be readily seen that,since water will not flow through gaps less thanabout 0.05 mm in diameter, there are very big dif-ferences in the drainage characteristics of coarseand fine sands (Figure 13.4). Consequently, soilsdominated by coarse sand are usually free drain-ing but have poor water retention, whereas thosecomposed mainly of unaggregated fine sand holdlarge quantities of water against gravity.The waterheld on all sand particles is readily removed byroots (see water, available).

Clay

Clay particles are those less than 0.002 mm indiameter. There are many different clay minerals,e.g. kaolinite, montmorillinite, vermiculite, andmica, all of which are derived from rock mineralsby chemical weathering. Most clay minerals have alayered crystalline structure and are plate like inappearance (Figure 13.4). The clay particles havesurface charges that give clay its very important andcharacteristic property of cation exchange. Thecharges are predominantly negative which meansthat the clay platelets attract positively chargedcations in the soil solution.These include the nutri-ents potassium, as K+; ammonia, as NH4

+; magne-sium, as Mg++; and calcium, as Ca++, as well as


smallest coarse sand grainsRoothair

COARSE SAND

FINE SAND

0.6 mm

SILT0.002 mm

CLAY

0.2 mmlargest fine sand haslargest pore space ofless than 0.05 mm

TYPICALCLAY PARTICLE(�100,000 actual size)

smallest porespace(0.05 mm across)betweencoarse sandgrains

0.06 mm

Figure 13.4 The relative sizes of coarse sand, fine sand,silt and clay particles (based on SSEW classification)with root hairs drawn alongside for comparison. Notethat even the smallest pore spaces between unaggregatedspherical coarse sand grains still allow water to bedrawn out by gravity and allow some air in at fieldcapacity, whereas most pores between unaggregated finesand grains remain water filled (pores less than 0.05 mmdiameter).

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hydrogen and aluminium ions. These ions are heldin an exchangeable way so that they remain avail-able to plants but are prevented from being leachedunless displaced by other cations. The greater thecation exchange capacity, the greater the reservesof cations held this way.

Hydrogen and aluminium cations make the soilacid.The other cations Ca++, Mg++, K+ and Na+ arecalled bases and make soils more alkaline (see soilpH). The proportion of the cation exchange cap-acity occupied by bases is known as its percentagebase saturation. A soil’s buffering capacity, i.e. itsability to resist changes in soil pH, also depends onthese surface reactions.The presence of high levelsof exchangeable aluminium and hydrogen meansthat very large quantities of calcium, in the form oflime, are required to raise the pH of acid clays. Incontrast only small quantities of lime are neededto raise the pH of a sand by the same amount (seeliming).

The clay particles are so small that the minuteelectrical forces on the surface become dominant(Figure 13.5); thus clay and water mixtures behaveas colloids. Note that as a particle is subdivided the total surface area increases; the surface areadoubles each time the sides of the individual

pieces are reduced by half. In particles that are less than 0.001 mm in diameter, which includesmost clay, colloidal properties are observed; mostnotably the properties of their surface dominatetheir chemical behaviour. Colloids are mixturesthat are in permanent suspension (see Table 13.1).Water-based colloids, such as clay, are ‘runny’ whenmixed with plenty of water (a ‘sol’) but with lesswater they are stiff and jelly like (a ‘gel’), e.g. paste.As they dry, these mixtures become sticky andeventually hard. Many natural materials such asgelatin, starch, gums and protoplasm (mainly pro-tein and water) are colloidal. This gives clay soilproperties of cohesion, plasticity, shrinkage andswelling. The small particles can pack and sticktogether very closely and in a continuous massthey restrict water movement.

The water-holding capacity of clay-dominatedsoils is very high because of the large surface areasand because many of the particles are porous.However, a high proportion of the water is heldtoo tightly for roots to extract (see water, available).Moist balls of clay are plastic, i.e. can be moulded.On drying, they harden and some may shrink. Inthe soil, the cracks that form on shrinking becomean important network of drainage channels. The


COARSE SAND MEDIUM SAND CLAY

1mm

1mm

1mm

1 mm3 1 mm3 1 mm3 1 mm3Volume1 2 8 1,000 millionNo of pieces6 mm2 8 mm2 12 mm2 6,000 mm2Total surface area

Figure 13.5 Surface area of soil particles. The effect of subdividing a cube corresponding in size to a grain of coarsesand. The same volume of medium sand is made up of over eight times more pieces that have a total surface area morethan double that of coarse sand. It requires over a 1000 million of the largest clay particles to make up the volume of onegrain of coarse sand and their total surface area is approximately 1000 times greater.

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cracks remain open until the soils are re-wetted andthe clay swells. Humus and calcium appear tocombine with clay in such a way that when thecombination dries, extensive cracking occurs andfavourable growing conditions result. Some claysare non-shrinking and are consequently more dif-ficult to manage, although they present less prob-lems with regard to building foundations.

Silt

Silt particles are those between 0.002 and 0.06 mmin diameter. Most in this size range are inert andnon-porous like sands but many particles, includ-ing felspar fragments, have the properties of clay.Soils dominated by silt do have a small cationexchange capacity but in the main they behavemore like a very fine sand. They have very goodwater-holding capacity and plants can take up a highproportion of this water.When wetted they have adistinctly soapy or silky feel. Silt soils are made upof particles that readily pack closely together buthave little ability to form stable crumbs (see soilaggregates). This makes them particularly difficultto manage.

Stones and gravel

Particles larger than 2 mm in diameter are knownvariously as grit, gravel, stones and boulders,according to size.The effect of stones on cultivatedareas depends on the type of stone, size and theproportion in the soil. The presence of even a few

stones larger than 20 cm prevents cultivation.Stonesin general have detrimental effects on mechanizedwork; ploughshares, tines and tyres are worn morequickly, especially if the stones are hard and sharpsuch as broken flint. Stones interfere with drillingof seeds and the harvesting of roots. Close cuttingof grasses is more hazardous where there are protruding stones. Mole draining becomes lesseffective in stony soils and large stones make itimpossible.

Stones can accumulate in layers and becomeinterlocked to form stone pans. In very gravellysoils the water-holding capacity is much reducedand the increased leaching leads to acid patches.Nutrient reserves are also reduced by the dilutionof the soil with inert material. However, stones canhelp water infiltration, protect the surface fromcapping and check erosion by wind or water.

SOIL TEXTURE

Soil texture describes the composition of a soil. Inmost cultivated soils the mineral content formsthe framework and exerts a major influence on itscharacteristics.

Although there is no universally accepted defin-ition of soil texture, it can usefully be defined asthe relative proportions of the sand, silt and clayparticles in the soil. Examples of different texturesare given in Figure 13.6.Texture can be consideredto be a fixed characteristic and provides a usefulguide to a soil’s potential. Fine-textured soils suchas clays, clay loam, silts and fine sands have good


Table 13.1 Colloidal systems

Mixture Colloidal system Examples

Solid dispersed in gas Solid aerosol Smoke, dustsSolid dispersed in liquid Sol or gel Paste, clay, humus,

protoplasmLiquid dispersed in gas Liquid aerosol Mist, fogLiquid dispersed in liquid Emulsion MilkGas dispersed in gas AtmosphereGas dispersed in liquid Foam Fire extinguishers, soap

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water-holding properties; whereas coarse-texturedsoils have low water-holding capacity but gooddrainage. This also means that soil temperaturesare closely related to soil texture because waterhas a very much higher specific heat value thansoil minerals. Consequently freely draining, coarsesand warms up more quickly in the spring but isalso more vulnerable to frosts than wetter soils.

Soils with high clay contents have good generalnutrient retention, whereas nutrients are readilylost from sandy soils, especially those with a highcoarse sand fraction. The application rate of pesti-cides and herbicides is often related to soil texture.The power requirement to cultivate a clay soil isvery much greater than that for a sandy soil. Theexpression ‘heavy’ for clay and ‘light’ for sandy soilsis derived from this difference in working proper-ties rather than the actual weight of the soil. Thetexture of a soil also influences the soil structureand soil cultivations. In general the texture of a

soil can be considered to be a fixed character. Theaddition of a calcareous clay to a sandy topsoil, apractice known as marling, can improve its water-holding capacity as well as reducing wind erosion,but it requires the incorporation of 500 t of dryclay per hectare to convert it to a sandy loam. Thepractice of adding clay is now largely confined tothe building of cricket squares. To ‘lighten’ a clayloam topsoil to a sandy loam, more than 2000 t ofdry sand is needed on each hectare. The additionof smaller quantities of sand is often an expensiveexercise to no effect; at worst it can make theresultant soil more difficult to manage.

Mechanical analysis of soils

Soil texture can be determined by finding the par-ticle size distribution. There are several methodsbut all depend on the complete separation of the


0

20

20

0100 80 60 40 20 0

40

4060

60

80

80

100

100

(after SSEW)

% SILT0.002–0.06 mm2–60 �m

% SAND0.06–2 mm60–2000 �m

% CLAY�0.002 mm�2 �m

clay

silty claysandy clay

sandy clayloam

silty clayloam

clay loam

silt loamsandy silt loamsandy loam

sandsandloamy

Figure 13.6 Soil textural triangle. The soil texture can be identified on this type of chart when at least two of the majorsize of fractions are known, e.g. 40 per cent sand, 30 per cent silt and 30 per cent clay is a clay loam (SSEW Soil-ParticleSize Classification).

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particles, the destruction of organic matter and theremoval of particles greater than 2 mm in diam-eter. Sieving can separate the stones, coarse sand,medium sand and fine sand fractions.

Finer particles are usually separated by takingadvantage of their different settling velocities whenin suspension. The settling velocity of a particledepends on its density and radius, the viscosity anddensity of the liquid and the acceleration due togravity, the method is simplified by assuming thatsoil particles are spherical and have the same dens-ity and the investigations are conducted in waterat 20°C.

Particles that are less than 0.001 mm in diameterare kept permanently in suspension by the bom-bardment of vibrating water molecules and arereferred to as colloids, e.g. most clay particles. Allsand particles will have fallen more than 10 cm after50 seconds: so a sample taken at that depth can beused to calculate the clay plus silt left in the sus-pension.A sample taken at this depth after 8 hourswill only contain particles less than 0.002 mm indiameter, i.e. clay alone. The soil texture can bededuced from this information using a textural triangle (Figure 13.6), which is the basis of identify-ing soil types.

Texturing by feel

A more practical method of determining soil tex-ture, especially in the field, is by feel.This can, withexperience, be a very accurate means of distin-guishing between over 30 categories.

A ball of soil about the size of a walnut is mois-tened and worked between the fingers to removeparticles greater than 2 mm and to break down thesoil crumbs. It is essential that this preparation isthorough or the effect of the silt and clay particleswill be masked.

Sands are soils that have little cohesion. Sandhas little tendency to bind even when wetted andit cannot be rolled out into a ‘worm’. A loamy sandhas sufficient cohesiveness to be rolled into a‘worm’ but it readily falls apart. What is generallyknown as loam, moulds readily into a cohesive ball

and it has no dominant feel of grittiness, silkinessor stickiness. If grittiness is detected and the ball isreadily deformed it is a sandy loam. If it is readilydeformed but has a silky feel it is silty loam. Clayloams bind together strongly, do not readily deform,and take a polish when rubbed with the finger.Claysbind together and are very difficult to deform.A clay soil readily takes a very marked polish. Ifthere is also a feeling of silkiness it is a silty clay,but if grittiness, it is a sandy clay. Wherever gritti-ness is detected, the designation sand can be furtherqualified by stating whether it is coarse, medium orfine sand, e.g. coarse sandy loam. Table 13.2 showsthe range of textural groupings commonly used.

Determining texture by feel has the limitationthat the influence of organic matter and chalk can-not be eliminated. Chalk tends to give a soil a silkyor gritty feel but the fact that a soil is known to bechalky should not influence the texturing. Its tex-tural class may be prefixed ‘calcareous’, e.g. cal-careous silty clay. Organic matter tends to increasethe cohesiveness of light soils, reduce the cohe-siveness of heavy soils and large quantities canimpart a silky or greasy feel. The prefix ‘organic’can be used for describing mineral soils with 6–20per cent organic matter. Soils with 20–35 per centorganic matter are peaty loams, 35–50 per centorganic matter loamy peats and soils with greaterthan 50 per cent organic matter are termed peat(see organic soils).

SOIL STRUCTURE

Soil structure is the arrangement of particles in thesoil. In order to provide a suitable root environ-ment for cultivated plants the soil must be con-structed in such a way as to allow good gaseousexchange whilst holding adequate reserves ofavailable water.There should be a high water infil-tration rate, free drainage and an interconnectednetwork of spaces allowing roots to find water and nutrients without hindrance. There should beno large cavities that prevent thorough contactbetween soil and roots, and allow roots to dry outin the seedbed.The soil should be managed so that


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erosion is minimized. Good structural stabilityshould be maintained so that the structure doesnot deteriorate and limit crop growth.

Porosity

The plant roots and soil organisms live in the poresbetween the solid components of the growingmedium. In the same way that a house is mainlyjudged by the living accommodation created by thebricks, wood, plaster, cement, etc., so a soil is evalu-ated by examining the spaces created. In generalthe smallest pores, micropores, contain only waterthat rarely dries out and is unavailable to plants(see permanent wilting point). The middle-sizedpores, mesopores, contain water available to plantsand the air moves in as it is removed by plant

roots.Pores greater than about 0.05 mm in diameter,called macropores, can drain easily to allow in airwithin hours of being saturated, i.e. fully wetted.Ideally there should be sufficient mesopores toensure good retention of available water, but suffi-cient macropores to allow free drainage, gaseousexchange and thorough root exploration as shownin Figure 13.7.

The key to managing most growing media is inmaintaining a high proportion of air-filled poreswithout restricting water supply.An important indi-cator of a satisfactory growing medium is its air-filled porosity or air capacity, i.e. the percentagevolume filled with air when it has completed drain-ing, having been saturated with water.

Bulk density is the mass of soil per unit volumeand it can be measured by taking a core of soil of known volume and weighing it after thorough


Table 13.2 Soil texture classification based on hand texturing

Textural class Symbol Textural group* ‘Sands’ ‘Other’†

Coarse sand CSSand SFine sand FS SandsVery fine sand VFSLoamy coarse sand LCS

‘Sands’Loamy sand LSLoamy fine sand LFS Very light soilsCoarse sandy loam CSL

Loamy very fine sand LVFSSandy loam SL Light soilsFine sandy loam FSL

Very fine sandy loam VFSLSilty loam ZyL Medium soilsLoam LSandy clay loam SCL

Clay loam CL ‘Other’Silt loam ZL Heavy soilsSilty clay loam ZyCL

Sandy clay SCClay C Very heavy soilsSilty clay ZyC

*Commonly used for determining soil-acting herbicide application rates.†Commonly used for determining fertilizer application or flooding levels.

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drying. In normal mineral soils results are usuallybetween 1.0 and 1.6 g/ml. The difference is largelyattributable to variation in total pore space. Finer-textured soils tend to have more pore space andtherefore lower bulk density than sands, but for all soils higher values indicate greater packing orcompaction.

This information is not only useful to diagnosecompaction problems but can also be used to cal-culate the weight of soil in a given volume.Assum-ing a cultivated soil to have a bulk density of1.0 g/ml, the weight of dry soil in 1 ha to a ploughdepth of 15 cm is 1500 tonnes; when compacted, thesame volume weighs 2400 tonnes. Similarly, 1 m3 ofa typical topsoil with a bulk density of 1.0 willweigh 1 tonne (1000 kg) when dry and up to half asmuch again when moist.

Soil structures

The pore space does not depend solely upon thesize of the soil particles as shown in Figure 13.4because they are normally grouped together.These aggregates, or peds, are groups of particlesheld together by the adhesive properties of clayand humus. The ideal arrangement of small and

large pores is illustrated in Figure 13.7 alongside adusty tilth with too few large pores and a cloddytilth that has too many large pores.

A soil with a simple structure is one in whichthere is no observable aggregation. If this is becausenone of the soil particles are joined together, as insands or loamy sands with low organic matter levels,it is described as single grain structure. Where allthe particles are joined with no natural lines ofweakness the structure is said to be massive. Aweakly developed structure is one in which aggre-gation is indistinct and the soil, when disturbed,breaks into very few whole aggregates but a lot ofunaggregated material.This tends to occur in loamysands and sandy loams. Soils with a high clay con-tent form strongly developed structures in whichthere are obvious lines of weakness and, when dis-turbed, aggregates fall away undamaged. The pris-matic, angular blocky, round blocky, crumbs andplaty structures which are found in soils are illus-trated in Figure 13.8.

Development of soil structures

Soil structure develops as the result of the actionon the soil components of natural structure-formingagents: wetting and drying, freezing and thawing,root growth, soil organisms and cultivations. Thewetting and drying cycle can affect the whole root-ing depth. Cracks open up in heavier soils as theclay shrinks. As well as being a major factor in the drying of deeper soil layers, the roots play animportant part in soil structure by growing into thecracks and keeping them open. They help estab-lish the natural fracture lines. In strong structures,a close-fitting arrangement of prismatic (see Plate23) or angular blocky aggregates is readily seen.In soils with low clay content the roots are vital inmaintaining an open structure.The exploring rootsprobe the soil, opening up channels where the soilis loose enough and producing sideways pressureas they grow. On death, the root leaves behindchannels stabilized by its decomposed tissue forother roots to follow. Fine granular structures aredeveloped under pastures by the action of the


DUSTY

sand

claysilt

IDEAL CLODDY

Figure 13.7 Tilth. The ideal tilth for most seedbeds ismade up of soil aggregates between 0.5 and 5mm diame-ter. Within these crumbs are predominantly small pores(less than 0.05 mm) that hold water and between thecrumbs are large pores (greater than 0.05 mm) that alloweasy water movement and contain air when soil is at fieldcapacity (actual size).

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fibrous rooting over many years.The soil structureis greatly improved by the root ball. Its physicalinfluence is most easily appreciated by shaking outthe soil crumbs from around the root of a tuft ofwell-established grass and comparing them withthe structure of soil taken from a nearby barepatch. The shattering of clods by frost action, pro-ducing a frost mould, is largely confined to the sur-face layers and is vital in the management of clays.Freshly exposed land is often referred to as raw;when weathered it becomes mellow. Once mellow,a seedbed is more easily prepared.The weatheringprocess and influence of cultivation tend to pro-duce rounded blocky structures and roundedgranules in the cultivated zone (Figure 13.8).

Earthworms and other soil organisms play animportant part in loosening the soil, maintaining

the network of drainage channels and stabilizingthe soil structure. Cultivation of soil by hand ormechanized implements is undertaken to producea suitable rooting environment for plants, to destroypests and weeds, and to mix in plant residues,manures and fertilizers. However, the use of culti-vators can lead to the formation of platy layers orpans, which are characterized by the lack of verti-cal cracks and form an obstruction to root andwater movement. Natural pans develop in somesoils as a result of fine material cementing a layerof soil together. In some sandy soils rich in ironoxide, these oxides cement together a layer ofsand where there has been a fluctuating watertable, to produce an iron pan. The collapse of sur-face crumb structure can lead to the formation ofa surface pan, crust or ‘cap’ (see Plate 21).


granules

roundedblocky

angularblocky

platy

platy

massive

massive

cultivation pan

surface pan or ‘cap’

prismatic

roundedprismatic(columnar)

Figure 13.8 Soil structures. The soil profile on the left is composed of soil particles aggregated into structures that pro-duce good growing conditions. Examples of structures that create a poor rooting environment are shown in the profileon the right.

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Structural stability

The structural stability of soil refers to its ability to resist deformation when wet. Soil aggregateswith little or no stability collapse spontaneously asthey soak up water, i.e. they slake. Those high in fine sand or silt are particularly vulnerable to slak-ing.Aggregates with better stability maintain theirshape when wetted for a short time but grad-ually pieces fall off if left immersed in water.Aggregates with good structural stability are ableto resist damage when wet unless vigorously dis-turbed. Soils with a high clay content have betterstability than those with low levels. Stability is also increased by the presence of calcium carbon-ate (chalk), iron oxides and, most importantly,humus.

Tilth

The soil surface or seedbed should be carefullymanaged to produce the required tilth: the struc-ture of the top 50 mm of the soil. Sandy soils areeasily broken down to the right size with cultiva-tion equipment. Heavier soils are less easy to cul-tivate and benefit from weathering to produce afrost ‘mould’.

The fineness of a seedbed should be related tothe size of seeds but ideally consists of aggregateor crumbs between 0.5 and 5 mm in diameter(Figure 13.7). Cloddy surfaces lead to poor germin-ation as well as poor results from soil herbicidetreatments.The rain on the soil surface breaks downtilth. As soil crumbs break up, the particles fill inthe gaps; this reduces infiltration rates. As the sur-face dries, a cap or pan is formed. Thus fine ‘dusty’tilths should be avoided and the soil crumbs shouldbe stable so that they can withstand the effect ofrain until plants are established. This is particu-larly important on fine sandy and silty soils, whichtend to have poor structural stability. In general,fine tilths should be avoided outdoors until wellinto spring when conditions are becoming morefavourable and growth through any developingcap is rapid.

CULTIVATIONS

In temperate areas the conventional preparationof land for planting is a thorough disturbance ofthe top 20–30 cm of soil. Digging or ploughingburies residues of previous plantings and weeds,and with repeated passes of rakes or harrows asuitable tilth is created. This procedure is verydemanding on energy, labour and time. Many ofthe cultivations tend to override the natural struc-ture-forming agents and when undertaken at thewrong time they create pans or leave a bare, loosesoil vulnerable to erosion. Some of the compactionproblems are overcome by cultivating in beds whichconfines traffic to well-defined paths between thegrowing areas. The advent of effective herbicideshas, in certain cases, enabled the inversion of soilto be eliminated. The use of powered implementshas speeded up work and reduced the number oftillage passes. In some areas of horticulture theadoption of minimum or zero tillage has preservednatural structure while beneficially concentratingorganic matter levels in the surface layers andreducing wind and water erosion.

Ploughing and digging

Ploughing and digging are used to loosen and invertthe soil. The land is broken up into clods and anincreased area is exposed to weathering. As thesoil is inverted, weeds, plant residues and bulkymanures are incorporated.The depth of ploughingor digging should be related to the depth of topsoilbecause bringing up the subsoil reduces fertility inthe vital top layers, seriously affecting germinationof seeds and establishment of plants. If deeperlayers are to be loosened, a subsoiler should beused. In plastic soil conditions the plough cansmear the soils, particularly when the wheels ofthe tractor spin in the furrow bottom. Theseplough pans tend to develop with successiveploughing to the same depth. Ploughing at differ-ent depths or attaching a subsoil tine can reducetheir incidence. Digging with a spade does not pro-duce a cultivation pan and is still used on small


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areas. Spading machines or rotary diggers imitatethe digging action without the disadvantages ofploughing, but tend to be very slow.

Rotary cultivators

Rotary cultivators are used to create a tilth onuncultivated or on roughly prepared ground. Thetype of tilth produced depends on suitable adjust-ment of forward speed, rotor speed, blade designand layout, shield angle and depth of working.The‘hoe’ blade is normally used for seedbed produc-tion but does have the disadvantage of smearingplastic soils at the cultivation depth, producing arotovation pan. ‘Pick’ tines produce a rougher tilthbut less readily cause a pan. Subsoil tines can befitted to prevent these pans developing.

Harrowing and raking

Harrowing and raking are methods of levellingsoils, incorporating fertilizers and producing asuitable tilth on the roughly prepared ground. Thesoil must be in a friable condition for this oper-ation and it is made easier if the top layers havebeen suitably weathered. The impact of the tinesbreaks the clods.

‘Progressive’-type cultivators were introducedessentially to loosen coarse structured clays bydrawing through the soil banks of tines, increasingin depth from the front, to cultivate the soil fromthe top down in one pass. Although this requirespowerful tractors to pull, especially if subsoilingtines are attached, it is a time-saving operationand the re-compaction inherent in multiple passmethods is reduced. These cultivators should notbe used on well-structured soils where full depthloosening is unnecessary.

‘Under-loosening’ cultivators have been designedto loosen compact topsoils without disturbing thesurface,which ensures a level, clod free and organic-matter-rich tilth. Used under the right conditionsthese implements improve water movement andplant growth. However, loosened soils are moresusceptible to compaction and consequently the

equipment should only be used when compactionis known to be present.

Subsoiling

Subsoiling is used to improve soil structure belowplough depth by drawing a heavy cultivation tinethrough the soil to establish a system of deep cracksin compacted zones. This helps the downwardmovement of water, circulation of air and penetra-tion of roots (see Figure 13.9). The operation ismost effective when the subsoil is friable and thesurface dry enough to be able to withstand theheavy tractor that is needed (see load bearing).

Effective subsoiling is made easier if the top sur-face is loosened by prior cultivation. Although thedraught is higher, subsoil disturbance is increasedsubstantially by attaching inclined blades or wings.Successful subsoiling is accompanied by a lift in the soil surface (soil heave) which usually makes itunsuitable for improving conditions in playing fields.

Subsoiling should only be used when the causeof any waterlogging is related to a soil structurefault (see also drainage). Slow subsoil permeabil-ity caused by high clay content is usually rectifiedwith mole drainage. If the soil is too sandy orstony, a subsoiler can be used so long as the crackscreated lead the water into a natural or artificialdrainage system. Subsoilers used in the right con-ditions readily burst massive structures and soilpans created by machinery but some natural pansare too strong for normal equipment.The problemof cultivation pans can be dealt with by using con-ventional subsoilers or by attaching small subsoiltines to the cultivation equipment. This tends toincrease the power requirement but eliminates thepan as it is created.

MANAGEMENT OF MAIN SOIL TYPES

Sandy soils

Sandy soils are usually considered to be easily cul-tivated but serious problems can occur because


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the particles readily pack together, especially whenorganic matter levels are low. Consequently manysandy soils are difficult to firm adequately withoutcausing over-compaction. Pans near the surfacecaused by traffic and deeper cultivation pans fre-quently occur on sandy soils, resulting in reducedrooting and water movement. Subsoiling is fre-quently undertaken on a routine basis every 4–6years, although the need can be reduced by keep-ing machinery off land while it has low load-bearingstrength and by encouraging natural structure-forming agents.

Coarse sands have low water-holding capacity,which makes them vulnerable to drought, particu-larly in drier areas. This is not such a disadvantageif irrigation equipment is installed and water isreadily obtainable. In many categories of horticul-ture there is a demand for soils with good work-ability. Coarse sands, loamy sands and sandy loamshave the advantage of good porosity and can becultivated at field capacity. Sands tend to go acid

rapidly and are vulnerable to overliming becauseof their low buffering capacity (see page 159).

Silts and fine sands

These can be very productive soils because of theirgood water-holding capacity and, while organicmatter levels are kept above 4 per cent, their easeof working. However silts and fine sands presentsoil management problems, especially when usedfor intensive plantings, because they have weakstructure, are vulnerable to surface capping, and areeasily compacted to form massive structures. Toachieve their high potential, efficient drainage isvital to maximize the rooting depth. Fine tilths inthe open should be avoided, especially in autumnand early spring, because frosts and heavy rainfallreduce the size of surface crumbs. For the same rea-son,care should be taken with irrigation droplet sizethat, if too large, can damage the surface structure.


1 Conventional

2 Winged

approx 1 metre

40–5

0 cm

40–5

0 cm

Figure 13.9 Subsoiling. The subsoiler is drawn through the soil to burst open compacted zones. It leaves cracks whichremain open to improve aeration, drainage and root penetration. The cracks created should link up with artificialdrainage systems unless the lower layers are naturally free draining.

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Improving soil structure is not easy after winterroot crop harvesting or orchard spraying on wetsoils because low clay content results in very littlecracking during subsequent wetting and dryingcycles. Improvement therefore depends on othernatural structure-forming agents or on subsoiling.

Clay soils

Clay soils tend to be slow draining, slow to warm upin Spring, and have poor working properties (seeworkability). A serious limitation is that the soil isstill plastic at field capacity, which delays soil prep-aration until it has dried by evaporation. Permanentplantings are established to avoid the need torework the soil. Playing surfaces created over clayshave severe limitations, particularly when requiredfor use in all-weather conditions.Where high stand-ards have to be maintained, as in golf greens, fineturf is established in a suitable growing mediumoverlying the original soil.However,a high clay con-tent is an advantage for the preparation of cricketsquares where a hard, even surface is required but isplayed on only in drier weather. Increasingly, heav-ily used areas are replaced by artificial surfaces.

Horticultural cropping of clays is limited to sum-mer cabbage, Brussels sprouts and to some top fruitin areas where the water table does not restrictrooting depth. Under-drainage is normally neces-sary. In wetter areas most clays are put down tograss. Timeliness, encouraging the annual dryingcycle of the soil profile and maximizing the effectof weathering to help cultivations are essential forsuccessful management of clay soils.

Peat soils

Peat soils have very many advantages over min-eral soils for intensive vegetable and outdoorflower production. Fenland soils and LancashireMoss of England; peatlands of the midland coun-ties of Ireland; the ‘muck’ soils of North America;and similar soils in the Netherlands, Germany,Poland and Russia have proved valuable when their

limitations to commercial cropping have beenovercome.

Well-drained peat at the correct pH is an excel-lent root environment. It has a very much higherwater-holding capacity than the same volume ofsoil and yet gaseous exchange is good. Root devel-opment is uninhibited because friable peat offershardly any mechanical resistance to root penetra-tion. This leads to high quality root crops that areeasily cleaned. These cultivated peat lands warmup quickly at the surface because the Sun’s energyis efficiently absorbed by their dark colour, withconsequent rapid crop growth. These soils have avery low power requirement for cultivation, arefree of stones, and can be worked over a widemoisture range.

Plant nutrition is complicated by natural traceelement deficiencies and the effect of pH on plantnutrient availability. Peat has poor load-bearingcharacteristics and specialized equipment is oftenneeded to harvest in wet conditions. Whilst peatwarms up quickly on sunny days, its dark surfacemakes it vulnerable to air frost because it acts as anefficient radiator. Firming the surface and keepingit moist combat this. Weeds grow well and theircontrol is made more difficult by the ability of peatto absorb and neutralize soil-acting herbicides.The high organic matter levels also make the peatsand sandy peats vulnerable to wind erosion inspring when the surface dries out and there is nocrop canopy to protect it.

FURTHER READING

Brown, L.V. Applied Principles of Horticultural Science,2nd edition (Butterworth-Heinemann, 2002).

Coker, E.G. Horticultural Science and Soils (Macmillan,1970).

Curtis, L.F., Courtney, F.M. and Tredgill, S. Soils in theBritish Isles (Longman, 1976).

Davies, D.G., Eagle, D.T. and Finley, J.B. Soil Manage-ment, 5th edition (Farming Press Books and Videos,1993).

Handreck, K. and Black, N. Growing Media forOrnamental Plants and Turf (New South WalesUniversity Press, 1989).


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Ingram, D.S. et al (editors) Science and the Garden,(Blackwell Science Ltd., 2002).

Mackney, D. (editor) Soil Type and Land Capability, SoilSurvey Technical Monograph No. 4 (HMSO, 1974).

MAFF. Code of Good Agricultural Practice for theProtection of Soil, (DEFRA, formerly MAFFPublications, 1998) http://www.defra.gov.uk/environ/cogap/soilcode.pdf.

Simpson, K. Soil (Longmans Handbooks in Agriculture,1983).

White, R.E. Introduction to the Principles and Practiceof Soil Sciences, 3rd edition (Blackwell ScientificPublications, 1997).


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WETTING OF A DRY SOIL

Rainfall is recorded with a rain gauge (see Figure2.10) and is measured in millimetres of water.Thus‘1 mm of rain’ is the amount of water covering anyarea to a depth of 1 mm. Therefore ‘1 mm of rain’

on 1 hectare of land is equivalent to 10 m3 or10 000 litres of water per hectare (area 10 000 m2 �depth 0.001 m).

As rain falls on a dry surface the water eithersoaks in (infiltration) or runs off over the surfaceas surface run-off. Ponding is the accumulation ofwater on the surface as a result of infiltration ratesslower than rainfall. Ponding leads to capping,which further reduces infiltration rates. Soil surfacescan be protected with mulches and care should betaken with water application rates during irrigation.

Saturated soils

As water soaks into the dry soil the surface layersbecome saturated or waterlogged, i.e. water fills allthe pore spaces. As water continues to enter thesoil it moves steadily downwards, with a sharpboundary between the saturated zone and dry, air-filled layers, as shown in Figure 14.1. So long aswater continues to soak into the soil, this wettingfront moves to greater depths.When rainfall ceasesthe water in the larger soil pores continues to movedownwards. The water that is removed from wetsoil by gravity is known as gravitational water.

Water is held in the soil in the form of waterfilms around all the soil particles and aggregates.Forces in the surface of the water films, surfacetension, hold water to the soil particles against theforces of gravity and the suction force of roots. Asthe volume of water decreases, its surface area and

14

Soil Water

Plants require a constant supply of water tomaintain growth. Water is taken up by theplant with minerals by the roots and is lost bytranspiration. Approximately 500 kg of wateris needed to replace the water lost from theleaves over a period when the plant grows by1 kg of dry weight. A mature tree can take upabout 1000 l a day. A single hectare of fullcanopy crop grown under glass can use over1000 m3 (1 million litres) of water over a year.

In this chapter the characteristics of soilwater are established whilst following theprocess of wetting a dry growing medium,then the drying out of a fully wetted one. Theconcept of available water is related to theability of the plant root to extract water fromdifferent growing media with different mois-ture contents. Irrigation and drainage areexplained as the means of maintaining opti-mum water levels in the soil. The chapter isconcluded by an examination of the nature ofwater quality and water conservation.

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hence its surface tension becomes proportionallygreater until, in very thin films of water, it preventsthe reduced volume of water from being removedby gravity. A useful comparison can be seen whenone’s hands are lifted from a bowl of water. Theydrip until the forces in the surface of the thin filmbecome equal to the forces of gravity acting on theremaining small volume of water over the hands.

Field capacity (FC)

A soil that has been saturated, then allowed todrain freely without evaporation until drainageeffectively ceases, is said to be at field capacity(FC). This may take 2 days or less on sandy soils,but far longer on clay where the process ofdrainage may continue indefinitely.


1 Dry soil wetted.2 Saturated zone extends downward as rain continues.3 No rain; gravitational water moves downwards until water film tension equal to pull of gravity.4 Further rain saturates top layers before more soil wetted.5 Water table forms if obstruction to gravitational water.6 Drained soil, at FIELD CAPACITY down to drains.7 At PERMANENT WILTING POINT.

� Water

� Rain

1 2 3 4 5

6

7

DrainWatertable

Figure 14.1 Water in the soil.

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Soil Water 173

At FC the soil pores less than about 0.05 mmremain full of water, whereas in the macroporesair replaces the gravitational water as illustratedin Figure 14.1 (see porosity).

The amount of water held at FC is known as the water-holding capacity (WHC) or moisture-holding capacity (MHC). Examples are given inTable 14.1. The WHC is expressed in millimetresof water for a given depth of soil.Thus a silty loamsoil 300 mm deep holds 65 mm of water when atFC. Conversely, if a silty loam had become com-pletely dry to 300 mm depth, it would require65 mm of rain or irrigation water to return it to FC.Since 1 mm of water is equivalent to 10 m3/ha, ahectare of silty loam would hold 650 m3 water inthe top 300 mm when at FC (see rainfall). Theprinciple described enables WHC or irrigationrequirement to be determined for any soil depth.The amount of water required to return a soil toFC is called the soil moisture deficit (SMD).

Watertables

Groundwater occurs where the soil and underlyingparent material are saturated (see Figure 14.1); thewatertable marks the top of this saturated zone,

which fluctuates over the seasons, normally beingmuch higher in winter. In wetlands the watertableis very near the soil surface and the land is not suit-able for horticulture until the watertable of thewhole area is lowered (see drainage).

Where water flows down the soil profile and isimpeded by an impermeable layer such as satu-rated clay or silty clay, a perched watertable isformed. Water from above cannot drain throughthe impermeable barrier and so a saturated zonebuilds up over it. Springs appear at a point on thelandscape where an overlying porous materialmeets an impermeable layer at the soil surface, e.g.where chalk slopes or gravel mounds overlie clay.

Capillary rise

Capillary rise occurs only from saturated soils.Water is drawn upwards from the watertable in acontinuous network of pores. The height to whichwater will rise depends on the continuity of pores,and on their diameter. In practice the rise from thewatertable is rarely more than 20 mm for coarsesands, typically 150 mm in finer textured soils butcan be substantially greater in silty soils and inchalk.

The upward movement of water in these veryfine pores is very slow. The principle of capillaryrise is used in watering plants grown in containers(see capillary benches). Several ‘self-watering’containers also depend on capillary rise from awater store in their base (see aggregate culture).

DRYING OF A WET SOIL

Soil water is lost from the soil surface by evap-oration and from the rooting zone by planttranspiration.

Evaporation

The rate of water loss from the soil by evaporationdepends on the drying capacity of the atmosphere

Table 14.1 Soil water. The amount of water in a givendepth of soil at FC or PWP can be calculated by simple proportion

Soil texture Water held in 300 mm soildepth (mm)

at FC, at availablei.e. WHC PWP water

(AWC)

Coarse sand 26 1 25Fine sand 65 5 60Coarse sandy loam 42 2 40Fine sandy loam 65 5 60Silty loam 65 5 60Clay loam 65 10 55Clay 65 15 50Peat 120 30 90

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just above the ground and the water content in thesurface layers. The evaporation rate is directlyrelated to the net radiation (see page 13) from thesun which can be measured with a solarimeter (seepage 27). Evaporation rates increase with higherair temperatures and wind speed or lower humid-ity levels. As water evaporates from the surface,the water films on the soil particles become thin-ner. The surface tension forces in the film surfacebecome proportionally greater as the water vol-ume of the film decreases. This leaves water filmson the particles at the surface with a high surfacetension compared with those in the films on parti-cles lower down in the soil. Water moves slowlyupwards to restore the equilibrium.

Whilst the surface layers are kept moist bywater moving slowly up from below, the losses byevaporation in contrast are quite rapid. Conse-quently the surface layers can become dry and theevaporation rate drops significantly after ‘5 mm ofwater’ is lost. Evaporation virtually ceases afterthe removal of ‘20 mm of water’ from the soil. Adry layer on the soil surface helps conserve mois-ture in the lower layers.

Mulches (see page 193) can also reduce waterloss from the soil surface. The evaporation fromthe soil surface is almost eliminated by a leafcanopy that shades the surface, thus reducing air-flow and maintaining a humid atmosphere overthe soil (see Humidity, page 25).

Evapotranspiration

As a leaf canopy covers a soil the rate of water lossbecomes more closely related to transpirationrates. The potential transpiration rate representsthe estimated loss of water from plants grown inmoist soil with a full leaf canopy. It can be calcu-lated from weather data (see Table 14.2).

As roots remove water it is slowly replaced bythe water film equilibrium, but rapid water uptakeby plants necessitates root growth towards a watersupply in order to maintain uptake rates. At anypoint when water loss exceeds uptake, the plantloses turgor and may wilt. This tends to happen invery drying conditions even when the growingmedium is moist. Wilting is accompanied by areduction in carbon dioxide movement into theleaf, which in turn reduces the plant’s growth rate(see photosynthesis). The plant recovers from thistemporary wilt as the rate of water loss falls belowthat of the uptake, which usually occurs in the coolof the evening onwards. Continued loss of watercauses the soil to reach the permanent wiltingpoint (PWP) which is when plants wilt but do notregain turgor overnight because their roots canextract no more water within the rooting zone.When the soil has reached the PWP there is stillwater in the smallest of the soil pores, within clayparticles, and in combination with other soil con-stituents, but it is too tightly held to be removed by


Table 14.2 Potential transpiration rates.The calculated water loss (mm) from a crop grown in moist soil with a full leafcanopy, over different periods of time and based on weather data collected in nine areas in the British Isles

Area April May June July August September Summer Winter Annual

Ayr 46 81 90 83 65 38 405 70 475Bedford 50 78 89 91 80 43 430 70 500Cheshire 53 75 83 88 76 44 420 80 500Channel Isles 51 86 91 99 84 46 457 103 560Essex (NE) 50 79 98 98 83 45 450 80 530Hertford 49 79 91 94 80 43 435 75 510Kent (Central) 50 79 93 96 83 44 445 65 510Northumberland 44 64 81 76 60 34 360 70 430Dyfed 46 75 84 81 74 44 405 105 510

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roots. Typical water contents of different types ofsoil at their PWP are given in Table 14.1.

Available water

Roots are able to remove water held at tensionsup to 15 atmospheres within the rooting zone, andgravitational water drains away. Consequentlythe available water for plants is the moisture in therooting depth between FC and the PWP. Theavailable water content (AWC) of different soiltextures is given in Table 14.1. Fine sands havevery high available water reserves because theyhold large quantities of water at FC and there isvery little water left in the soil at PWP (see sands,page 158). Clays have lower available waterreserves because a large proportion of the waterthey hold is held too tightly for roots to extract(see clay, page 159).

Roots remove the water from films at FC veryeasily. Even so, plants can wilt temporarily and anyrestriction of rooting makes wilting more likely.Water uptake is also reduced by high soluble saltconcentrations (see osmosis) and by the effect ofsome pests and diseases (see vascular wilt dis-eases).As the soil dries out, the water films becomethinner and the water becomes more difficult forthe roots to extract. After about half the AWC hasbeen removed, temporary wilting becomes signifi-cantly more frequent. Irrigating when availablewater levels fall below this point minimizes slowingof growth rates. Plants grown under glass are oftenirrigated more frequently to maintain a suitablyformulated growing medium near to FC. Thisensures maximum growth rates since the rootshave access to ‘easy’ water, i.e. water removed bylow suction force.

Workability of soils

The number of days each year that are available forsoil cultivation depends on the weather but morespecifically on soil consistency. This describes theeffect of water on those physical properties of thesoil influencing the timing and effect of cultivations.

It is assessed in the field by prodding and handlingthe soil. A very wet soil can lose its structure andflow like a thick fluid. In this state it has no load-bearing strength to support machinery. As the soildries out the soil becomes plastic; the particlesadhere and are readily moulded. In general, the soilis difficult to work in this condition because it sticksto the cultivation equipment, has insufficient load-bearing strength, is readily compacted and is easilysmeared by cultivating equipment.As the soil driesfurther it becomes friable. At this stage the soil isin the ideal state for cultivation because it has ade-quate load-bearing strength but the soil aggregatesreadily crumble. If the soil dries out further to a harsh consistency the load-bearing strengthimproves considerably, but whilst coarse sands andloams still readily crumble in this condition, soilswith high clay, silt or fine sand content form hard-resistant clods.The friable range can be extended byadding organic matter (see humus).At a time whenbulky organic matter is more difficult to obtain itis important to note that a fall in soil humus con-tent narrows the friable range. This allows less lat-itude in the timing of cultivations and increasesthe chances of cultivations being undertaken whenthey damage the soil structure. Timeliness is thecultivation of the soil when it is at the right consis-tency. Whereas many sands and silts can be culti-vated at FC, clays and clay loams do not becomefriable until they have dried out to well below FC.

DRAINAGE

Drainage is the removal of gravitational water fromthe soil profile. As this water leaves the macropores,air that takes its place enables gaseous exchangeto continue. Horticultural soils should return to atleast 10 per cent air capacity in the top half metrewithin 1 day of being saturated (see porosity).Some soils, notably those over chalk or gravel, arenaturally free draining but many have underlyingmaterials which are impermeable or only slowlypermeable to water. In such cases artificial drainage,sometimes referred to as field drainage or underdrainage, is put into carry away the gravitational

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water (see Figure 14.2).This helps the soil to reachits FC rapidly but does not reduce its WHC.

Well-drained soils are those that are rarely satu-rated within the upper 900 mm except during orimmediately after heavy rain.Uniform brown,red oryellow colours indicate an aerobic soil, i.e. a soil inwhich oxygen is available (see Plate 22). Imperfectlydrained soils are those that are saturated in the top600 mm for several months each year. These soilstend to have less bright colours than well-drainedsoil. Greyish or ochreous colours are distinct at450 mm, giving a characteristic rusty mottledappearance (see Plate 23). Poorly drained soils aresaturated within the upper 600 mm for at least halfthe year and are predominantly grey (see Plate 22).

Symptoms of poor drainage

These include restricted rooting; reduced workingdays for cultivation; weed, pest and disease prob-lems, and excess fertilizer requirements. Soil pits dug in appropriate places reveal the extent of thedrainage problem and help pinpoint the cause,which is the basis of finding the solution.The level ofwater that develops in the pit indicates the currentwatertable. Further indications of poor drainage are the presence of high organic matter levels (seeorganic soils) and small black nodules of manganesedioxide. Topsoils waterlogged for long periods inwarm conditions have a smell of bad eggs (seesulphur cycle). The presence of compacted zonesshould be looked for as they indicate obstructions tothe flow of gravitational water (see bulk density).

Soil colours show the history of waterlogging inthe soil. Whereas free drainage is indicated by uni-form red, brown or yellow soil throughout the sub-soil, the iron oxide which gives soils these colours inthe presence of oxygen is reduced to grey or blueforms in anaerobic conditions, i.e. when no oxygenis present. Zones of soil that are saturated for pro-longed periods have a dull grey appearance, referredto as gleying. Reliance on colour alone as an indi-cation of drainage conditions is not recommendedbecause it persists for a long time after efficientdrainage is established.


Figure 14.2 Drainage.

(a) Simple Interceptor Drainage System:

(b) Outfalls

MAINSLOPE

permeable backfill

outfall pipe clearof ditch flow

clay pipes wellbutted up

(c) Silt trap/Inspection Chamber

silt trap set at change of gradient

removable cover

‘silt’

ditch

maindrain

laterals mole drains or subsoiling

headwall

vermincover

ditch

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Structural damage whether caused by water (seestability), machinery (see cultivation) or by accu-mulations of iron (see natural pans) is an obstruc-tion to water flow in the soil profile. Platy structuresnear the surface can be broken with cultivatingequipment on arable land or spiking on grassland;but subsoiling should burst those deeper in the soil.If water cannot soak away from well-structuredrooting zones, artificial drainage is required.

Low permeability soils

Low permeability of subsoils is the major reason forunder-drainage in horticultural soils (see porosity).Clay, clay loam and silty clays when wetted becomealmost impermeable as the clay swells and thecracks close.

Pipe drainage. Lines of pipes are placed in thesubsoil to intercept the trapped water.An even grad-ient from their highest point to the outfall in a ditchshould be established to prevent silting up and silttraps should be placed at regular intervals to help toservice the system at points where there is a changeof gradient or direction (see Figure 14.2).

The spacing between the lines of pipes dependson the permeability of the soil, a maximum of 5 mintervals being necessary in clay subsoils. Soil per-meability and the land use dictate the depth of thedrains which is normally more than 60 cm. Drainsshould be set deeply in cultivated land whereheavy equipment and deep cultivation might dis-turb the pipes. Shallow drains can be used whererapid drainage is a high priority and the pipes arenot likely to be crushed by heavy vehicles or severedby cultivating equipment, e.g. sports grounds.The diameter of the pipe depends on the gradientavailable and the amount of water to be carriedwhen wet conditions prevail. Most frequently usedare 75 and 100 mm diameter pipes, usually leadinginto larger drains.

Pipes are made of clay or plastic. Tiles (clay-ware) are usually 300 mm sections of pipe buttedtightly together to allow entry of water but not soilparticles. It is recommended that they are coveredwith permeable fill, usually stones or clinker, toimprove water movement into the drains. Plastic

pipes usually consist of very long lengths of pipeperforated by many small holes.

Water has to be discharged into a ditch abovethe wet season water level to allow water out ofthe pipes. The outlet of the drains is very vulne-rable to damage and so it should consist of a strong,long pipe set flush in a concrete or brick headwallso that it is not dislodged by erosion in the ditchnor by people using it as a foothold. Pipes not setflush should be glazed to prevent frost damage.Vermin traps should be fitted to prevent pipesbeing blocked by nests or dead animals.

Mole drainage is very much cheaper than pipedrainage.A mole plough draws a 75 mm ‘bullet’ fol-lowed by a 100 mm plug through the soil at a depthof 500–750 mm from a ditch up the slope of a fieldor across a pipe drain system with permeable back-fill (see Figure 14.2).The soil should be plastic at theworking depth so that a tunnel to carry water is cre-ated. The soil above should be drier so that somecracks are produced as the implement is drawnslowly along. These cracks improve the soil struc-ture and conduct water to the mole drain. Sandyand stony areas are unsuitable because tunnels arenot properly formed or collapse as water flows.Tunnels drawn in clay soils can remain useful for10–15 years but in wetter areas their useful life maybe nearer 5 or even as little as 2 years. Pipe drainageis usually combined with secondary treatments suchas mole drainage or subsoiling to achieve effectivedrainage at reasonable costs. Deep subsoilingimproves soil permeability and the pipes carry thewater away. Installation costs can be reducedbecause pipes can be laid further apart. Similarlymole drainage over and at right angles to the pipesenables them to be spaced 50–100 m apart.

Sandslitting is used on sports grounds toremove water from the surface as quickly as pos-sible. It involves cutting trenches at frequent inter-vals in the soil and infilling with carefully gradedsand that conducts water from surface to a freedraining zone under the playing surface.

Water that spills out onto lower ground fromsprings can be intercepted with ditches placed at thejunction of the permeable and impermeable layers.French drains can be placed around impermeable

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surfaces such as concrete hard standings and stonepatios to intercept the run-off.

Maintenance of drainage systems

Under-drainage is very expensive to install andmust be serviced to ensure that the investment isnot wasted. Ditches need regular attention becausethey are open to the elements.Weed growth shouldbe controlled, rubbish cleared out and collapsedbanks repaired, because obstructions lead to siltingup or the undercutting of the bank. The design ofthe ditches depends on the soil type and should bemaintained when being repaired. Drain outlets area particularly vulnerable part of the drainage sys-tem, especially if not set into a headwall. Theyshould be marked with a stake (holly trees weretraditionally used in some areas) and inspected reg-ularly after the soil returns to FC. Blockages shouldbe cleared with rods and vermin traps refittedwhere appropriate.Wet patches in the field indicatewhere a blockage in a pipe has occurred. The pipeshould be exposed and the cause of the obstructionremedied. Silted-up pipes can be rodded, brokensections replaced or dislodged pipes realigned. Silttraps need to be cleaned out regularly to preventaccumulated soil being carried into the pipes.

At all times it should be remembered that thedrains only carry away water that reaches thepipes. Every effort must be made to maintain goodsoil permeability and to avoid compaction prob-lems. Subsoilers (see page 167) should be used toremedy subsoil structural problems. Once drainagehas been installed the soil dries more quickly, lead-ing to better soil structure because cracks appearmore extensively and for longer periods. Deeperlayers of the soil are dried out as roots explore theimproved root environment, which gives anotherturn in the improving cycle.

IRRIGATION

Irrigation is used to prevent plant growth being limited by water shortage. Irrigation should be seenas a husbandry aid in addition to otherwise sound

practice. It is assumed in the following that water isbeing added to a well-drained soil unaffected bycapillary rise. The need for irrigation depends onavailable water in the rooting zone and the effect ofwater stress on the plant’s stage of growth.Responseperiods are the growth stages when the use of irri-gation during periods of rainfall deficiency is likelyto show economic benefits. In general all plantsbenefit from moist seedbeds, and eliminating waterstress maximizes vegetative growth. Initiation offlowering and fruiting is favoured by drier condi-tions. The response periods of a range of plantsgrown in the UK is given in Table 14.3.

The very large quantities of water required forcommercial production are illustrated clearly in theestimates for growing in protected culture whereall the requirements have to be delivered to thecrop by irrigation. In the British Isles, the daily con-sumption of water from a full cover crop such astomatoes or cucumbers about 20 000 l/ha in Marchrising to double that in June.This amounts to about9000 m3 per year (approximately 750 000 gallons/acre). A more exact estimate can be obtained bymeasuring the light levels outside the greenhouse;2200 l/ha are required for each megajoule persquare metre (see page 27). This can be comparedwith the 6000 m3 of rainfall that could be collected,on average, from the roof of a hectare of glass inthe south-east of England (see page 171). To takeadvantage of this contribution there would need tobe substantial storage facilities and the water qual-ity issues would need to be addressed.

Irrigation plan

In general terms water is added to a soil when mois-ture levels fall to 50 per cent of AWC in the rootingzone. Outdoors 25 mm of water is the minimumthat should be added at any one time in order toreduce the frequency of irrigation, to reduce waterloss by evaporation and to prevent the develop-ment of shallow rooting. On most soils the amountof water added should be such as to return the soilto FC. Addition of water to clays and clay loamsshould be minimized so as not to reduce the vital


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drying and wetting cycles, and if they have to be irri-gated they should not be returned to FC in caserain follows (see ponding). Irrigation should neverresult in fertilizers being leached from the rootingdepth unless it is the specific objective, as in flood-ing of greenhouse soils (see conductivity).

Most recommendations are given in a simplifiedform taking the above points into account. Therecommended plan is usually expressed in termsof how much water to apply, at a given SMD, for anamed crop on a soil of stated AWC. Thus for out-door-grown summer lettuce crop grown on soils of

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Table 14.3 Irrigation guide

Plants Response periods: growth stages at which Irrigation plan to irrigate and time of year when they occur (mm of water at mm SMD)

A B CLow* Medium High**AWC AWC AWC

Beans, runner Early flowering onwards June to August 25 at 25� 50 at 50� 50 at 75

Brussels sprouts When lower buttons August to October 40 at 40 40 at 40 40 at 4015 to 18 mm diameter

Carrots Throughout life June to September 25 at 25 40 at 50

Cauliflowers early summer Throughout life April to June 5 at 25 25 at 25

Flowers annuals and Throughout life April to early 25 at 25 25 at 25 25 at 25biennials September

Perennial Throughout life April to 25 at 25 50 at 75 50 at 75September

Lettuce summer Throughout life April to August 25 at 25 25 at 25 25 at 50

Nursery stock trees and (a) To establish newly April to June 25 at 25 25 at 50 25 at 50�shrubs planted stock

(b) Established stock May to July 25 at 25 25 at 50 25 at 50�(c) To aid early lifting September 25 at 25� 25 at 50� 25 at 50�

Potatoes, first early After tuberization May to June 25 at 25 25 at 50 25 at 50reaches 10 mm diameter

Maincrop and second From time tubers reach June to August 25 at 25 25 at 50 25 at 50earlies marble stage onwards

Rhubarb When pulling has stopped May to September 40 at 50� 40 at 50� 50 at 75�

Strawberries fruit September 50 at 50 50 at 75 50 at 75to October

Top fruit When SMD is more than 50 mm applyApples July to September 50 mm of water to suffice for Pears July to August 2 weeks. Then continue irrigation

to make the total water supply (rain � irrigation) equal to 50 mm/ fortnight for the remainder of July,40 mm/fortnight in August and 25 mm/fortnight in September.

*Less than 40 mm available water per 300 mm soil, e.g. gravels, coarse sands.**More than 65 mm available water per 300 mm soil, e.g. silts, peats.

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a medium AWC, 25 mm of water should be addedwhen a 25 mm SMD occurs. This would require the application of 250 000 l/ha or 25 l/m2. Further examples are given in Table 14.3.

SMD

SMD is the amount of water required to returnthe growing medium to FC. SMD can be calcu-lated by keeping a soil water balance sheet. Theaccount is conveniently started after rain returnsthe soil to FC, i.e. when SMD is zero. In Britain itis assumed that, unless it has been a dry winter, thesoil is at FC until the end of March. From the firstday of April a day-by-day check can be made ofwater gains and losses.

Rainfall varies greatly from year to year from onelocality to the next and so it should be determined

on site (see rain-gauge) or obtained from a localweather station.Water loss for each month does notvary very much over the years and so potential tran-spiration rates based on past records can be used inthe calculation. There are potential transpirationrate figures available for all localities having weatherstations. Examples are given in Table 14.2.These fig-ures can be used when calculating water loss butuntil there is 20 per cent leaf canopy, a maximumSMD of 20 mm is not exceeded, because in the earlystages water loss is predominantly from the soil sur-face by evaporation.

A worked example of a weekly water balancesheet is given in Table 14.4; a daily water balancesheet may be more appropriate in some situations.

In protected cropping all the water that plantsrequire has to be supplied by the grower, whomust therefore have complete control over irriga-tion. With experience the grower can determine


Table 14.4 A weekly soil balance sheet for established nursery stock grown on sandy loam(AWC 55 mm per 300 mm) in Essex. The irrigation plan is to apply 25 mm water if a 50 mmSMD is reached (see Table 14.3). Water loss estimated from Table 14.2

Week Water Water gains in mm SMD at end of week (mm)beginning loss (mm) Rainfall (irrigation)

March 31 0

April 7 11 10 (0) 0 � 11 � (10 � 0) � 114 11 8 (0) 1 � 11 � (8 � 0) � 421 12 16 (0) 4 � 12 � (16 � 0) � 028 12 5 (0) 0 � 12 � (5 � 0) � 7

May 4 16 28 (0) 7 � 16 � (28 � 0) � 0*11 17 10 (0) 0 � 17 � (10 � 0) � 718 18 14 (0) 7 � 18 � (14 � 0) � 1125 18 4 (0) 11 � 18 � (4 � 0) � 25

June 1 18 10 (0) 25 � 18 � (10 � 0) � 338 23 14 (0) 33 � 23 � (14 � 0) � 42

15 23 18 (0) 42 � 23 � (18 � 0) � 4722 23 20 (0) 47 � 23 � (20 � 0) � 50**29 23 10 (25) 50 � 23 � (10 � 25) � 38

July 6 22 8 (0) 38 � 22 � (8 � 0) � 52**13 22 18 (25) 52 � 22 � (18 � 25) � 3120 22 24 (0) 31 � 22 � (24 � 0) � 2927 22 5 (0) 29 � 22 � (5 � 0) � 46

*SMD cannot be less than zero because water above FC drains away.**Irrigation might have been delayed if prolonged heavy rain forecast.

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water requirements by examining plants, soil, rootballs or by tapping pots. A tensiometer can beused to indicate the soil water tension but, whilethis is useful to indicate when to water, it does not show directly how much water is needed.Evaporimeters distributed through the plantingcan give the water requirement by showing howmuch water has been evaporated. A solarimetermeasures the total radiation received from the sunand the readings obtained can be used to calculatethe water losses, often expressed in litres per squaremetre for convenience.

Methods of applying water

These should be carefully related to plant require-ments, climate and soil type. On a small scale,watering cans or hoses fitted with trigger lances canbe used,but care should be taken to avoid damagingthe structure of the growing medium.Water can besprayed from fixed or mobile equipment but it isessential that the rate of application is related tosoil infiltration rate. The droplet size in the sprayshould not be large enough to damage the surfacestructure (see tilth). Indoors, spray lines can be fit-ted with nozzles to control the direction and quan-tity of water. Overhead lines can lead to very highhumidity levels and wet foliage, predisposing someplants to disease (see grey mould). Consequently,it should be restricted to watering low level crops,e.g. lettuce,deliberately increasing humidity (‘damp-ing down’ or ‘spraying over’) or winter flooding(see conductivity). Trickle lines deliver water veryslowly to the soil, leaving plant foliage and the soilsurface dry, which ensures a drier atmosphere andreduced water loss. However, care is neededbecause there is very little sideways spread of waterinto coarse sand, loose soil or a growing mediumthat has completely dried out. Drip irrigation is a variation on the trickle method but the water is applied through pegged down thin, flexible‘spaghetti’ tubes to exactly where it is needed, e.g.in each pot or base of each plant.

Simple flooded benches are sometimes used towater pot plants, the shallow tray of the benched

area is filled with water from which the pots take up water after which the water is drained off (‘ebband flow’). This tends to produce a high humidityaround the plants and capillary benches have cometo be preferred.The pots stand on, and the contentsmake contact with, a level 50 mm bed of sand keptsaturated at the base by an automatic water supply.Water lost by evaporation at the surface and fromthe plant is replaced by capillary rise.The sand mustbe fine enough to lift the water but coarse enoughto ensure that the flow rate is sufficient. Capillarymatting made of fibre woven to a thickness andpore size to hold, distribute and/or lift water hasmany uses in watering containerized crops indoorsor outside. Containers with built-in water reservesand easy watering systems utilize capillarity to keepthe rooting zone moist. Sub-irrigated sand beds areused for standing out container plants in nurseryproduction and are less wasteful of water that themore usual overhead spray lines.

WATER QUALITY

Water used in horticulture is taken from differentsources and has different dissolved impurities.Soft water has very few impurities, whereas hardwater contains large quantities of calcium and/ormagnesium salts which raise the pH of the grow-ing medium, especially where the impurities accu-mulate (see liming). Even small quantities ofmicro-elements such as boron or zinc have to beallowed for when making up nutrient solutionswhich are to be recirculated (see hydroponics).Water taken from boreholes in coastal areas canhave high concentrations of salt that can lead to salt concentration problems. The quantity ofdissolved salt in water can be measured by itsconductivity; the higher the salt concentration thegreater its electrical conductivity. Providing thelevels of useful salts are not too high, the watercan be used as long as the additional nutrientlevels (fertilizers) are suitably adjusted (seeconductivity).

In the recirculation systems that are becomingmore prevalent in protected culture, the salts not

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used by plants become concentrated in the water.These dissolved salts can interfere with the uptakeof useful salts such as potassium, make it difficultto create a balanced feed within the safe conducti-vity limits and reduce the plant growth rates asthey become too concentrated.

Rainwater is increasingly being used as a majorsource of water. It is usually of high quality, i.e. lowconductivity but there can be contamination relatedto the location or the method of collection or stor-age, e.g. high levels of zinc when collected throughgalvanized gullies.

Good quality rainwater can be used to dilute otherwise unsuitable water to bring it into use.Alternatively, poor quality water can be treatedusing reverse osmosis; water under pressure isforced through a membrane which holds back mostof the dissolved salts. Alternatively, deionizationcan be used, which involves passing the water overresins to remove the unwanted salts. In both cases,an environmentally sound method for disposal ofthe concentrated solution produced remains aproblem. The high-energy distillation and electro-dialysis methods are generally too expensive forcleaning water for growing.

Water drawn from rivers, lakes or even on-sitereservoirs may contain algal, bacterial or fungalpollution, which can lead to blocked irrigationlines or plant disease (see hygienic growing).

To avoid disease problems, water supplies can besterilized. On a commercial scale this is usuallydone by heat sterilization. Ultraviolet light or ozonetreatments are usually more expensive and the useof hydrogen peroxide tends to be less effective.

WATER CONSERVATION

The need to manage water efficiently is a majorconcern in the use of scarce resources. Respon-sible action is increasingly supported by legisla-tion and the higher price of using water. Clearlythe major factors that determine the level of wateruse are related to the choice of plant species to begrown and the reasons for growing. The selectionof drought tolerant rather than water intensive

plantings is fundamental. Likewise the recycling ofwater and the capture of rainwater are importantconsiderations in the choice of water source. Somegrowing systems are inherently less water intensive,but in most of them there are many ways in whichwater use can be reduced if certain principles arekept in mind and acted upon appropriately.

Whenever possible water should not be used;alternative strategies should be considered. If ithas to be applied, recycled water should be con-sidered. Water must be used efficiently.

Water is lost primarily through drainage orevaporation. Where application is partially con-trolled, the correct relationship between waterapplied and water holding (see WHC) helps toprevent leaching, which leads to nutrient loss (seenitrogen). Thus returning an outdoor soil to lessthan FC helps avoid losses to drainage and lossesby run off in the event of unexpected rainfall.

Reducing the action of the drying atmospherecan minimize evaporation losses. Overhead appli-cation of water in the open can be limited byincreasing the growing medium’s water reservoir.Soils have different available WHC but most canbe improved by the addition of suitable organicmatter. Most importantly, maintaining good soilstructure can increase the rooting depth. Plantsshould be encouraged to establish as quickly aspossible but, after the initial watering-in, infre-quent applications will encourage the plant to putdown deep roots by searching for water. Whenwater does have to be applied overhead, thisshould be undertaken in cool periods. However,care should be taken to avoid creating conditionsthat encourage diseases such as Botrytis.

Water is lost more rapidly from a moist than adry soil surface. After just 5 mm of water has beenlost from the surface, the rate of evaporation fallssignificantly. Infrequent application thus helps, buteven more effective is the delivery of water to spe-cific spots next to plants (see trickle lines) or frombelow through pipes to the rooting zone. Avoidbringing moist soil to the surface. If hoeing isundertaken it should be confined to the very toplayers, this also reduces the risk of root damage.Losses from the surface can be reduced considerably


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by plant cover and almost eliminated by the use ofmulches. Loss of water from the plants themselvesis reduced when they are grouped together ratherthan spaced out.

Unless maximum growth rates are the main con-sideration, reduced application saves water, moneyand staff time without detriment to most plant-ings. In production horticulture, the introduction ofsophisticated moisture-sensing equipment and com-puter control has enabled water to be deliveredmore precisely when and where it is needed. Thishas led to considerable reductions in water use.

Nutrient loss and run off from overhead water-ing used in container nursery stock production canbe minimized by matching application to each ofthe following: rainfall, growing medium containersize, plant species, stage of growth and the time ofyear. Nozzles should be maintained to ensure evenwater application. There tends to be less loss fromsub-irrigated capillary sand beds. Recirculation(closed) systems should be considered in newdevelopments.

The quantity of water required for flooding soilsin protected culture (e.g. to remove excess nutri-ents when a crop sensitive to high salt levels, suchas lettuce, is to be grown after a tolerant one, suchas tomatoes) can be reduced by discontinuing theliquid feeding of the first crop as soon as possible.

In non-recirculating (open) hydroponics systemsexcessive water waste should be avoided by usingflow meters to measure the quantity of run-off andcomparing it with standard figures for the growingsystem used.A run-off of over 30 per cent is usuallyconsidered to be excessive and the amount and fre-quency of nutrient applications delivered by thenozzles or drippers should be reviewed. Closed sys-tems (see NFT, page 218) recirculate the nutrient

solution but this is not always practical.Where theyare, the system must not be emptied illegally intowatercourses or soakaways. It is recommended thatthe volume in the system be run down before dis-charge and the waste nutrient solution be sprayedonto crops during growing season. Permission toempty into public sewers might be granted but isusually subject to a charge depending on volumeand contamination level.

FURTHER READING

Baily, R. Irrigated Crops and Their Management(Farming Press, 1990).

Brown, L.V. Applied Principles of Horticulture, 2nd edi-tion, (Butterworth-Heinemann, 2002).

Castle, D.A., McCunnell, J. and Tring, I.M. FieldDrainage: Principles and Practice (Batsford Academic,1984).

Farr, E. and Henderson, W.C. Land Drainage, LongmanHandbooks in Agriculture, (Longman, 1986).

Hope, F. Turf Culture: A Manual for the Groundsman(Cassell, 1990).

Ingram, D.S. et al. (Editors) Science and the Garden(Blackwell Science Ltd., 2002).

MAFF Advisory Booklets:No. 2067 Irrigation Guide (1979).No. 2140 Watering Equipment for Glasshouse Crops(1980).

MAFF Advisory Leaflets:Nos 721–740 Getting Down to Drainage (various dates).No. 776 Water Quality for Crop Irrigation (1981).

MAFF Advisory Reference Book No. 138 Irrigation(1976).

MAFF Technical Bulletin No. 34 Climate and Drainage(1970).

MAFF Code of Good Agricultural Practice for theProtection of Water (1991).

Soil Water 183

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LIVING ORGANISMS IN THE SOIL

As in any other plant and animal community theorganisms that live in the soil form part of the foodwebs (see page 6). The main types present in anysoil are the primary producers which are thosecapable of utilizing the sun’s energy directly, syn-thesizing their own food by photosynthesis, suchas green plants (see photosynthesis), the primaryconsumers which are those organisms which feeddirectly on plant material and secondary consumerswhich feed only on animal material. In practice,

there are some organisms that feed on both plantsand animals and also parasites living on organismsin all categories.

Decomposers are an important group, whichhave the special function within a community ofbreaking down dead or decaying matter into sim-pler substances with the release of inorganic salts,making them available once more to the primaryproducers. Primary decomposers are those organ-isms that attack the freshly dead organic matter.These include insects, earthworms and fungi. Fungiare particularly important in the initial decom-position of fibrous and woody material. Secondarydecomposers are those organisms that live on thewaste products of other decomposers and includebacteria and many species of fungi.

Plant roots

These are important as contributors to the organicmatter levels in the soil. They shift and move soilparticles as they penetrate the soil and grow in size.This rearrangement changes the sizes and shapesof soil aggregates and when these roots die anddecompose,a channel is left which provides drainageand aeration. Root channels are formed over andover again unless the soil becomes too dense forroots to penetrate. Roots absorb water from soilsand dry it, causing those with a high clay content toshrink and crack. This helps develop and improvestructures on heavier soils (see page 164).

15

Soil Organic Matter

Organic matter exerts a profound influence oncrop nutrition, soil structure, and cultivations.In this chapter the three main categories oforganic matter are described; the living organ-isms, dead but identifiable organic matter andhumus. The nutrient cycles describe how min-erals are released from plant tissue and madeavailable in the soil as plant nutrients. Theinterlocked nature of the different nutrientcycles is illustrated by considering the effect ofthe carbon:nitrogen ratio. The organic matterlevels in different soils and the factors thatcause the variations are examined. The benefi-cial effects of organic matter are establishedand the characteristics of different sources ofbulky organic matter are considered.

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Soil Organic Matter 185

Earthworms

There are 10 common species of earthworm inBritain that vary in size from Lumbricus terrestris,which can be in excess of 25 cm, to the many smallspecies less than 3 cm long. The main food ofearthworms is dead plant remains. Casting speciesof earthworms are those that eat soil as well asorganic matter and their excreta consist of intim-ately mixed, partially digested, finely dividedorganic matter and soil. Many species never pro-duce casts and only two species regularly cast onthe surface giving the worm casts that are a prob-lem on fine grass areas, particularly in the autumn.It has been estimated that in English pastures theproduction of casts each year is 20–40 t/ha, theequivalent of 5 mm of soil deposited annually.Thissurface casting also leads to the incorporation ofthe leaf litter and the burying of stones. However,L. terrestris is the organism mainly responsible forthe burying of large quantities of litter by draggingplant material down its burrows.

The network of burrows, which develops as aresult of worm activity, is an important factor inmaintaining a good structure, particularly in unculti-vated areas and in soils of low clay content. Somespecies live entirely in the surface layers of the soils,others move vertically establishing almost perman-ent burrows down to 2 m.

Earthworm activity and distribution is largelygoverned by moisture levels, soil pH, temperature,organic matter and soil type. Most species tend tobe more abundant in soils where there are goodreserves of calcium. Earthworm populations areusually lower on the more acid soils, but most thrivein those near neutral. Worm numbers decrease indry conditions, but they can take avoiding actionby burrowing to more moist soil or by hibernating.Each species has its optimum temperature range;for L. terrestris this is about 10°C, which is typicalof soil temperatures in the spring and autumn inthe UK. Soils with low organic matter levels sup-port only small populations of worms. In contrast,compost heaps and stacks of farmyard manure(FYM) have high populations. In oak and beechwoods where the fallen leaves are palatable to

worms, their populations are large and they canremove a high proportion of the annual leaf-fall.Thisalso happens in orchards unless harmful chemicalssuch as copper have reduced earthworm popula-tions. Light and medium loams support a highertotal population than clays, peat and gravelly soils.

Slugs and snails, arthropods such as millipedes,springtails and mites, and nematodes are also foundin high numbers and play an important part in thedecomposition of organic matter. Several speciesare also horticultural pests (see Chapter 10).

Bacteria

Bacteria are present in soils in vast numbers.About 1000 million or more occur in each gram offertile soil. Consequently, despite their microscopicsize, the top 150 mm of fertile topsoil carries about1 t of bacteria per hectare. There are many differ-ent species of bacteria to be found in the soil andmost play a part in the decomposition of organicmatter. Many bacteria attack minerals; this leads to the weathering of rock debris and the release ofplant nutrients. Detoxification of pesticides andherbicides is an important activity of the bacterialpopulation of cultivated soils.

Soil bacteria are inactive at temperatures below6°C but their activities increase with rising tempera-ture up to a maximum of 35°C. Actively growingbacteria are killed at temperatures above 82°C, butseveral species can form thick-walled resting sporesunder adverse conditions. These spores are veryresistant to heat and they survive temperatures up to 120°C. Partial sterilization of soil can kill the actively growing bacteria but not the bacterialspores. The growth rate and multiplication dependalso upon the food supply. High organic matterlevels support high bacterial populations so long asa balanced range of nutrients is present. Bacteriathrive in a range of pH 5.5–7.5; fungi tend to domin-ate the more acid soils. Aerobic conditions shouldbe maintained because the beneficial organisms aswell as plant roots require oxygen, whereas manyof the bacteria that thrive under anaerobic condi-tions are detrimental.

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Fungi

The majority of fungi live saprophytically on soilorganic matter. Some species are capable only ofutilizing simple and easily decomposable organicmatter whereas others attack cellulose as well.There are some important fungi that can decom-pose lignin; making them one of the few primarydecomposers of wood and fibrous plant material.Several fungi in the soil are parasites and examplesof these are discussed in Chapter 11. Fungi appearto be able to tolerate acid conditions and low cal-cium better than other micro-organisms and areabundant in both neutral and acid soils. Most arewell adapted to survive in dry soils but few thrivein very wet conditions. Their numbers are high insoils rich in plant residues but decline rapidly asthe readily decomposable material disappears.Thebacteria persist longer, where present, and eventu-ally consume the fungal remains.

The rhizosphere

The rhizosphere (see also page 8) is a zone in thesoil that is influenced by roots. Living roots changethe atmosphere around them by using up oxygenand producing carbon dioxide (see respiration).Roots exude a variety of organic compounds thathold water and form a coating that bridges the gapbetween root and nearby soil particles. Micro-organisms occur in greatly increased numbers andare more active when in proximity to roots. Someactually invade the root cells where they live assymbionts. The Rhizobium spp. of bacteria livessymbiotically with many legumes.

Symbiotic associations involving plant roots andfungi are known as mycorrhizae. There is consider-able interest in exploiting the potential of mycor-rhizae, which appear to be associated with a highproportion of plants especially in less fertile soils.In this symbiotic relationship the fungus obtains itscarbohydrate requirements from the plant. In turn,the plant gains greater access to nutrients in thesoil, especially phosphates, through the increasedsurface area for absorption and because the fungus

appears to utilize sources not available to higherplants. Most woodland trees have fungi coveringtheir roots and penetrating the epidermis. Orchidsand heathers have an even closer association inwhich the fungi invade the root and coil up withinthe cells. The association appears to be necessaryfor the successful development of the seedlings.Mycorrhizal plants generally appear to be moretolerant of transplanting and this is thought to bean important factor for orchard and container-grown ornamentals.

NUTRIENT CYCLES

All the plant nutrients are in continuous circulationbetween plants, animals, the soil and the air. Theprocesses contributing to the production of simplerinorganic substances such as ammonia, nitrites,nitrates, sulphates and phosphates are sometimesreferred to as mineralization. Mineralization yieldschemicals that are readily taken up by plants fromthe soil solution. The formation of humus, organicresidues of a resistant nature, is known as humifica-tion. Both mineralization and humification are inti-mately tied up in the same decomposition processbut the terms help identify the end product beingstudied. Likewise it is possible to follow the circula-tion of carbon in the carbon cycle and nitrogen inthe nitrogen cycle, although these nutrient cyclesalong with all the others are interrelated.

Carbon cycle

Green plants obtain their carbon from the carbondioxide in the atmosphere and during the processof photosynthesis are able to fix the carbon, con-verting it into sugar. Some carbon is returned to theatmosphere by the green plants themselves duringrespiration, but most is incorporated into plant tissue as carbohydrates, proteins, fats, etc. The car-bon incorporated into the plant structure is eventu-ally released as carbon dioxide, as illustrated inFigure 15.1.

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All living organisms in this food web releasecarbon dioxide as they respire. The sugars, cellu-lose, starch and proteins of succulent plant tissue,as found in young plants, are rapidly decomposedto yield plant nutrients and have only a short-termeffect. In contrast, the lignified tissue of olderplants rots more slowly. Besides the release ofnutrients, humus is formed from this fibrous andwoody material, which has a long-term effect onthe soil. Plants grown in the vicinity of vigorouslydecomposing vegetation, e.g. cucumbers in strawbales, live in a carbon dioxide enriched atmos-phere. Carbon dioxide is also released on combus-tion of all organic matter, including the fossil fuelssuch as coal and oil. Organic materials such asparaffin or propane, which do not produce harm-ful gases when burned cleanly, are used in pro-tected culture for carbon dioxide enrichment.

Nitrogen cycle

Plants require nitrogen to form proteins. Althoughplants live in an atmosphere largely made up ofnitrogen they cannot utilize gaseous nitrogen.Plants take up nitrogen in the form of nitrates and,to a lesser extent, as ammonia. Both are releasedfrom proteins by a chain of bacterial reactions asshown in Figure 15.2.

Ammonifying bacteria convert the proteins theyattack to ammonia. Ammonia from the breakdownof protein in organic matter or from inorganic


CARBONDIOXIDE

GREEN PLANTS

SOIL ANIMALS

MICRO-ORGANISMS

HUMUS

FOSSIL FUELS

PEATS

SOILORGANICMATTER

combustion

photosynthesis

respiration

Figure 15.1 Carbon cycle. The recycling of the elementcarbon by organisms is illustrated. Note how all the car-bon in organic matter is eventually released as carbondioxide by respiration or combustion. Green plants con-vert the carbon dioxide by photosynthesis into sugarswhich form the basis of all the organic substances requiredby plants, animals and micro-organisms.

GASEOUSNITROGEN

N-FIXINGBACTERIA

PLANT PROTEIN

ANIMALS

MICRO-ORGANISMS

denitrification

lightning

NITRATE

NITRITE

AMMONIA

NITROGEN FERTILIZERS

ROCK DEPOSITS

HaberProcess

industrial process

Figure 15.2 Nitrogen cycle. The recycling of the elementnitrogen by organisms is illustrated. Note the importanceof nitrates that can be taken up and used by plants to manufacture protein. Micro-organisms also have thisability but animals require nitrogen supplies in proteinform. Gaseous nitrogen only becomes available toorganisms after being captured by nitrogen-fixing organ-isms or via nitrogen fertilizers manufactured by man.In aerobic soil conditions, bacteria convert ammonia tonitrates (nitrification), whereas in anaerobic conditionsnitrates are reduced to nitrogen gases (denitrification).

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nitrogen fertilizers is converted to nitrates by nitri-fying bacteria. This is accomplished in two stages.Ammonia is first converted to nitrites by Nitro-somonas spp. Nitrites are toxic to plants in smallquantities but they are normally converted tonitrates by Nitrobacter spp. before they reach harm-ful levels. Ammonifying and nitrifying bacteriathrive in aerobic conditions.

Where there is no oxygen, anaerobic organismsdominate. Many anaerobic bacteria utilize nitratesand in doing so convert them to gaseous nitrogen.This denitrification represents an important lossof nitrate from the soil, which is at its most seriousin well-fertilized, warm and waterlogged land.

Nitrogen fixation. Although plants cannot utilizegaseous nitrogen it can be converted to plantnutrients by some micro-organisms. Azotobactorare free-living bacteria that obtain their nitrogenrequirements from the air. As they die anddecompose, the nitrogen trapped as protein isconverted to ammonia and then nitrates by othersoil bacteria. The Rhizobia spp. which live in rootnodules on some legumes, also trap nitrogen to thebenefit of the host plant. Finally, nitrogen gas canbe converted to ammonia industrially in the Haberprocess, which is the basis of the artificial nitrogenfertilizer industry.

Sulphur cycle

Sulphur is an essential constituent of plants whichaccumulates in the soil in organic forms. This sul-phur does not become available to plants untilaerobic micro-organisms, which yield soluble sul-phates, mineralize the organic form. Under anaer-obic conditions there are micro-organisms whichutilize organic sulphur and produce hydrogen sul-phide which has a characteristic smell of bad eggs.

Carbon to nitrogen ratio

All nutrients play a part in all nutrient cycles simplybecause all organisms need the same range of

nutrients to be active. Normally there are adequatequantities of nutrients, with the exception of carbonor nitrogen, which are needed in relatively largequantities. A shortage of nitrogenous material wouldlead to a hold-up in the nitrogen cycle but would alsoslow down the carbon cycle, i.e. the decomposition oforganic matter is slowed because the micro-organ-isms concerned suffer a shortage of one of theiressential nutrients. A useful way of expressing therelative amounts of the two important plant foods isin the carbon to nitrogen (C:N) ratio.

Plant material has relatively wide C:N ratios, butthose of micro-organisms are much narrower. Thisis because micro-organisms utilize about three-quarters of the carbon in plants during decompos-ition as an energy source. The carbon utilized thisway is released as carbon dioxide, whereas, usually,all the nitrogen is incorporated in the microbialbody protein.This concentrates the nitrogen in thenew organism that is living on the plant material.

Sometimes the C:N ratio is so wide that somenitrogen is drawn from the soil and ‘locked up’ inthe microbial tissue. This is what happens whenstraw (and similar fibrous or woody material suchas wood chips and bark) with a ratio of 60:1 is duginto the soil. For example, if a thousand 12 kg balesof straw are dug into 1 ha of land then the additionto the soil will be 12 000 kg of straw containing4800 kg of carbon and 80 kg of nitrogen. Three-quarters of the carbon (3600 kg) is utilized for energyand lost as carbon dioxide and a quarter (1200 kg)is incorporated over several months into microbialtissue. Microbial tissue has a C:N ratio of about 8:1which means that by the time the straw is used upsome 150 kg of nitrogen is locked up with the1200 kg of carbon in the micro-organisms. Sincethere was only 80 kg of nitrogen in the straw puton the land, the other 70 kg has been ‘robbed’ fromthe soil. This nitrogen is rendered unavailable toplants (‘locked up’) until the micro-organisms dieand decompose.To ensure rapid decomposition orto prevent a detrimental effect on crops the addi-tion of straw must be accompanied by the additionof nitrogen.

Nitrogen is released during decomposition if theorganic material has a C:N ratio narrower than

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30:1, such as young plant material, or with nitrogen-supplemented plant material such as FYM.

In general, fresh organic matter decomposes veryrapidly so long as conditions are right, but the olderresidues tend to decompose very slowly.

HUMUS

Active micro-organisms gradually decompose thedead organic matter of the soil until it consists offinely divided plant and micro-organism remains. Itis a black colloidal material which coats soil particlesand gives topsoil its characteristic dark colour. Thisprocess of humification leads to the formation ofhumus,a collection of humic acids,which is the slowlydecomposing residue of soil organic matter. It ismainly derived from the decomposition of fibrousvegetation that is rich in lignin such as straw. Thiscolloidal material has a high cation exchange capac-ity and therefore can make a major contribution tothe retention of exchangeable cations, especially onsoils low in clay (see sands, page 157). It also adheresstrongly to mineral particles, which makes it a valu-able agent in soil aggregation. In sandy soils it pro-vides a means of sticking particles together whereasin clays it forms a clay–humus complex that makesthe heavier soils more likely to crumble. Its presencein the soil crumbs makes them more stable, i.e. moreable to resist collapse when wetted and it increasesthe range of soil workability (see page 175). Bacteriaeventually decompose humus so the amount in thesoil is very much dependent on the continued add-ition of appropriate bulky organic matter.

ORGANIC MATTER LEVELS

The routine laboratory method for estimatingorganic matter levels depends upon finding thetotal carbon content of the soil. A simpler methodis to dry a sample of soil and burn off the organicmatter. After cooling, the soil can be re-weighedand the loss in weight represents destroyed organicmatter. These methods give an overall total of soilorganic matter excluding the larger soil animals.

Most topsoil contains between 1 and 6 per cent oforganic matter, whereas subsoil usually containsless than 2 per cent. The distribution of organicmatter under grass in normal temperate areas isshown in Figure 15.3. Soil organic matter is con-centrated in the topsoil because most of the rootsoccur in this zone and the plant residues tend to be added to the surface forming the leaf litterlayer. The organic matter level in any part of the soildepends upon how much fresh material is addedcompared with the rate of decomposition. It is stable when these two processes are balanced andthe equilibrium reached is determined mainly byclimate, soil type and treatment under cultivation.

Climate

Climate affects both the amount of organic matteradded and the rate of decomposition. Below 6°Cthere is no microbial activity, but it increases with

% Organic matterLitter layer 1 2 3 4 5

Topsoil

Subsoil

1 metre

Figure 15.3 Distribution of organic matter in an unculti-vated soil. Organic matter content of soil decreases fromthe soil surface downwards. Note that the topsoil is signifi-cantly richer in humus, which gives it a characteristicallydarker colour.

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increasing temperature so long as conditions areotherwise favourable. In dry areas there is notonly less plant growth, resulting in less organicmatter being added to the soil, but also less micro-bial action. In warm climates, where there isadequate moisture, low organic matter resultsfrom very much increased decomposition. In coolerareas there tends to be an accumulation of organicmatter because the decreased plant growth ismore than offset by the reduced micro-organismactivity that occurs over the long winter periods.Organic matter also tends to accumulate in wetterconditions.Where waterlogging is prevalent, 10–20per cent organic matter levels develop; wherewaterlogging is permanent, organic matter accu-mulates to give rise to peat.

Soil type

Generally, soils with lower clay contents have lowerorganic matter levels. Coarse sands and sandyloams tend to be warmer than finer-textured soilsand have better aeration, which results in highermicrobial activity. Such soils often support lessplant growth because of poor fertility and poorwater-holding capacity. These factors combine togive soils with low organic matter levels. In culti-vation these same soils become a problem unlesslarge quantities of organic matter are applied atfrequent intervals to maintain adequate humuslevels. Such soils are often referred to as ‘hungrysoils’ because of their high demand for manure.On finer-textured soils the higher fine sand, silt orclay content increases the water-holding capacity.This reduces soil temperature, resulting in lessmicrobial activity. The presence of clay directlyreduces the rate of decomposition because it com-bines with humus and protects it from microbialattack.

Cultivation

On first cultivation the increased aeration andnutrients stimulate micro-organisms and a new

equilibrium with lower soil organic matter levels prevails. Once under cultivation, grasses andhigh-producing legumes tend to increase levelsbut most crops, particularly those in which com-plete plant removal occurs, lead to decreasedorganic matter levels. Only large, regular dressingsof bulky organic matter such as straw, FYM,leaf mould and compost can improve or main-tain the level of soil organic matter on cultivatedsoils.

Organic matter can accumulate under grass andform a mat on the surface where the carbon cycleis slowed because of nutrient deficiency usuallyinduced by surface soil acidity or excess phosphatelevels.This is part of the reason for the developmentof ‘thatch’ in turf.

Organic soils

While all soils contain some organic matter, mostare classified as mineral soils. Organic soils arethose that have enough organic matter present todominate the soil properties and they developwhere decomposition is slow because the activityof micro-organisms is reduced by cold or water-logged conditions. Peat is formed of partiallydecomposed plant material. This occurs in water-logged conditions, usually low in nutrients, wheredecomposition rates are low. There are great dif-ferences between peats because of the variationsin conditions where they occur and the species ofplants from which they are formed. Some peat isformed in shallow water, as found in poorly draineddepressions or infilling lakes. In such circum-stances the water drains from surrounding mineralsoils and consequently has sufficient nutrients tosupport vegetation often dominated by sedges,giving rise to sedge peat. As the waterlogged area,pond or lake becomes full of humified organicmatter it forms a bog, moor or fen. In the drierconditions of eastern England some of these areashave been drained to form the productive fenland.In the wetter west, sphagnum moss, which is ableto live on very low nutrient levels that prevail, growson top of the infilled wet land. The dead vegetation


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becomes very acid and decomposes slowly. Itbuilds up to form a high moor.

Some of the peatlands, which are enriched withminerals, prove very valuable when drained. Manyof these soils are used to produce vegetables andother high value crops. Unfortunately the increasedaeration allows the organic matter to be decom-posed at a rate faster than it can be replenished.Furthermore, when the surface dries out the lightparticles are vulnerable to wind erosion. Conse-quently, the soil level of these areas is falling at therate of 3 m every 100 years. Keeping the watertableas high as possible and providing protection againstwind erosion can minimize subsidence.

BENEFITS OF ORGANIC MATTER

There are three main types of organic matter in the soil; the living organisms, the dead ones invarying degrees of decomposition and the humus.

The presence of partially decomposed organicmatter creates an open soil structure and, on many soils, increases the water-holding capacity.As it decomposes it can act as a dilute slow releasesource of nutrients.

The humus coats the soil particles and modifiestheir characteristics. On sandy and silty soils thehumus enables stable crumbs to be formed. Stable,well-structured fine sands and silts are only pos-sible under intensive cultivation if high humuslevels are maintained by the addition of largequantities of bulky organic matter. The surfacecharges on humus are capable of combining withthe clay particles, thereby making heavy soils lesssticky and more friable.These surface charges alsoenable humus to hold cations against leaching,which is very important in soils low in clay. Humusalso improves water-holding properties of the soil.Its darkening effect increases heat absorption.

The living organisms in the soil play their part inthe conversion of plant and animal debris to min-erals and humus. The Rhizobia and Azotobacterspp. fix gaseous nitrogen while other bacteria playan important role in the detoxification of harmfulorganic materials such as pesticides and herbicides.

Soil structure is modified by the influence of plantroots and earthworms.

ADDITION OF ORGANIC MATTER

It is normal in horticulture to return residues tocultivated areas where possible. Whether or notthe plant remains are worked into the soil in whichthey have been grown depends upon their nature.The residue of some crops such as tomatoes in thegreenhouse are removed to reduce the diseasecarryover and because it cannot easily be incorpo-rated into the soil. Other crops such as hops areremoved for harvesting and some of the processedremains, spent hops, can be returned or used else-where. Wherever organic matter is removed,whether it is just the marketed part such as topfruit from the orchard or virtually the whole cropsuch as cucumbers from a greenhouse, the nutri-ents removed must be replaced to maintain fertil-ity (see fertilizers).

Although some forms do not return nutrients tothe soil, bulky organic matter such as compost,straw, FYM and peat is an important means ofmaintaining organic matter and humus levels. Themain problem is obtaining cheap enough sourcesbecause their bulk makes transport and handling amajor part of the cost.They can be evaluated on thebasis of their small nutrient value and their effecton the physical properties of soil as appropriate.

Composting

Composting refers to the rotting down of plantresidues before they are applied to soils. Many gar-deners depend on composting as a means of usinggarden refuse to maintain organic matter levels intheir soils. On a larger scale there is interest in theuse of composted town refuse for horticultural pur-poses. Many councils are now supplying compost-ing equipment to encourage householders torecycle organic matter as well as glass and metals.Horticulturists are increasingly concerned with therecycling of wastes and attention is being given to


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modern composting methods. For successful com-posting, conditions must be favourable for thedecomposing micro-organisms. The material mustbe moist and well aerated throughout.As the heapis built, separate layers of lime and nitrogen areadded as necessary to ensure the correct pH andC:N ratio. Organic waste brought together in largeenough quantities under ideal conditions can gener-ate enough heat to take the temperature to over70°C within 7 days. If all the material is brought tothis temperature it has the advantage of killingharmful organisms and weed seeds.

Straw

Straw is an agricultural crop residue readily avail-able in many parts of the country, but care shouldbe taken to avoid straw with harmful herbicideresidues. It is ploughed in or composted and thenworked in. There appears to be no advantage incomposting if allowance is made for the demandon nitrogen by soil bacteria.About 6 kg of nitrogenfertilizer needs to be added for each tonne of strawfor composting or preventing soil robbing (seepage 188). Chopping the straw facilitates its incor-poration and while under-composed it can open upsoils. On decomposition, it yields very little nutri-ent for plant use but makes an important contribu-tion to maintaining soil humus levels. Straw balessuitably composted on site are the basis of produc-ing an open growing medium for cucumbers.

FYM

This is the traditional material used to maintainand improve soil fertility. It consists of straw, orother bedding, mixed with animal faeces and urine.The exact value of this material depends upon theproportions of the ingredients, the degree of decom-position and the method of storage. Samples varyconsiderably. Much of the FYM is rotted down inthe first growing season but almost half survivesfor another year, and half of that goes onto a thirdseason and so on. A full range of nutrients is

released into the soil and the addition of the majornutrients should be allowed for when calculatingfertilizer requirements. The continued release oflarge quantities of nitrogen can be a problem,especially on unplanted ground in the autumn,when the nitrates formed are leached deep intothe soil over the winter.

FYM is most valued for its ability to provide theorganic matter and humus for maintaining orimproving soil structure. As with any other bulkyorganic matter, FYM must be worked into soilswhere conditions are favourable for its continueddecomposition. Where fresh organic matter isworked into wet and compacted soils the need foroxygen outstrips supply and anaerobic conditionsdevelop to the detriment of any plants present.Where this occurs a foul smell (see sulphur) andgrey colourings occur. FYM should not be workedin deep, especially on heavy soil.

Horticultural peats

Sphagnum moss peats have a fibrous texture, highporosity, high water retention and a low pH. Theyare used extensively in horticulture as a source of bulky organic matter and are particularly valuedas an ingredient of potting composts because, withtheir stability, excellent porosity and high waterretention, they can be used to create an almostideal root environment.

Sedge peats tend to contain more plant nutri-ents than sphagnum moss. They are darker, moredecomposed, and have a higher pH level but aslightly lower water-holding capacity.They tend tobe used for making peat blocks.

Considerable efforts are being made to findalternative materials to replace peat in order toavoid destroying valued wetland habitats fromwhich they are harvested.

Leaves

Leaf mould is made of rotted leaves of deciduoustrees. It is low in nutrients because nitrogen and


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phosphate are withdrawn from the leaves beforethey fall and potassium is readily leached from theageing leaf. They are often composted separatelyfrom other organic matter and much valued inornamental horticulture for a variety of uses suchas an attractive mulch or, when well rotted down,as a compost ingredient.They are commonly com-posted in mesh cages, but many achieve success byputting them in polythene bags well punched withholes. The leaves alone have a high C:N ratio sodecomposition is slow and it is not usually until thesecond year that the dark brown crumbly materialis produced although the process can be speededup by shredding the leaves first.

Unless they are from trees growing in very acidconditions, the leaves are rich in calcium and theleaf mould made from them should not be usedwith calcifuge plants.

Pine needles are covered with a protective layerthat slows down decomposition. They are low incalcium and the resins present are converted toacids. This extremely acid litter is almost resistantto decomposition. It is valued in the propagationand growing of calcifuge plants such as rhodo-dendrons and heathers, and as a material for con-structing decorative pathways.

Air-dried digested sludge

This consists of sewage sludge that has been fer-mented in sealed tanks, drained and stacked to dry. The harmful organisms and the objectionablesmells of raw sewage are eliminated in this process.

It provides a useful source of organic matter butis low in potash. Advice should be taken beforeusing sewage sludges because in some regions theycontain large quantities of heavy metals such aszinc, nickel and cadmium that can accumulate inthe soil to levels toxic to plants.

Leys

The practice of ley farming involves grassing downareas and is common where arable crop produc-tion can be closely integrated with livestock. At

the end of the ley period the grass or grass andclover sward is ploughed in. The root action of thegrasses and the increased organic matter levels canimprove the structure and workability of problemsoils. There are some pest problems peculiar tocropping after grass that should be borne in mind(see wireworms), and generally the ley enterprisehas to be profitable in its own right to justify itsplace in a horticultural rotation. It is practised insome vegetable production and nursery stock areas.

Mulching

Mulches are materials applied to the surface ofthe soil to suppress weeds, modify soil tempera-tures, reduce water loss, protect the soil surfaceand reduce erosion. Many organic materials areused for this purpose, including straw, leaf mould,peat, compost, lawn mowings and spent mushroomcompost. The organic mulches increase earth-worm activity at the surface, which promotes bet-ter and more stable soil structure in the top layers.Soil compaction by water droplets is reduced and,as the organic mulches are incorporated, the soilstructure can be improved. If thick enough, mulchescan suppress weed growth but it is counter-productive to introduce a material that containsweeds. Likewise, care should be taken not to intro-duce pests and disease or use a material such ascompost where slugs can be a problem.

When organic matter is added as a mulch it isacting, in effect, as an extra layer of loose soil.Thus, water loss from the soil surface is reducedbecause it is covered with a dry layer (see evapo-ration). Soil temperatures lag behind the surfacetemperatures because of its insulatory propertieswith the greater lag at greater depth. They tend toreduce soil temperatures in the Summer but retainwarmth later in the Autumn.

Manufactured materials such as paper, metalfoil or, most commonly, polythene are also usedbut these have very little insulatory effect. How-ever, these materials are particularly effective inreducing water loss by evaporation at the surface(see water conservation). The colour of the mulch


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is important because light-coloured material willreflect radiation, whereas dark material can leadto earlier cropping by warming up the soil earlier.

ORGANIC PRODUCTION

Organic production in the European Union issubject to regulations laid down in 1991. These

require that the fertility and the biological activityof the soil must be maintained or increased by thecultivation of legumes, green manures or deep-rooting crops in an appropriate multi-annual rota-tion and by the incorporation of organic matterincluding FYM, composted or not, from holdingsproducing according to the same regulations. Ifadequate nutrition or soil conditioning cannot beachieved by these means then other, specified,organic or mineral fertilizers, as listed in Table 15.1,may be applied. For compost activation, appropri-ate micro-organisms or plant-based preparationsmay be used. These organic or mineral fertilizersmay be applied only to the extent that the ade-quate nutrition of the crop being rotated, or soilconditioning, is not possible by the preferredmethods.

FURTHER READING

Advisory Reference Book 210 Organic Manures (MAFF,1976).

Bragg, N. Peatland: Its Alternatives (Horticultural Devel-opment Council, 1991).

Brown, L.V. Applied Principles of Horticulture, 2ndedition (Butterworth-Heinemann, 2002).

Edwards, C.A. and Lofty, J.R. Biology of Earthworms,2nd edition (Halsted Press, 1977).

Hills, L.D. Organic Gardening (Penguin Books Ltd.,1977).

Jackson, R.M. and Raw, F. Life in the Soil. (EdwardArnold).

Postgate, J. Nitrogen Fixation – Studies in Biology No. 92,3rd edition (Ed Arnold, 1998).

Robinson, D.W. and Lamb, J.G.D. (Editors). Peat in Horticulture (HEA/Academic Press, 1975).

Russell, E.J. The World of the Soil (Collins New Naturalist, 1957).

Table 15.1 Sources of nutrients for use in organicgrowing

Farmyard and poultry manureSlurry or urineComposts from spent mushroom and vermiculture

substratesComposts from organic household refuseComposts from plant residuesProcessed animal products from slaughterhouses and

fish industriesOrganic by-products of foodstuffs and textile

industriesSeaweeds and seaweed productsSawdust, bark and wood wasteWood ashNatural phosphate rockCalcinated aluminium phosphate rockBasic slagRock potashSulphate of potash*LimestoneChalkMagnesium rockCalcareous magnesium rockEpsom salt (magnesium sulphate)Gypsum (Calcium sulphate)Trace elements (boron, copper, iron, manganese,

molybdenum and zinc)*Sulphur*Stone mealClay (bentonite and perlite)

*Need recognized by control body.

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CONTROL OF SOIL pH

Life in the soil is greatly influenced by the soil pH,either directly or indirectly.The pH scale is a meansof expressing the degree of acidity or alkalinity. pHvalues less than 7 are acid and the lower the figure,the greater the acidity.Values greater than 7 indicateincreasing alkalinity and pH 7 is neutral.Temperatearea soils usually lie between pH 4 and 8; the vastmajority are between 5.5 and 7.5.

For ideal growing conditions most plants requirea soil of about pH 6.5, which is slightly acid.At thispoint most of the plant nutrients are available foruptake by the roots.Alkaline conditions are usuallycreated by the presence of large quantities of

16

Plant Nutrition

Green plants require 15 elements in order togrow normally including carbon, oxygen andhydrogen (see Chapter 5). The 12 essentialminerals enter the plant tissue in the form ofions from the growing media or to a lesserextent through the leaves. In established plantcommunities such as forest or grassland theminerals are recycled through the complexfood webs and the community developsaccording to the many factors that affect plantgrowth, including the net gain or loss ofminerals.

Removing a plant or part of it breaks thenatural cycle and prevents minerals containedin the plant from returning to the growingmedium for re-use. These minerals can bereplaced in many different ways. Where only a small quantity of nutrient is needed it can be supplied through suitable types of bulkyorganic matter which release sufficient nutri-ent when mineralized. It is often impracticableto supply large quantities of nutrients by theaddition of vast quantities of bulky organicmatter; organic or inorganic fertilizers, whichare more concentrated sources of majorplant nutrients, resolve this dilemma. Manygrowing media, especially those used in com-posts, are very deficient in minerals and there-fore supplementation with plant nutrients isessential. Ensuring mineral uptake is a major

concern. The roots must be able to find thenutrients (see soil structure), and soil pH andmineral balance should be adjusted to pro-vide optimum availability of nutrients.

This chapter describes the nature of soil pH,its effect on nutrient uptake and the methodsof changing it to maintain optimum growingconditions.The methods of applying nutrientsare examined and the characteristics of majorand minor (‘trace’) elements in growing mediaare outlined. Considering how to apply the cor-rect quantities of nutrient to different plantsconcludes the chapter.

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calcium (‘lime’), which interferes with the uptakeand utilization of several of the plant nutrients.

Calcicoles, or ‘lime-loving’ plants, have evolveda different metabolism and are tolerant of high soilpH. In contrast, the calcifuge or ‘lime-hating’ plants,such as rhododendrons and some heathers, do noteven tolerate the level of calcium in soils at pH 5.5.Consequently, they must be grown in more acidconditions. At very low pH some substances suchas aluminium become soluble at levels that are toxicto plants.

Acidity and alkalinity

Pure water is neutral, i.e. neither acid nor alkaline.It is made up of two hydrogen atoms and one oxy-gen atom, expressed as the familiar formula H2O.Most of the water is made up of water moleculesin which the atoms stay together, but a tiny pro-portion disassociates, i.e. they form ions. Equalnumbers of positive ions, called cations, and nega-tive ions, called anions, are formed. In the case ofwater, an equal number of hydrogen cations, H+,and hydroxyl anions, OH�, exist within the clus-ters of water molecules (H2O). When, as in water,the concentration of hydrogen ions is the same as that of hydroxyl ions, there is neutrality. As the concentration of hydrogen ions is increased,the concentration of hydroxyl ions decreases andacidity increases. Likewise, addition of hydroxylions and a decrease in hydrogen ions lead toincreased alkalinity.

Many compounds form ions as they dissolve andmix intimately with water to produce a solution.Allacids release hydrogen ions when dissolved in water.Whereas a strong acid (such as hydrochloric acid)dissociates when dissolved, only a part of a weakacid (such as carbonic or acetic acid) breaks up intoions. Bases, such as caustic soda, dissolve in waterand increase the concentration of hydroxyl ions.

The pH scale expresses the amount of acidity oralkalinity in terms of hydrogen ion concentration.In order to present the scale simply, negative loga-rithms are used: ‘pH’ is the negative logarithm,the mathematical symbol for which is ‘p’, of the

hydrogen ion concentration, abbreviated as ‘H’. Itis important to note that, as the scale is logarith-mic, a one-unit change represents a 10-foldincrease or decrease in hydrogen ion concentra-tion, and two units a 100-fold change. Thus a solu-tion of pH 3 is 10 times more acid than one of pH 4,100 times more acid than pH 5, and a 1000 timesmore than pH 6.

The buffering capacity of a substance is its abil-ity to resist change in pH. Pure water has nobuffering capacity: the addition of minute quan-tities of acid or alkali has an immediate effect onits pH. In contrast, the cation exchange capacity ofclays reduces the effect because the hydrogen ionsexchange with calcium ions on the clay’s colloidsurface. Since the number of hydrogen ions beingreleased or absorbed is small compared with theclay’s reserve, the pH changes very little. Highhumus soils similarly have the advantage of a highbuffering capacity. A related buffer effect is seenwhen acids, such as the carbonic acid of rain, areincorporated into soils with ‘free’ lime present: theacid dissolves some of the carbonate with noaccompanying change in pH.

Soil acidity

The balance of hydrogen ions and basic ionsdetermines soil acidity. A clay particle with abun-dant hydrogen ions acts as a weak acid, whereasif fully charged with bases (such as calcium, Ca)it has a neutral or alkaline reaction (see basesaturation). Consequently, soil pH is usually regu-lated by the presence of calcium cations; soilsbecome more acid as calcium is leached fromthe soil faster than it is replaced, e.g. by mineralweathering. This is the tendency in temperateareas where rainfall, which is a weak acid (see car-bonic acid), exceeds evaporation over the year.Thus hydrogen and aluminium ions take over thesoil’s cation exchange sites and the pH falls. Soilswith large reserves of calcium, such as those derivedfrom chalky boulder clay, do not become acidbecause they are kept base saturated. In contrast,calcium ions are readily leached from free-draining


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Plant Nutrition 197

sands in high rainfall areas and these soils tend togo acid rapidly.

In addition to the carbonic acid in rainfall, thereare several other sources of acid that affect the soil.Acid rain (polluted rain and snow) is directly harm-ful to vegetation but also contributes to the fall insoil pH.

Organic acids derived from the microbial break-down of organic matter, e.g. humic acids, also leadto an increase in soil acidity. The bacterial nitrifi-cation of ammonia to nitrate yields acid hydrogenions. Consequently fertilizers containing ammo-nium salts prevent calcium from attaching to soilcolloids and cause calcium loss in the drainagewater. Other fertilizers have much less effect.Calcium and magnesium are plant nutrients and

the soil’s lime reserves are therefore graduallyreduced by crop removal.

Effects of liming

When lime is added to an acid soil, the calcium ormagnesium replaces the exchangeable hydrogenon the soil colloid surface and neutralizes the soluble acids (see cation exchange). Eventuallyhydrogen ions are completely replaced by basesand base saturation is achieved, producing a soilpH of about 7. The influence of soil pH on plantnutrient availability is demonstrated in Figure16.1. It can be seen that for mineral soils the mostfavourable level for ensuring the availability of all

MINERAL SOILS PEATS

pH 4.0 5.0 6.0 7.0 8.0

4.0 5.0 6.0 7.0 8.0

pH 4.0 5.0 6.0 7.0 8.0

4.0 5.0 6.0 7.0 8.0

Neutral Neutral

Nitrogen

Phosphorus

Potassium

Sulphur

Calcium

Magnesium

Iron

Manganese

Boron

Copper

Zinc

Molybdenum

Nitrogen

Phosphorus

Potassium

Sulphur

Calcium

Magnesium

Iron

Manganese

Boron

Copper

Zinc

Molybdenum

Optimum ‘pH’for most peats

Optimum ‘pH’for most mineral soils

Figure 16.1 Effect of soil pH on nutrient availability. The availability of a given amount of nutrient is indicated by thewidth of the band. The growing media should be kept at a pH at which all essential nutrients are available. For mostplants the optimum pH is 6.5 in mineral soils and 5.8 in peats.

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nutrients is pH 6.5, while peat soils should belimed to pH 5.8 in order to maximize overall nutri-ent availability. In acid conditions some nutrientssuch as manganese and other soil minerals such asaluminium may become toxic.

Beneficial soil organisms are affected by soilacidity and liming.A few soil-borne disease-causingorganisms tend to occur more frequently on lime-deficient soils (see clubroot), whereas others aremore prevalent in well-limed soils. Calcium some-times improves soil structure and soil stability. It is probable that this is mainly because it createsconditions favourable for decomposition of organicmatter, yielding humus, and encourages root activity. Free lime in clay soils sometimes, but notalways, leads to better crumb formation on dryingand shrinking.

Plant tolerance

Tolerance to soil pH and calcium levels variesconsiderably, but all plants are adversely affectedwhen the soil becomes too acid. Table 16.1 shows

the point below which the growth of commonhorticultural plants is significantly reduced. In thecase of calcifuges (i.e. ‘lime-hating’ plants), the high-est point before growth is affected by the presenceof calcium should be noted, e.g. for Rhododendronsand some Ericas this is at pH 5.5.

Lime requirement

The lime requirement of soil depends on therequired rise in pH and the soil texture (see buffer-ing capacity). Lime requirement is expressed asthe amount of calcium carbonate in tonnes perhectare required to raise the pH of the top 150 mmto the desired pH. A pH of 6.5 is recommended fortemperate plants on mineral soils, pH 5.8 on peats.The amount of a liming material needed to meetthe lime requirement will depend on its neutraliz-ing value (NV) and sometimes fineness.

Liming materials

Liming materials can be compared by consideringtheir ability to neutralize soil acidity, fineness andcost to deliver and spread. The NV of a lime indi-cates its potency. It is determined in the laboratoryby comparing its ability to neutralize soil aciditywith that of the standard, pure calcium oxide. ANV of 50 signifies that 100 kg of that material hasthe same effect on soil acidity as 50 kg of calciumoxide. The fineness of the lime is important becauseit indicates the rate at which it affects the soil acid-ity (see surface area). It is expressed, where rele-vant, in terms of the percentage of the sample thatwill pass through a 100 mesh sieve. Liming mater-ials commonly used in horticulture are listedbelow with some of their properties.

Calcium carbonate is the most common limingmaterial. Natural soft chalk or limestone that ishigh in calcium carbonate is quarried and ground.It is a cheap liming material, easy to store and safeto handle. A sample in which 40 per cent will passthrough a 100 mesh sieve can be used at the stand-ard rate to meet the lime requirement. Coarsersamples, although cheaper to produce, easier tospread and longer lasting in the soil, require heavier


Table 16.1 Soil acidity and plant tolerance

Plants pH below which plant growth may be restricted

on mineral soils

Beans 6.0Cabbage 5.4Carrots 5.7Celery 6.3Lettuce 6.1Potato 4.9Tomato 5.1

Apple 5.0Blackcurrant 6.0Raspberry 5.5Strawberry 5.1

Carnation 6.0Chrysanthemum 5.7Daffodil 6.1Hydrangea (pink) 5.9Hydrangea (blue) 4.1Rose 5.6

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dressings. Shell sands have NV from 25 to 45.Whilstthe purest samples can be used at nearly the samerate as calcium carbonate, twice as much of thepoorer samples is required to have the same effect.

Calcium oxide, also known as quicklime, burntlime, cob lime or caustic lime, is produced whenchalk and limestone are very strongly heated in alimekiln.Calcium oxide has a higher calcium contentthan calcium carbonate and consequently a higherNV. Pure calcium oxide is used as the standard toexpress NV (100) and the impure forms have lowervalues (usually 85–90). If used instead of groundlimestone, only half the quantity should be applied.In contact with moisture, lumps of calcium oxideslake, i.e. react spontaneously with water to producea fine white powder, calcium hydroxide, with releaseof considerable heat. Although rarely used now cal-cium oxide should be used with care because it is afire risk, burns flesh and scorches plant tissue.

Calcium hydroxide, hydrated or slaked lime isderived from calcium oxide by the addition ofwater. The fine white powder formed is popular in horticulture. It has a higher NV than calciumcarbonate and its fineness ensures a rapid effecton the growing medium. Once exposed to theatmosphere it reacts with carbon dioxide to formcalcium carbonate.

It should be noted that all forms of processedlime quickly revert to calcium carbonate whenadded to the soil. Calcium carbonate, which is insol-uble in pure water, gradually dissolves in the weakcarbonic acid of the soil solution around the roots.

Magnesian limestone, also known as Dolomiticlimestone, is especially useful in the preparation ofcomposts because it both neutralizes acidity andintroduces magnesium as a nutrient. Magnesiumlimestone has a slightly higher NV (50–55) thancalcium limestone, but tends to act more slowly.

Lime application

Unless very coarse grades are used, lime raises thesoil pH over a 1–2 year period, although the fulleffect may take as long as 4 years; thereafter pHfalls again. Consequently lime application should

be planned in the planting programme. It is nor-mally worked into the top 150 mm of soil. If deeperincorporation is required, the quantity used shouldbe increased proportionally. The lime should beevenly spread and regular moderate dressings arepreferable to large infrequent applications. Verylarge applications needed in land restoration workshould be divided for application over several years.

Care should be taken that the surface layers ofthe soil do not become too acid even when thelower topsoil has sufficient lime.Top layers are thefirst to become depleted with consequent effect onplant establishment. This tendency has to be care-fully watched out for in turf management.

Applications of organic manures or ammoniumfertilizers should be delayed until lime has beenincorporated. If mixed they react to release ammo-nia which can be wasteful and sometimes harmful.

Decreasing soil pH

This is sometimes necessary for particular plants,e.g. Ericas requiring acid soils. In some circum-stances it is appropriate to grow plants in a raisedbed of acid peat or to work large quantities of peatinto the topsoil. Adding acids can reduce either inconjunction with this approach or as an alterna-tive, the base saturation of the mineral soil. Someacid industrial by-products can be used but themost usual method is to apply agricultural sul-phur, which is converted to sulphuric acid by soilmicro-organisms.The sulphur requirement dependson the pH change required and the soil’s bufferingcapacity. The application of large quantities oforganic matter gradually makes soils more acid.Nitrogen fertilizers releasing ammonia consider-ably reduce soil pH over a period of years in out-door soils (see soil acidity) and can be used inliquid feeding to offset the tendency of hard waterto raise pH levels in composts.

FERTILIZERS

Fertilizers are concentrated sources of plant nutri-ents that are added to growing media. Straight

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fertilizers are those that supply only one of themajor nutrients: nitrogen, phosphorus, potassiumor magnesium (see Table 16.2). The amount ofnutrient in the fertilizer is expressed as a percent-age. Nitrogen fertilizers are described in terms ofpercentage of the element nitrogen in the fertilizer,i.e. per cent N. Phosphate fertilizers have beendescribed in terms of the equivalent amount ofphosphoric oxide, i.e. per cent P2O5, or increasinglyas percentage phosphorus, per cent P. Likewisepotash fertilizers, i.e. per cent K2O, or percentagepotassium, per cent K. Magnesium fertilizers aredescribed in terms of per cent of Mg. The percent-age figures clearly show the quantities, in kg, ofnutrient in each 100 kg of fertilizer.

Compound fertilizers are those that supply twoor more of the nutrients nitrogen, phosphorus andpotassium. The nutrient content expressed as forstraight fertilizers is, by convention, written on thebag in the order nitrogen, phosphorus and potas-sium. For example, 20–10–10 denotes 20 per centN, 10 per cent P2O5 and 10 per cent K2O.

Fertilizer regulations require that further detailsof trace elements, pesticide content and phospho-rus solubility should appear where applicable onthe invoice. Fertilizers and manures are availablein many different forms. Generally the termorganic implies that the fertilizer is derived fromliving organisms, whereas inorganic fertilizers arethose derived from non-living material. However,in the context of organic growing it is necessary tolook at specific requirements of the regulations(Table 15.1).

Application methods

Fertilizers are applied in several different ways.Base dressings are those that are incorporated inthe growing medium. Combine drilling with seedsand fertilizer running into the same drill can achievethis. In horticulture, however, band placement offertilizers is far more common, involving equipmentthat drills the seeds in rows and places a band of


Table 16.2 Nutrient analysis of fertilizers

N P2O5(P) K2O (K) Mg Ca S Na% % % % % % %

Ammonium nitrate 33–35Ammonium sulphate 20–21 24Bone meal 3 20 (9)Calcium nitrate 15.5 20Calcium sulphate 23 18Chilean potassium nitrate 15 10 (8) 20Dried blood 12–14Hoof and horn 12–14Kieserite 15 21Magnesium sulphate 10 13Meat and bone meal 5–10 18 (8)Monoammonium phosphate 12 37 (15)Phosphoric acid 54 (24)Potassium chloride 59 (49)Potassium nitrate 14 46 (38)Potassium sulphate 50 (42) 17Shoddy (wool waste) 2–15Superphosphate 18–20 (8–9) 20 12–14Triple superphosphate 47 (20) 14Urea 46

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fertilizer in parallel a few centimetres below andto one side. The risk of retarded germination orscorch of young plants due to highly soluble fertil-izers placed near seeds is thus avoided (see saltconcentration). There is much less risk if fertilizeris surface broadcast, i.e. scattered on preparedsoil surface, or broadcast on the surface to becultivated-in during the final stages of seedbedpreparation. Top-dressings are fertilizers added tothe soil surface but not incorporated. Such fertiliz-ers must be soluble and not fixed by soil becausethe nutrient is carried to the roots by soil water.Nitrogen is the material most frequently appliedby this method mainly because the large applica-tions to crops require a base dressing and one ormore topdressings to minimize the risk of scorchand loss by leaching. Fertilizers are carefully incor-porated in composts during the mixing of theingredients (see compost mixing). Liquid feedingis the application of fertilizer diluted in water tothe root zone. Foliar feeding is the application of a liquid fertilizer in suitably diluted form to betaken up through leaves. This technique is usuallyrestricted to the application of trace elements.

Formulations

Quick-acting fertilizers contain nutrients in a formwhich plant roots can take up, and dissolve as soonas they come in contact with water, e.g. ammoniumnitrate and potassium chloride. Many are obtain-able as powders or crystals. These are difficult tospread or place evenly but several are formulatedthis way to help in the preparation of liquid feeds.For this purpose they must be readily soluble andfree of impurities that might lead to blockages inthe feed lines. Some of the less soluble fertilizers,as well as lime, are spread in a finely divided formfor maximum effect on soil. One of the major prob-lems with many of the fertilizers is their hygroscopicnature, i.e. they pick up water from the atmosphereand create storage and distribution problems.Powdered forms in particular go sticky as they takeup water then ‘cake’ (form hard lumps) as theydry. Fertilizers formulated as granules are more

satisfactory for accurate placement or broadcasting.They flow better, can be metred, and are thrownmore accurately. Prilled fertilizers represent animprovement on granules because of their uni-form spherical shape.

Slow-release fertilizers are those in which alarge proportion of the nutrient is released slowly.Several of these fertilizers, such as rock phosphate,are insoluble or only slightly soluble and the nutri-ents are released only after many months, evenyears. Micro-organisms break down organic prod-ucts and the rate at which nutrients become avail-able depends on their activity (see bacteria). Someslow-release artificial fertilizers, such as thosebased on urea formaldehyde, dissolve slowly inthe soil solution whilst others are formulated insuch a way that the soluble fertilizer they containdiffuses slowly through a resin coat, e.g. Osmocote,or sulphur coating, e.g. Gold-N. Some of theseslow-release fertilizers have been formulated insuch a way as to release nutrients at a rate thatmatches a plant’s uptake and as such are some-times referred to as controlled-release fertilizer.Frits, made from fine glass powders containingnutrient elements, are used either to release sol-uble materials slowly or to overcome the trace ele-ment problem caused by the narrow limits betweendeficiency and toxicity (see trace elements). Fritsease the difficulties experienced in mixing tinyamounts evenly through large volumes of compost.Ion-exchange resins release their nutrients byexchange with cations in the surrounding water.These resins help to overcome the problems ofhigh salt concentration and leaching of nutrientsfrom growing media based on inert materials (seeaggregate culture).

Some plant nutrients are formed as chelates tomaintain availability in extreme conditions wherethe mineral salt is ‘locked up’. There are many dif-ferent chelating or sequestrating agents selected foreach element to be protected, and for each unavail-ability problem. Iron is chelated with EDDHA toform the product Chel 138 or Sequestrine 138 whichreleases the element in all soils including thosewith a high pH (see iron deficiency). Ethylenedi-aminetetraacetic acid (EDTA), effective where

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there are high levels of copper, zinc and manganese,is used to chelate iron to be applied in foliar sprays.

PLANT NUTRIENTS

Nitrogen

Nitrogen is taken up by plants as the nitrate and toa lesser extent, the ammonium ion. Nitrate is con-verted to ammonium in the plant where it is utilizedto form protein. Plants use large quantities and itis associated with vegetative growth. Consequentlylarge dressings of nitrogen are given to leafy crops,whereas fruit, flower or root crops require limitednitrogen balanced by other nutrients to preventundesirable characteristics occurring.

Excess nitrogen produces soft, lush growth mak-ing the plant vulnerable to pest attack and morelikely to be damaged by cold.Very large quantitiesof nitrogen are undesirable since they can harmthe plant by producing high salt concentrations atthe roots (see conductivity) and are lost by leaching.Large quantities are usually applied as a splitdressing, e.g. some in base dressing and the rest inone or more top-dressings.

Nitrates are mobile in the soil, which makesthem vulnerable to leaching. In the British Isles itis assumed that all nitrates are removed by thewinter rains so that virtually none is present untilthe soils warm up and nitrification begins or artifi-cial nitrogen is applied (see nitrogen cycle). Nitratesleached through the root zone may find their wayinto the groundwater that is the basis of the watersupply in some areas. Nitrification also leads to theloss of bases; for every 1 kg N in the ammonia formthat is oxidized to nitrate and leached, up to 7 kg ofcalcium carbonate or its equivalent is lost. Nitrogenis also lost from the root zone by denitrification,especially in warm, waterlogged soil conditions.When in contact with calcareous material, ammo-nium fertilizers are readily converted to ammoniagas which is lost to the soil unless it dissolves insurrounding water. For this reason urea or ammo-nia-based fertilizers should not be applied to suchsoils as a top dressing or used in contact with lime.

Nitrogen fertilizers used in horticulture and theirnutrient content are given in Table 16.2. Ammo-nium nitrate is now commonly used in horticul-ture. In pure form it rapidly absorbs moisture tobecome wet; on drying it ‘cakes’ and can be a firerisk. Pure ammonium nitrate can be safely han-dled in polythene sacks and as prills. Ammoniumsulphate has a very acid reaction in the growingmedium. Urea has a very high nitrogen content andin contact with water it quickly releases ammonia.Its use as a solid fertilizer is limited but it is utilizedin liquid fertilizer or in foliar sprays. The additionof a sulphur coating to urea not only creates acontrolled-release action, but also a fertilizer withan acid reaction. Other manufactured organic fer-tilizers include urea formaldehydes (nitroform,ureaform, etc.) which release nitrogen as they aredecomposed by micro-organisms, isobutylideneurea (IBDU) which is slightly soluble in water andreleases urea and crotonylidene (CDU, e.g.Crotodur).The latter breaks down very slowly andevenly, which makes it ideal for applying to turf.

Natural organic sources of nitrogen, includingdried blood, hoof and horn and, shoddy, amongstothers, are generally considered to provide slow-release nitrogen, but in warm greenhouse condi-tions decomposition is quite rapid.

Phosphorus

Phosphorus is taken up by plants in the form ofthe phosphate anion H2PO�

4. Phosphorus is mobilein the plant and is constantly being recycled fromthe older parts to the newer growing areas. Inpractice this means that, although seeds have richstores of phosphorus, phosphate is needed in theseedbed to help establishment. Older plants havea very low phosphate requirement compared withquick growing plants harvested young.

Most soils contain very large quantities of phos-phorus but only a small proportion is available toplants. The concentration of available phosphateions in the soil water and on soil colloids is at itshighest between pH 6 and 7. Phosphorus is releasedfrom soil organic matter by micro-organisms


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(see mineralization), but most of it and any othersoluble phosphorus, including that from fertilizers,is quickly converted to insoluble forms by aprocess known as phosphate fixation. Insolublealuminium, iron and manganese phosphates areformed at low pH and insoluble calcium phos-phate at high pH. The carbonic acid in the vicinityof respiring roots and organisms (see respiration)in the rhizosphere such as mycorrhizae (see page 9)facilitate phosphorus uptake. The low solubility ofphosphorus in the soil makes it virtually immobile,with the result that roots have to explore for it.Soils should be cultivated to allow roots to exploreeffectively; compacted or waterlogged areas denyplants phosphorus supplies. Phosphate added tothe soil should be placed near developing roots(see band placement) in order to reduce phospho-rus fixation and ensure that it is quickly found. Ifapplied to the surface, phosphate fertilizers shouldbe cultivated into the root zone.

Unlike soils, most artificial growing media haveno reserves of phosphorus and when added in sol-uble form it remains mobile and subject to leach-ing. Incorporating phosphorus in liquid feeds inhard water is complicated by the precipitation ofinsoluble calcium phosphates that lead to blockednozzles. Slow-release phosphates are often selectedin these situations to reduce losses and to elim-inate the need for phosphorus in the liquid feeds.

Phosphorus nutrition used to be based on organicsources such as bones, but now phosphate fertil-izers are mainly derived from rock phosphate ore.Slow-acting forms such as rock phosphate, bonemeal and basic slag can be analysed in terms oftheir ‘citric soluble’ phosphate content, this beinga good guide to their usefulness in the first season.Such materials should be finely ground to enhancetheir effectiveness. These phosphates are appliedmainly to grassland, tree plantings and in thepreparation of herbaceous borders, to act as long-term reserves, particularly on phosphate-deficientsoils. Magnesium ammonium phosphate (MagAmp,Enmag), calcium metaphosphate and potassiummetaphosphate contain other nutrients but areslow-release phosphates for use in soilless grow-ing media. Treating rock phosphates with acids

produces water-soluble phosphates. Superphos-phate, derived from rock phosphate by treatingwith sulphuric acid, is composed of a water-solublephosphate and calcium sulphate (gypsum), whereastriple superphosphate, derived from a phosphoricacid treatment, is a more concentrated source ofphosphorus with less impurity. Both superphos-phate and triple superphosphate are widely usedin horticulture and are available in granular orpowder form. Whilst they have a neutral effect onsoil pH they tend to reduce the pH of composts.High-grade monoammonium phosphate is used asa phosphorus source in liquid feeds because it islow in iron and aluminium impurities that lead toblockage in pipes and nozzles.

Potassium

Potassium is taken up by the roots as the potassiumcation and is distributed throughout the plant ininorganic form where it plays an important role inplant metabolism. For balanced growth the nitro-gen to potassium ratio should be 1:1 for mostcrops, but 2:3 for roots and legumes. Leafy cropstake up large amounts of potassium, especiallywhen given large amounts of nitrogen. Wherepotassium supplies are abundant some plants,especially grasses, take up ‘luxury’ levels, i.e. morethan needed for their growth requirements. Conse-quently, if large proportions of the plant are takenoff the land, e.g. as grass clippings, there is a rapiddepletion of potassium reserves.

Potassium forms part of clay minerals and isreleased by chemical weathering. The potassiumin soil organic matter is very rapidly recycled andexchangeable potassium cations held on the soilcolloids and in the soil solutions are readily avail-able to plant roots. Potassium is easily leached fromsands low in organic matter and from most soilless-growing media.

Potassium and magnesium ions mutually inter-fere with the uptake of each other. This ion antag-onism is avoided when the correct ratio between3:1 and 4:1 (available potassium to magnesium) is present in the growing medium. Availability of

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potassium is also reduced by the presence ofcalcium (see induced deficiency).

The main potassium fertilizers used in horticul-ture are detailed in Table 16.2. Although cheaperand widely used in agriculture, potassium chloridecauses scorch in trees and can lead to salt concen-tration problems because the chloride ion accu-mulates as the potassium is taken up. Commercialpotassium sulphate can be used in base dressingsfor composts but only the more expensive refinedgrades should be used in liquid feeding. More usu-ally potassium nitrate is used to add both potas-sium and nitrate to liquid feeds, but is hygroscopic.Most potassium compounds are very soluble sothat the range of slow-release formulations islimited to resin-coated compounds.

Magnesium

Magnesium is an essential plant nutrient in leavesand roots and taken up as a cation.There are largereserves in most soils, especially clays, and thosesoils receiving large dressings of farmyard manure.Deficiencies are only likely on intensively croppedsandy soils if little organic manure is used. Magne-sium ion uptake is also interfered with by largequantities of potassium ions or calcium ions becauseof ion antagonism. Chalky and over-limed soils areless likely to yield adequate magnesium for plants.

Magnesium fertilizers include magnesian lime-stones containing a mixture of magnesium andcalcium carbonate that raises soil pH (see liming).Magnesium sulphate as kieserite provides magne-sium ions without affecting pH levels and in apurer form, Epsom salts, is used for liquid feedingand foliar sprays.

Calcium

Calcium is an essential plant nutrient taken up bythe plant as calcium cations. Generally a satisfac-tory pH level of a growing medium indicates suit-able calcium levels (see liming). Gypsum (calciumsulphate) can be used where it is desirable to

increase calcium levels in the soil without affect-ing soil pH. Deficiencies are infrequent and usu-ally caused by lime being omitted from composts.Inadequate calcium in fruits is a more complexproblem involving the distribution of calcium withinthe plant. Calcium nitrate or chloride solutionscan be applied to apples to ensure adequate levelsfor safe storage (see plant tissue analysis).

Sulphur

Sulphur taken up as sulphate ions is a nutrientrequired in large quantities for satisfactory plantgrowth. It is not normally added specifically as afertilizer because the soil reserves are replenishedby re-circulated organic matter and air pollution.Furthermore, several fertilizers used to add othernutrients are in sulphate form, e.g. ammonium sul-phate, superphosphate and potassium sulphate,and as such supply sulphur as well.As air pollutionis reduced and fewer sulphate fertilizers are used,it is becoming necessary for growers to take posi-tive steps to include sulphur in their fertilizerprogramme.

TRACE ELEMENTS

Trace elements, also known as micro-elements orminor elements, are present in plants in very smallquantities but are just as essential for healthygrowth as major elements. Furthermore, they canbe toxic to plants if too abundant. This means thatrectifying deficiencies with soluble salts has to beundertaken carefully.

Deficiencies

Simple deficiencies are those in which too little ofthe nutrient is present in the growing medium.Most soils have adequate reserves of trace elementsand so simple deficiencies in them are uncommon,especially if replenished with bulky organic matter.Sandy soils tend to have low reserves and so too


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have several organic soils from which traceelements have been leached. In horticulture sim-ple deficiencies of trace elements are mainly associ-ated with growing in soilless composts whichrequire careful supplementation.

Induced deficiencies are those in which sufficientnutrient is present but other factors such as soilpH or ion antagonism interfere with plant nutrientavailability. On mineral soils boron, copper, zinc,iron and manganese become less available inalkaline soils, whereas molybdenum availability isreduced severely in soils with pH levels below 5.5as shown in Figure 16.1. Trace element problemsare aggravated in dry soils or where waterlogging,root pathogens or poor soil structure reduces rootactivity.

Iron deficiency is induced by the presence oflarge quantities of calcium and this ‘lime-induced’chlorosis (yellowing) occurs on overlimed soilsand calcareous soils.The natural flora of chalk andlimestone areas is calcicoles. Other plants grownin such conditions usually have a typically yellowappearance. Deficiencies can also be induced byhigh levels of copper, manganese, zinc and phos-phorus.Top fruit and soft fruit are particularly sus-ceptible, as well as crops grown in complete nutrientsolutions. The problem is overcome by using ironchelates.

Boron deficiency tends to occur when pH isabove 6.8. It is readily leached from peat. Cropsgrown in peat are particularly susceptible whenpH levels rise (Figure 16.1). Boron can be appliedto soils before seed sowing in the form of borax or‘Solubor’.

Manganese deficiency is more frequent onorganic and sandy soils of high pH. Plant uptakecan be reduced by high potassium, iron, copperand zinc levels. Manganese availability is greatlyincreased at low pH and can reach toxic levelswhich most commonly occur after steam steriliza-tion of acid, manganese-rich soils. High phosphoruslevels can be used to reduce the uptake of man-ganese in these circumstances.

Copper deficiency usually occurs on peat andsands, notably reclaimed heathland, and in thinorganic soils over chalk. High rates of nitrogen can

accentuate the problem. Soils can be treated withcopper sulphate or plants can be sprayed withcopper oxychloride.

Zinc deficiencies are not common and are usu-ally associated with high pH.

Molybdenum deficiency occurs on most soil typesat a low pH. Availability becomes much reducedbelow pH 5.5 especially in the presence of highmanganese levels. Cauliflowers are particularlysusceptible and soils are limed to solve the problem.Sodium or ammonium molybdate can be added togrowing media or liquid feeds where molybdenumsupplies are inadequate.

FERTILIZER PROGRAMME

The fertilizer applications required to produce thedesired plant growth vary according to the type ofplant, climate, season of growth and the nutrientstatus of the soil. General advice is available inmany publications including those of the nationaladvisory services and horticultural industries.Examples are given in Table 16.3.

Growing medium analysis

The nutrient status of growing media variesgreatly between the different materials and withinthe same materials as time passes. The nutrientlevels change because they are being lost by plantuptake, leaching and fixation and gained by theweathering of clay, mineralization of organic mat-ter and the addition of lime and, fertilizers.

There are many visual symptoms which indicatea deficiency of one or more essential nutrients (seeminerals) but unfortunately by the time theyappear the plant has probably already suffered acheck in growth or change in the desired type ofgrowth.The concentration of minerals in the plantand the nature of growth are linked so that planttissue analysis, usually on selected leaves, can pro-vide useful information, particularly in the diagnosisof some nutrient deficiencies, e.g. it is used to iden-tify the calcium levels in apples in order to check

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Table 16.3 Examples of fertilizer requirements

Carrots

N, P or K index 0 1 2 3 4 over 4

(kg/ha)

Carrots, early bunching N 60 25 Nil – – –P2O5 400 300 250 150 125 NilK2O 200 125 100 Nil Nil Nil

Carrots, fen peats N Nil Nil Nil – – –(maincrop) other soils N 60 25 Nil – – –

all soils P2O5 300 250 200 125 60 NilK2O 200 125 100 Nil Nil Nil

Carrots on sandy soils respond to salt: 150 kg/ha Na (400 kg/ha salt) should be applied and potashreduced by 60 kg/ha K2O. Salt must be worked deeply into the soil before drilling or be ploughed in.

Dessert apples: mature trees

Summer Cultivated Grass/ Grassrainfall or overall herbicide

herbicide strip

(kg/ha per year)

Nitrogen (N) more than 30 40 90350 mmless than 40 60 120350 mm

P, K or Mg index 0 1 2 3 over 3

(kg/ha per year)

P2O5 80 40 20 20 NilK2O 220 150 80 Nil –Mg 60 40 30 Nil –

For the first 3 years, fertilizer is not required by young trees grown in herbicide strips, provided that deficiencies ofphosphate, potash and magnesium are corrected before planting by thorough incorporation of fertilizer.

Lettuce: base dressings for border soils under glass

Nitrate Ammonium Triple Sulphate KieseriteP, K or nitrate superphosphate of potashMg index

g/m2

0 30 150 160 1101 15 140 110 802 Nil 130 50 303 Nil 110 Nil Nil4 Nil 80 Nil Nil5 Nil 45 Nil NilOver 5 Nil Nil Nil Nil

Increase the nitrogen application by 50 per cent for summer grown crops. Lettuce is sensitive to low soil pH andthe optimum pH is the range 6.5–7.0. It is also sensitive to salinity, and growth may be retarded on mineral soilswhen the soil conductivity index is greater than 2.

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their storage qualities. However, nutrient supply isusually assessed by analysis of the growing medium.There is general agreement about the method-ology for analysing soils. However, there needs tobe some care where analysis of other growing mediais undertaken because there are considerable dif-ferences between the methods particularly withregard to dilution of the nutrient extractant.

A representative sample of the growing mediumis taken and its nutrient status determined. Thisinvolves extracting the available nutrients andmeasuring the quantities present. The pH level isdetermined and, where appropriate, the limerequirement. The conductivity of growing mediafrom protected culture is also measured.The nitro-gen status of soils is usually determined from pre-vious cropping because, outdoors, nitrates arewashed out over winter and their release fromorganic matter reserves is very variable. In pro-tected planting, nitrate and ammonia levels areusually determined.

Results are often given in the form of an indexnumber in order to simplify their presentation andinterpretation. The ADAS soil analysis index isbased on a 10 point scale from 0 (indicating levelscorresponding to probable failure of plants if nutri-ent is not supplied) to 9 (indicating excessivelyhigh levels of nutrient present).

Fertilizer recommendations

The results of the growing medium analysis areinterpreted with the appropriate nutrient require-ment tables to determine the actual amount of fer-tilizer to apply. These tables usually have growingmedium nutrient status indices to aid interpretationand results are normally given in kg of nutrientper hectare or grams of nutrient per square metre(Table 16.3). In some cases the amount of namedfertilizer required is stated; if another fertilizer isto be used to supply the nutrient the quantityneeded must be calculated using the nutrient con-tent figures (Table 16.3). It is important thatthroughout the fertilizer planning process the sameunits are used, i.e. per cent P2O5 or P per cent; K2O

or per cent K. Conversion figures are:

% P2O5 � % P � 2.29

% P � % P2O5 � 0.44

% K2O � % K � 1.20

% K � % K2O � 0.83

Sampling

Normally only a small proportion of the wholegrowing medium is submitted for analysis andtherefore it must be a representative sample of thewhole.This is not easy because of the variability ofgrowing media, particularly soils. It is recommendedthat each sample submitted for testing shouldbe taken from an area no greater than 4 ha (seeFigure 16.2). The material sampled must itself beuniform and so only areas with the same charac-teristics and past history should be put in the samesample. Irrespective of the area involved, from smallplot to 4 ha field, at least 25 sub-samples shouldbe taken by walking a zig-zag path avoiding theatypical areas such as headlands, wet spots, oldpaths, hedge lines, old manure heaps, etc.The sameamount of soil should be taken from each layer toa depth of 150 mm.This is most easily achieved witha soil auger or tubular corer.

Peat bags should be sampled with a cheese-typecorer by taking a core at an angle through theplanting hole on the opposite side of the plant tothe drip nozzle from each of 30 bags chosen froman area up to a maximum of 0.5 ha. Discard the top20 mm of each core and if necessary take morethan 30 cores to make up a 1 l sample for analysis.

Samples should be submitted to analytical labora-tories in clean containers capable of completelyretaining the contents. They should be accompan-ied by name and address of supplier, the date ofsampling and any useful background information.All samples must be clearly identified. Furtherdetails of sampling methods in greenhouses ororchards, bags, pots, straw bales, water, etc. areobtainable from the advisory services used.Remember, the result of the analysis can be no

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better than the extent to which the sample is rep-resentative of the whole.

SOIL CONDUCTIVITY

The soil solution is normally a weaker solutionthan the plant cell contents. In these circumstancesplants readily take up water through their roots byosmosis. As more salt, such as soluble fertilizer, isadded to the soil solution, salt concentrations are

increased and less water, on balance, is taken upby roots.When salt concentrations are balanced asmuch water passes out of the roots as into them.When salt concentrations are greater in the soilthe roots are plasmolysed. The root hairs, then theroots, are ‘scorched’, i.e. irreversibly damaged, andthe plant dries up.

Symptoms of high salt concentration aboveground are related to the water stress created.Plants wilt more often and go brown at the leafmargin. Prolonged exposure to these conditionsproduces hard, brittle plants, often with a blueytinge. Eventually severe cases become desiccated.

Salt concentration levels are measured indirectlyusing the fact that the solution becomes a betterconductor of electricity as salt concentration isincreased. The conductivity of soil solution ismeasured with a conductivity meter.

Salt concentration problems are most commonwhere fertilizer salts accumulate, as in climates withno rainfall period to leach the soil; and in pro-tected culture. Periods when rainfall exceedsevaporation, as in the British Isles during winter,ensure that salts are washed out of the ground.Any plant can be damaged by applications of excessfertilizer. Some plants such as tomatoes and celeryare more tolerant than others but seedlings are verysensitive.Young roots can be scorched by the closeproximity of fertilizer granules in the seedbed (seeband placement).

In protected culture large quantities of fertilizerare used and residues can accumulate, particularlyif application is not well adjusted to plant use.Sensitive plants such as lettuce are particularly atrisk when following heavily fed, more tolerant plantssuch as tomatoes or celery. Salt concentrationlevels should be carefully monitored and feedingadjusted accordingly, applying water alone if neces-sary. Soils can be flooded with water betweenplantings to leach excess salts. Large quantities ofwater are needed, but should be applied so that thesoil surface is not damaged. Every effort should bemade to minimize the effect on the environmentand quantities of water needed to flush out theexcess salts by reducing the nutrient levels as thecrop comes to an end.


SOIL A.SOIL B.

CorerAuger

400

mm900

mm

headlandsite ofold manureheap

SAMPLING TOOLS

gateway

wetspot

� points from which cores are taken.

� areas to avoid when sampling.

Figure 16.2 Sampling growing media. Suitable tools toremove small quantities of growing media are corers oraugers which have the advantage of removing equalquantities from the top and bottom of the sampled zone.The material to be sampled must be clearly identified,then 25 cores should be removed in a zig-zag that avoidsanything abnormal.

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FURTHER READING

ADAS/ARC. The Diagnosis of Mineral Disorders inPlants, Vol. 1 Introduction, General editor Robinson,JBD. (HMSO, 1982).

ADAS/ARC. Vegetable Crops, Vol. 2 General editorRobinson, JBD. (HMSO, 1987).

ADAS/ARC. Winsor, G. and Adams, P. (Editors).Glasshouse Crops, Vol. 3 (HMSO, 1987).

Archer, J. Crop Nutrition and Fertilizer Uses, 2nd edition(Farming Press, 1988).


Hay, R.K.M. Chemistry for Agriculture and Ecology(Blackwell Scientific Publications, 1981).

Ingram, D.S. et al. (Editors) Science and the Garden(Blackwell Science Ltd., 2002).

MAFF Reference Book No. 209 FertilizerRecommendations (1988).

Roorda von Eysinga,J.P.N.L.and Smilde,K.W. NutritionalDisorders in Chrysanthemums (Centre for AgriculturalPublishing and Documentation, Wageningen, 1980).

Roorda von Eysinga,J.P.N.L.and Smilde,K.W.NutritionalDisorders in Glasshouse Tomatoes, Cucumbers andLettuce (Centre for Agricultural Publishing andDocumentation, Wageningen, 1981).

Simpson, K. Fertilizers and Manures (LongmanHandbooks in Agriculture, 1986).

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GROWING IN CONTAINERS

The importance of supplying water to plants in arestricted root volume is usually understood, butthe difficulties associated with achieving it whilstmaintaining adequate air-filled porosity (AFP)are less well appreciated (see page 163).

Roots require oxygen to maintain growth andactivity. As temperatures rise, the plant requiresmore but the amount of oxygen that can be

dissolved in water decreases. Even in cool condi-tions, the oxygen that can be extracted from thewater provides only a fraction of the root require-ments. So, unless the plants have special modifica-tions to transport oxygen through their tissues, asin aquatics, there has to be good gaseous move-ment through the growing medium. Many largeinterconnected pores allow rapid entry of oxygen(see soil structures).

It is generally agreed that 10–15 per cent AFP isneeded for a wide range of plants.Azaleas and epi-phytic orchids require 20 per cent or more, whereasothers, including chrysanthemums, lilies and poin-settia, tolerate 5–10 per cent AFP and carnations,conifers, geraniums, ivies and roses can be grown atlevels as low as 2 per cent.

Creating successful physical conditions dependson the use of components, which provide a highproportion of macropores. Even in mixes whichretain a good reservoir of water, very large quan-tities of water have to be applied over the courseof a season so that the materials chosen must havevery good stability; fine sand and silt soils collapsetoo quickly and reduce the size of the pore spaces.The sizes of the components used must be selectedcarefully to ensure that they create macropores,but also so that the gaps between the larger par-ticles are not subsequently filled in by smaller par-ticles (‘fines’).This is most easily achieved by usingclosely graded coarse particles. The reverse isachieved when combining many different-sizedparticles, as one would in mixing concrete where

17

Alternatives to Growing in the Soil

The soil has many advantages over alternativegrowing media, not least because it is usuallythe most available, and plants normally growin it. Furthermore, it tends to be expensive tomodify a soil or use an alternative growingmedium. However, soil does have serious lim-itations for use in many aspects of horticultureand consequently the plants are frequentlygrown in suitable material put over the soil or,more commonly, in containers such as pots,liners, troughs, modules, cells, etc. In this chap-ter the principles underlying the use of con-tainers and the mixes used in them areestablished, the nature of alternative materialsand systems including hydroculture is exam-ined and the advantages and disadvantages ofthe different approaches are discussed.

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Alternatives to Growing in the Soil 211

the object is to minimize the airspaces as shown inFigure 17.1.

In addition to open ground or greenhouse bor-ders, plants may be grown in pots, troughs, bags andother containers where restricted rooting makesmore critical demands on the growing mediumfor air, water and nutrients.

Ensuring that a growing medium in a containerhas adequate AFP is made difficult because waterdoes not readily leave the container unless it is ingood contact through its holes with similar-sizedpore spaces. This is the case when placed on sandor capillary matting, but when standing out ongravel or wire the water will cling to the particlesin the container (see surface tension). This can betested by fully watering a pot of compost, holdingit until it has finished dripping then touching thecompost through a hole; normally a stream of

water will run down your finger. In fact the com-post acts as a sponge and if it is at less than con-tainer capacity it will ‘suck up’ water from below.Furthermore, the lower layers remain almost sat-urated irrespective of the height or width of thecontainer (You may understand this better if youfully wet a washing sponge and leave it to drain,after water has left the sponge, under the influenceof gravity, the lower layers remain saturated). Thismakes it particularly difficult to get good aerationin shallow trays, modules and blocks.

Providing the nutrients for the plant through asmall volume means that, if supplied in solubleform in one application, the salt concentrationproduced is often too high, especially for seedlings(see conductivity). Consequently feeding has to bemodified using split dressings, slow-release fertiliz-ers or liquid feeds.

(a)

(b)

(b)

(c)

(c)

CONCRETE

SAND MIXESwith ‘fines’ thatclog up pores, ORwashed,closely gradedgrains (90%between 0.1 and0.6 mm) togive highproportion ofmacropores

3 parts coarse aggregate2 parts sand1 part cementto achieve a mix with aminimum of air spaces

Figure 17.1 Pore spaces in (a) concrete mix (b) and (c) sand mixes.

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COMPOSTS

In horticulture the growing media used in contain-ers are usually referred to as ‘composts’. Thesematerials are also called plant substrates, plantgrowing media, or just ‘mixes’ or ‘media’. Over theyears growers have added a wide variety of mater-ials such as leaf mould, pine needles, spent hops,old mortar, crushed bricks, composted animal andplant residues, peat, sand and grit to selected soilsto produce a compost with suitable physical prop-erties. To supplement the nutrient released fromthe materials in the compost, if any, various slow-release organic manures or small dressings ofpowdered soluble inorganic fertilizers have beenadded to the mixtures to provide the necessarynutrition.

The correct physical and nutritional conditionsare vital to successful growing in a restricted rootingvolume. The most significant developments wereas a result of the work done in the 1930s at theJohn Innes (JI) Institute where the importance of‘sterile’ (pest and disease free), stable and uniformingredients was demonstrated. The range of com-posts that resulted from this work established themethods of achieving uniform production andreliable results with a single potting mixture suit-able for a wide range of plant species.

Loam composts

Loam composts, typified by JI composts, are basedon loam sterilized to eliminate the soil-bornefungi (see damping off) and insects that largelycaused the unreliable results from traditional com-posts. There is a risk of ammonia toxicity develop-ing after sterilization of soil with pH greater than6.5 or very high in organic matter (see nitro-gen cycle). Induced nutrient deficiencies are pos-sible in soils with a pH greater than 6.5 or less than5.5. Furthermore, loam should have sufficient clayand organic matter present to give good structuralstability. Peat and sand are added to furtherimprove the physical conditions, the peat giving ahigh water holding capacity and the coarse sand

ensuring free drainage and therefore good aer-ation.There are two main JI composts, one for seedsowing and cuttings, the other for potting.

JI seed compost consists of two parts loam, onepart peat and one part sand by volume. Well-drained turfy clay loam low in nutrients and a pHbetween 5.8 and 6.5, undecomposed peat graded3–10 mm with a pH between 3.5 and 5.0 and lime-free sand graded 1–3 mm should be used; 1200 g ofsuperphosphate and 600 g of calcium carbonateare added to each cubic metre of compost.

JI potting (JIP) composts consist of seven partsby volume loam,three parts peat and two parts sand.To allow for the changing nutritional requirementsof a growing plant, the nutrient level is adjusted byadding appropriate quantities of JI base fertilizerwhich consists of two parts by volume hoof andhorn, two parts superphosphate and one part potas-sium sulphate. To prepare JIP 1, 3 kg JI base fertil-izer and 600 g of calcium carbonate are addedto 1 m3 of compost. To prepare JIP 2 and JIP 3,double and treble fertilizer levels, respectively,are used.

Whilst the standard JI composts are suitable fora wide range of species, some modification isrequired for some specialized plants. For example,calcifuge plants such as Ericas and Rhododendronsshould be grown in a JI(S) mix in which sulphur isused instead of calcium carbonate.

All loam-based composts should be made upfrom components of known characteristics andaccording to the specification given. Such compostsare well proven and are relatively easy to managebecause of the water-absorbing and nutrient-retention properties of the clay present.

These composts are commonly used by ama-teurs, for valuable specimens, and for tall plantswhere pot stability is important; but loam-basedcomposts have been superseded in horticulturegenerally by cheaper alternatives.The main disad-vantage of loam-based composts has always beenthe difficulty in obtaining suitable quality loam aswell as the high costs associated with steam steril-izing. Furthermore, the loam must be stored drybefore use and the composts are heavy and difficultto handle in large quantities. Many loam-based

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composts currently produced have relatively lowloam contents and consequently exhibit few of itsadvantages.

Loamless or soilless composts

Loamless composts introduced the advantages ofa uniform growing medium, but with componentsthat are lighter, cleaner to handle, cheaper to pre-pare and which do not need to be sterilized (unlessbeing used more than once). Many have a lownutrient levels which enables growers to manipu-late plant growth more precisely through nutri-tion, but the control of nutrients is more critical asmany components have a low buffering capacity.

Peat has until recently been the basis of mostloamless composts. It is used alone or in combin-ation with materials such as sand to produce therequired rooting environment. Peats are derivedfrom partially decomposed plants and their char-acteristics depend on the plant species and theconditions in which they are formed (see Chapter15). Peats vary and respond differently to herbi-cides, growth regulators and lime. All peats have ahigh cation exchange capacity, which gives themsome buffering capacity. The less decomposedsphagnum peats have a desirable open structurefor making composts and all peats have highwater-holding capacities.

Considerable efforts are being made to reducethe destruction of wetland habitats by findingalternatives to peat for use in horticulture.A list ofsome of the materials used is given in Table 17.1.Much progress has been made by using suitablyprocessed bark or coconut fibre in composts.Along with several other organic sources they arewaste based and recycling them helps in conserv-ing resources.All such alternatives must be free oftoxins and pathogens, particularly those that maybe a hazard to humans. Most of the inorganic alter-natives are made from non-renewable resources(sand, loam and pumice) or consume energy intheir manufacture (plastic foams and polystyrene)or both (vermiculite, perlite and rockwool).It seems unlikely that versatile peat will be

replaced by a single alternative but rather differ-ent sectors will adopt substitutes best suited totheir requirements.

Alternatives to peat

Sand and gravel are used in composts, frequently incombination with peat. They have no effect on thenutrient properties of composts except by dilutingother materials. They are used to change physicalproperties. As sand or gravel is added to light-weight materials the density of the compost can beincreased, which is important for ballast when tallplants are grown in plastic pots. Sand is also used asan inert medium in aggregate culture. Sand shouldbe introduced with caution because it tends toreduce the AFP of the final mix. It is important thatthe sands used should have low lime levels, other-wise they may induce a high pH and associatedmineral deficiencies (see trace elements).

Pulverized bark has been used as a mulch andsoil conditioner for many years. More recently ithas been tried in compost mixtures as a replace-ment for peat. There are many different types of bark and they have different properties. Its


Table 17.1 Alternatives to peat

Organic materials Inorganic materials

Pine Expanded aggregatesCoir Extracted mineralsGarden compost HydroponicsHeather/bracken PerliteLeaf-mould PolystyreneLignite RockwoolRecycled landfill Dredgings/warpRefuse-derived humus VermiculiteSeaweed TopsoilSewage sludgeSpent hops and grainsSpent mushroom compostStrawVermicompostsWood chips/fibreWoodwastesWood fibre

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problems include the presence of toxins, over-come by composting, and a tendency to ‘lock-up’nitrogen (see carbon:nitrogen ratio), which can beoffset by extra nitrogen in the feed. When com-posted with sewage sludge, a material suitable as aplant-growing medium is produced. It is increas-ingly being incorporated into growing mixes in theattempt to reduce the use of peat.

However, the great variation of barks, especiallywhen they are from a mixed source, makes it diffi-cult to incorporate into growing mixes. Much ofthe conifer bark tends to be stringy. Consequentlythe main role of bark is in mulching. The importof bark is strictly controlled by the Forestry Com-mission to prevent the introduction of pests anddiseases.Wood-fibres based on stabilized shreddedwood are being used to increase the AFP ofmixes but they tend to be dusty and not easily dis-persed in compost mixes. Sawdust and off-cutsfrom the chipboard industry are also being testedfor use in growing, but there are problems associ-ated with their stability and fungal growth in thefreshly stored material.

Coconut wastes such as coir (the dust particles)are proving to be useful in growing mixes. Thematerial has good water-holding capacity, rewet-ting and AFP characteristics. It has a pH between5 and 6, which makes it suitable for a wide range ofplants, but cannot replace peat directly in mixesfor calcifuges. It has a carbon to nitrogen ratio of80:1 which means that allowance has to be madefor its tendency to lock-up nitrogen (see page 188).

Rockwool is an insulation material derived froma granite-like rock crushed, melted and spun intothreads. The resulting slabs of lightweight spongy,absorbent, inert and sterile rockwool provide idealrooting conditions with high water-holding cap-acity and good aeration. Shredded rockwool canbe used in compost mixes (see Plate 20). Its pH ishigh but easily reduced by watering with a slightlyacid nutrient solution.

It is frequently used in tomato and cucumberproduction and film-wrapped cubes are availablefor plant raising and pot plants. It is necessary to use a complete nutrient feed (see aggregate culture). It has some buffering capacity but this

is very low on a volume basis. The main problemareas lie in calcium and phosphorus supply andthe control of pH and salt concentration. Somerockwool has been formulated with clay to over-come some of these problems. This increases itscation exchange properties, making it very suit-able for interior landscaping. Rockwool is alsoavailable in water-absorbent and water-repellingforms. Mixtures of these enable formulators toachieve the right balance between AFP, waterholding and capillary lift. Rockwool is availableas granules that provide a flexible alternativefor those who produce their own mixes. How-ever, it is most usually supplied as wrappedslabs, cubes, propagation blocks and plugs thatare modularized to create a complete growingsystem.

Perlite is a mineral that is crushed and thenexpanded by heat to produce a white, lightweightaggregate (see Plate 20). The granules are porousand the rough surface holds considerably morewater than gravel or polystyrene balls. It tends tobe used to improve aeration of growing mediagenerally and the rewetting of peat. It is devoid ofnutrients and has no cation exchange capacity.Graded samples may be used in aggregate culture,but tend to be used to add to mixes to improve theuptake of water.

Vermiculite is a mica-like mineral expanded to20 times its original size by rapid conversion tosteam of its water content. The finished product isavailable in several grades all of which producegrowing media with good aeration and water-holding properties (see Plate 20). There is a ten-dency for the honeycomb structure to break downand go ‘soggy’. Consequently, for long-term plant-ing, it tends to be used in mixtures with the morestable peat or perlite. Some vermiculites are alka-line but the slightly acid samples are preferred inhorticulture.Vermiculite has a high cation exchangecapacity, which makes it particularly useful forpropagation mixes. Most samples contain someavailable potassium and magnesium.

Pumice is a porous volcanic rock that is pre-pared for use as a growing medium by crushing,washing (to remove salt and ‘fines’) and grading. It


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is most commonly used to grow long-term cropssuch as carnations in troughs or polysacks.

Expanded polystyrene balls or flakes provide avery lightweight inert material, which can beadded to soils or composts as a physical condi-tioner. It is non-porous and so reduces the water-holding capacity of the growing medium whileincreasing its aeration, thus making it less liable towaterlogging when over-watered. This has made itan attractive option for winter propagation mixes.However, it is less popular than it might bebecause it is easily blown away and sticks to mostsurfaces.

Plastic foams of several different types arebecoming popular for propagation because of theiropen porous structure.They are available as flakesand balls for addition to composts or as cubes intowhich the cuttings can be pushed.

Chopped straw has been used with some suc-cess. Generally the main types available, wheatand barley, break down too easily and a practic-able method of stabilizing them has not yet beenfound. Stable, friable material has been derivedfrom bean and oil seed rape straws although careis needed in mixes because of the high potassiumlevels.

Lignite is very variable soft brown coal formedfrom compressed vegetation, often found at thebase of the larger peat bogs.The dusts, ‘fines’, havebeen used as carriers for fertilizers and the moregranular material can be used to replace grit inmixes often bringing an improved water retention.

Absorbent polymers have the ability to holdvast quantities of water that is available to plants.However, this is considerably reduced in practicebecause water absorption falls as the salt concen-tration of the water increases and the release pat-terns appear to be very similar to that of somecompost ingredients such as sphagnum moss peats.

Wetters or non-phytotoxic detergents, areincluded in mixes to enable water to wet dry com-posts. They reduce the surface tension of thewater, which improves its penetration of the pores.This speeds up the wetting process and maximizesthe water holding capacity of the materials used.Wetters should be selected with care because the

different types need to be matched with the peatin the mix and above all must not be harmful tothe plants.

Compost formulations

Materials alone or in combination are preparedand mixed to achieve a rooting environment thatis free from pests and disease organisms and hasadequate AFP, easily available water and suitablebulk density for the plant to be grown.

While lightweight mixes are usually advanta-geous, ‘heavier’ composts are sometimes formu-lated to give pot stability for taller specimens. Thisshould not be achieved by compressing the light-weight compost but by incorporating denser mater-ials such as sand.Quick-growing plants are normallythe aim and loosely filling containers with thecorrect compost formulation and settling it withapplications of water obtain this. Firming with arammer reduces the total pore space whilst increas-ing the amount of compost and nutrients in the container. The reduction in available water andincrease in soluble salt concentration leads toslower growing, harder plants (see conductivity).

The addition of nutrients has to take intoaccount not only the plant requirements but alsothe nutrient characteristics of the ingredients used.Most loamless composts require trace element sup-plements and many, including those based on peat,need the addition of all major nutrients and lime.

Glasshouse Crops Research Institute (GCRI)developed general-purpose potting compostsbased on a peat/sand mix (see Table 17.2). Theycontain different combinations of nutrients andconsequently their storage life differs. One of therange has a slow-release phosphate, removingthe need for this element in a liquid feed (seephosphorus).

The GCRI seed compost contains equal partsby volume of sphagnum peat and fine, lime-freesand. To each cubic metre of seed compost isadded 0.75 kg of superphosphate, 0.4 kg potassiumnitrate and 3.0 kg calcium carbonate.

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Compost mixing

It is most important when making up the desiredcompost formulation to achieve a uniform prod-uct and, commercially, it must be undertaken witha minimum labour input. The ingredients of thecompost must be as near as possible to the specifi-cation for the chosen formulation. Materials mustnot be too moist when mixing because it thenbecomes impossible to achieve an even distribu-tion of nutrients.

There are several designs of compost mixer.Continuous mixers are usually employed by spe-cialist compost mixing firms and require carefulsupervision to ensure a satisfactory product. Batchmixers of the ‘concrete mixer’ design are pro-duced for a wide range of capacities to cover mostnursery needs. Many of the bigger mixers haveattachments which aid filling. Emptying equip-ment is often linked to automatic tray or pot-fill-ing machines.

Ingredients used in loamless composts or growing modules do not normally require partialsterilization unless they are being reused, but steril-izing equipment is certainly needed to prepareloams for inclusion in loam-based composts.Wheresteam is used it is injected through perforated pipeson a base plate and rises through the material beingsterilized. In contrast a steam–air mixture injectedfrom the top under an air-proof covering is forceddownwards to escape through a permeable base.

Storage of prepared composts should beavoided if possible and should not exceed 3 weeksif slow-release fertilizers are incorporated. Ifnitrogen sources in the compost are mineralized,ammonium ions are produced followed by asteady increase in nitrates (see Chapter 15). Thesechanges lead to a rise in compost pH followed bya fall. As nitrates increase, the salt concentrationrises towards harmful levels (see conductivity).Peat-based composts can become infested duringstorage by sciarid flies.

Table 17.2 GCRI composts

Constituents Seed compost Potting compostsUrea formaldehyde types* High P type**

General use Winter use Summer use

Peat:sand 1 2 3(per cent by volume) 50:50 75:25 75:25 75:25 75:25

Base dressings(kg/m3)Ammonium nitrate Nil 0.4 Nil Nil 0.2Urea formaldehyde Nil Nil 0.5 1.0 NilMagnesium ammonium Nil Nil Nil Nil 1.5

phosphatePotassium nitrate 0.4 0.75 0.75 0.75 0.4Normal superphosphate 0.75 1.5 1.5 1.5 NilGround chalk 3.0 2.25 2.25 2.25 2.25Ground magnesian Nil 2.25 2.25 2.25 2.25

limestoneFritted trace elements (WM225) Nil 0.4 0.4 0.4 0.4

*Composts containing urea formaldehyde should not be stored longer than 7 days.**For longer-term crops where there is a risk of phosphorus deficiency and liquid feeding with phosphate is notdesired, use commercial magnesium ammonium phosphate. This also contains 11 per cent K2O.

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PLANT CONTAINERS

The characteristics of the container affect the rootenvironment, as does the standing-out area. Claypots are porous and water is lost from the wallsby evaporation. Consequently clay pots dry outquicker than plastic ones, especially in the winterand, although air does not enter through the walls,this can help improve AFP. The higher evapor-ation rate also keeps the clay pots slightly cooler,which can be beneficial in hot conditions. Likewisethe contents of white plastic pots can be as muchas 4°C lower than in other colours. Pots of white orlight green plastic can transmit sufficient light toadversely affect root growth and encourage algalgrowth.

Biodegradable containers such as those madefrom paper have become popular because theycan be planted directly. Some materials decom-pose more rapidly than others and there can be atemporary ‘lock-up’ of nitrogen, but most peatcontainers are now manufactured with addedavailable nitrogen. It is essential that these con-tainers are soaked and surrounding soil is keptmoist after planting or the roots fail to escapefrom the dry wall.

The air to water characteristics of the mixture inthe container depends not only on the nature ofthe contents but also the characteristics of thebase on which the container stands. If containersare stood out on wire mesh or on stones relativelylittle water leaves so the oxygen content remainspoor. This is because the surface water filmsaround the finer particles in the compost hold thewater against gravity. More water can be drawnout of the container if the standing out material isof a similar particle size to the contents and thereis a good connection between the two. (You candemonstrate this to yourself if you fully wet a pot of compost and let it drain on a wire mesh or suspended in the air. When the water hasstopped dripping from the holes, touch the com-post in the pot through one of the drainage holesand be ready to have water running down yoursleeve.)

Blocks

Blocks are made of a suitable compressed growingmedium into which the seed is sown with no con-tainer or simply a net of polypropylene. Aerationtends to be poorer than in pots but this is a suc-cessful means of growing some vegetables on alarge scale. One type of block comes in the form ofa dry compressed disc that expands quickly readyto receive the seed in the shallow depression in thetop surface.

Modules

Increasingly traditional seedbed, bare-rooted orblock transplant techniques have become replacedby raising a wide variety of plants in modules.A module is made by adding a loose growingmedium mix to a tray of cells. The cells are vari-ously wedge or pyramid shaped, so designed toenable a highly mechanized transplanting processto be used. Fine, free-flowing mixes of peat, poly-styrene or bark are used to fill the cells, which havelarge drainage holes and no rim to hold free water.Roots in the wedge-shaped cells are ‘air-pruned’as they reach the edge of the cell, which encour-ages secondary root development. ‘Plugs’ are mini-modules in which each transplant develops in lessthan 10 cm3 of growing medium and are used forbedding plants as well as vegetable production.The rate of establishment is largely determined bythe water stress experienced by the transplant.Irrigation of the module or plug is found to bemore successful than applying water to the sur-rounding growing medium.

Hydroponics

Hydroponics (water culture) involves the growingof plants in water. The term often includes thegrowing of plants in solid rooting medium wateredwith a complete nutrient solution, which is moreaccurately called ‘aggregate culture’. Plants can be


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grown in nutrient solutions with no solid materialso long as the roots receive oxygen and suitableanchorage and support is provided.

The advantages of hydroponics compared withsoil in temperate areas include accurate control ofthe nutrition of the plant and hence better growthand yield. There is a constant supply of availablewater to the roots. Evaporation is greatly reducedand loss of water and nutrients through drainageis minimal in re-circulating systems. There can bea reduction in labour and growing medium costsand a quicker ‘turnround’ time between crops inprotected culture. The disadvantages include thehigh initial costs of construction and the controlsof the more elaborate automated systems.

Active roots require a constant supply of oxy-gen, but oxygen only moves slowly through water.

This can be resolved by pumping air through thewater that the plants are grown in, but it is usuallyachieved on a large scale by growing in thin filmsof water as created in the nutrient film technique(NFT) or a variation on the very much olderaggregate culture methods.

NFT

This is a method of growing plants in a shallowstream of nutrient solution continuously circu-lated along plastic troughs or gullies.The method iscommercially possible because of the developmentof relatively cheap non-phytotoxic plastics to formthe troughs, pipes and tanks (see Figure 17.2).There is no solid rooting medium and a mat of


probe

dosingpumps

underwaterpump

clip

NUTRIENTSOLUTION

CONTROLBOX

ACID

conductivity meter

alarms

pH meter flow regulator

firm graded supportor trays

re-circulation pipe

capillary mattingroots growing out ofpropagation pots

polythene gullies

Figure 17.2 The NFT layout. The nutrient solution is pumped up to the top of the gullies. The solution passes down thegullies in a thin film and is returned to the catchment tank. The pH and nutrient levels in the catchment tank are moni-tored and adjusted as appropriate.

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roots develops in the nutrient solution and in themoist atmosphere above it. Nutrient solution islifted by a pump to feed the gullies directly or viaa header tank.The ideal flow rate through the gul-lies appears to be 4 litres/min. The gullies have aflat bottom, often lined with capillary matting toensure a thin film throughout the trough. They arecommonly made of disposable black/white poly-thene set on a graded soil or on adjustable trays.There must be an even slope with a minimum gradient of 1 in 100, areas of deeper liquid stagnateand adversely affect root growth (see anaerobicconditions).

The nutrient solution can be prepared on sitefrom basic ingredients or proprietary mixes. It isessential that allowance be made for the localwater quality, particularly with regard to themicro-elements such as boron or zinc that canbecome concentrated to toxic levels in the circu-lating solution. The nutrient level is monitoredwith a conductivity meter and by careful observa-tion of the plants. Maintenance of pH between 6and 6.5 is also very important. Nutrient and pHcontrol is achieved using, as appropriate, a nutri-ent mix, nitric acid or phosphoric acid to lower pH(see mineral acids) and, where water supplies aretoo acid, potassium hydroxide to raise pH. Greatcare and safety precautions are necessary whenhandling the concentrated acids during preparation.

The commercial NFT installations have auto-matic control equipment in which conductivity andpH meters are linked to dosage pumps.The high andlow level points also trigger visual or audible alarmsin case of dosage pump failure. Dependence on theequipment may necessitate the grower installingfail-safe devices, a second lift pump and a standbygenerator. A variation on this method is to growcrops such as lettuce in gullies on suitably gradedgreenhouse floors (see Plate 18a and b).

Aggregate culture

In aggregate culture the nutrient solution is brokenup into water films by an essentially inert solidmedium such as coarse sand or gravel. More

commonly today materials such as rockwool, per-lite, polyurethane foam, duraplast foam or expandedclay aggregates (see Plate 20) are used. These are inthe form of polythene wrapped slabs or ‘bolsters’ ofgranules sitting on a polythene covered floor gradedacross the row. Polyurethane slabs are often placedunderneath them to help create even slopes andinsulate them from the cooler soil below.

These growing containers, on which the plantssit, are drip fed with a complete nutrient solutionat the top with the surplus running out throughslits at the bottom on the opposite side. When thismethod was first developed the NFT systems werecopied, i.e. the water was re-circulated, but it wassoon found to be difficult where the quality ofwater was poor and there was a risk of a build upof water-borne pathogens and trace elements. Itwas found that the surplus nutrient solution wasmost easily managed by allowing it to run to wasteinto the soil. However, this open system presentsenvironmental problems and increasingly a closedsystem has had to be adopted. It is now becomingmore usual to run the waste to a storage sump viacollection gullies or pipes. Some of this can beused to irrigate outdoor crops if nearby. To re-circulate the water it is necessary to have equipmentto remove the excess salts or accept a gradualdeterioration of the nutrient solution then flush itout to a sump when it becomes unacceptable.

Sources of infection such as Pythium are min-imized by isolation from soil and using clean water;the risks of re-circulating pathogens is addressedby using one of the four main methods of steriliza-tion (see water quality).

Rockwool slabs are a very successful way ofgrowing and widely used for a range of commer-cialcrops such as tomatoes, cucumbers, peppers, mel-ons, lettuce, carnations, roses, orchids and strawber-ries in protected culture. It is not biodegradable sothe vast quantity of rockwool now produced hascreated a serious disposal problem.The slabs can beused successfully several times, if sterilized on eachoccasion, but eventually they lose their structure.Tearing them up and incorporating in composts orsoils can deal with a limited amount, but far morecan now be recycled in the production of new slabs.


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Several types of expanded clay aggregates usedin the building industry such as Leca or Hortaghave been used particularly in interior landscaping(see Plate 20). Smooth but porous granules 4–8 mmin diameter, giving a capillary rise of about 100 mm,are used to create an ideal rooting environmentwith a dry surface which makes it an attractivemethod of displaying house plants (see Figure 17.3).All forms of aggregate culture require feedingwith all essential minerals. Trace element deficien-cies occur less frequently when clay aggregates areused. Ion exchange resins are an ideal fertilizerformulation in these circumstances because thenutrients are released slowly, remove harmfulchlorides and fluorides from irrigation water andaid pH control.

SPORT SURFACES

The specifications for sports playing surfaces aresuch that turf has increasingly given way to artificialalternatives typified by the trend towards playing‘lawn’ tennis on ‘clay’ courts. This is partly attrib-utable to maintenance requirements, but at thehigher levels of sport it is because the users or themanagement expect play to continue with a min-imum of interference by rainfall. The usual prob-lem is that the soil in which the turf grows does notretain its structure under the pounding it receivesfrom players and machinery, especially when it isin the wet plastic state. Turf is still preferred bymany, but to achieve the high standards required ithas to be grown in a much modified soil (see alsosand slitting) or, increasingly, in an alternativesuch as sand. The most extreme approach is togrow the turf in pure sand isolated from the soil,sometimes within a plastic membrane. The highcost of these methods is such that it is only used tocreate small areas such as golf greens.

Normally the existing topsoil is removed fromthe site and the subsoil is compacted to form afirm base and graded to carry water away to drains.Drainage pipes are laid, above which is placed adrainage layer usually consisting of washed, pea-sized gravel, as shown in Figure 17.4. As it is con-siderably coarser than the sand placed on it, thislayer prevents the downward percolation of water(see water films) and creates a perched watertable.This helps to give the root zone a large reserve ofavailable water whilst ensuring that gravitationalwater, following heavy rain or excess irrigation, isremoved very rapidly.

A 25–30 cm root zone of free-draining sand isplaced uniformly over the drainage layer, evenlyconsolidated. Allowance has to be made for con-tinued settling over the first year. It is essential thatthe sand used has a suitable particle size distribu-tion, ideally 80–95 per cent of the particles beingbetween 0.1 and 0.6 mm diameter. A minimum of‘fines’ is essential to avoid the clogging up ofthe pores in the root zone (see Figure 17.1).Sometimes a small amount of organic matter isworked into the top 5 cm to help establish the


nutrient battery (ion exchange resins)

water levelindicatorsuitablegrowingmediume.g. Leca

attractiveouter box

flower pot

water level

Figure 17.3 Plant pots with water reserves. Plants grownin a variety of growing media can be fitted with reservoirsthat supply water by capillarity. A water level indicator isfrequently incorporated and in some systems the nutri-ents are supplied from ion exchange resins. While thissystem can be used for any pot size it is particularlyattractive in large displays.

chap-17.qxd 6~4~04 2:28 PM Page 220


grass, although success is probably as easilyachieved with no more than regular light irriga-tion and liquid feeding.

Some very sophisticated all-sand systems, suchas the cell system, are constructed so that the rootzone is subdivided into bays with vertical plasticplates and supplied with drains that can be closedso that the water in each of them can be con-trolled. Tensiometers are used to activate valvesthat allow water back into the drainage pipes tosub-irrigate the turf.

FURTHER READING

Bragg, N. Grower Handbook 1 – Growing Media(Grower Books, 1998).

Bunt, A.C. Media and Mixes for Container GrownPlants (Unwin Hyman, 1988).

Cooper, A. The ABC of NFT (Grower Books, 1979).Handreck, K.A. and Black, N.D. Growing Media for

Ornamentals and Turf, Revised 3rd edition (NewSouth Wales University Press, 2002).

Mason, J. Commercial Hydroponics (Kangaroo Press,1990).

Pryce, S. The Peat Alternatives Manual (Friends of theEarth, 1991).

Smith, D. Grower Manual 2, Growing in Rockwool(Grower Books, 1998).

Foundation:compactedsub-soil

Drainage layer:pea gravel todrainage pipes

Root zone 25–30 cm deep:closely graded sand,0.1–0.6 mm diameter

Figure 17.4 Pure sand root zone.

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2, 4-D, 43, 94, 143

Abdomen, 101Abscisic acid, 62, 64Abscission, 61, 74, 79Acaricide, 142, 144Acarina, 114Achillea, 94Acid, 88, 196, 197, 198

rain, 198Acidity, 196–199Active ingredient, 142Adaptations, 37–38, 50, 56, 58,

74Adelgid, 107Adenine, 80Adventitious, 68Aerobic, 176

respiration, 59, 185, 186, 192Aerosol, 160After-ripening, 64Ageing, 48, 52, 58, 79Aggregates, 164Aggregate culture, 218,

219–210Aggregation, 189Agrobacterium tumifasciens, 86Agropyron repens, 92Air, 151Air capacity, 175Air drainage, 22Air-dried digested sludge, 193Air-filled porosity, 163, 210Air pollution, 204Alcohol, 60Aldicarb, 118, 145, 146Algae, 36, 88, 121Alkalinity, 196–199Allele, 81, 82

Alluvial, 156Alternate host, 91, 138Altitude, 21Aluminium, 158, 196Amitrole, 143Ammonia, 158, 187, 199, 206Ammonium:

carbonate, 128ions, 216nitrate, 200, 202sulphate, 200, 202

Anaerobic, 176conditions, 188, 176, 190respiration, 60, 65, 188, 190

Analysis:growing medium, 207plant tissue, 205soil, 207

Anatomy, 40–41Anchorage, 151Anemometer, 26Animal classification, 35Anions, 196, 202Annual rings, 44Annuals, 30, 34, 88Anthers, 76, 77, 81Anthocorid, 139Anticoagulant, 98Antidote, 98Antitranspirants, 49Apex, 43, 73Aphidius, 106, 140Aphids, 104–107, 132, 133Apical dominance, 72, 93Apomixis, 69Apple canker, 126Applicators:

dust, 147granule, 147

Aquatic:gardening, 1plants, 46

Arabis mosaic virus, 133Arachnida, 114Arboriculture, 1AHU, 20Armillaria mellea, 128Arthropods, 35, 101, 105, 185Asexual reproduction, 68,

121, 124, 125, 128Aspect, 22ATP, 41, 50ATU, 19Augers, 208Autumn colour, 59, 79Auxins, 61, 66, 71, 72, 73, 76,

78, 79, 143Axillary buds, 37Azotobacter spp, 188, 191

Bacillus thuringiensis, 148Backcross breeding, 85Bacteria, 36, 76, 129–131,

185ammonifying, 187nitrifying, 188

Bacterial:canker, 130slime, 130

Bait, 99, 146Band placement, 200Bark, 44, 213–214, 194

ringing, 44, 97, 98, 99Basalt, 152Base, 80, 159

dressings, 200saturation, 197

Basic slag, 194

Beaufort Scale, 26Beds, 138, 166Bees, 77, 149Beetle, 112–113, 126

Colorado, 135flea, 113raspberry, 113

Bentonite, 194Biennials, 30, 34Big bud mite, 115, 134Binomial, 30Biodiversity, 6, 7Biological control, 139Biological weathering, 153Biomass, 7Biome, 5Black bean aphid, 106Black leg, 130Black out, 74Black spot of rose, 125Blight, potato, 123–124Blocks, growing, 217Blood, dried, 200Bone meal, 200Boot-laces, 128, 138Bordeaux mixture, 142, 145Boron, 51, 197, 205Botrytis, 9, 126, 182Bottom heat, 70Boulder clay, 154Boulders, 160Bracts, 38, 76Breeding, 77, 84–85

backcross, 85inbreeding, 84mutation, 85outbreeding, 84pedigree, 84

Brickearth, 156

Index

(Page numbers of words that appear in bold in the text are bold in the index. Page numbers of words that appear as headings in thebook are italic in the index)

Index.qxd 6~4~04 2:29 PM Page 222

Broadcasting, 201Brown earths, 156Brown scale, 108Bud, 38, 58, 100

apical, 43, 72adventitious, 58, 93axillary, 38, 43, 70, 72blast, 108break, 76dormancy, 74scales, 43, 58terminal, 43

Buffering capacity, 159, 168,196, 213

Bulbs, 58Bulk density, 163Bulky manures, 191Bullfinch, 100Burning, 139Bumblebee, 77Butterflies, 109

C-3 process, 54C-4 process, 54CAM process, 54Cabbage white butterfly, 109Calcicoles, 196Calcifuges, 196Calcareous, 162Calcium, 50, 51, 158, 185, 196,

197, 198, 199, 200, 204carbonate, 198–199chloride, 204hydroxide, 199nitrate, 200, 204oxide, 199pectate, 40, 51sulphate, 200, 204

Callus, 44, 71, 72Calyx, 76Cambium, 42, 43, 69

ring, 43secondary, 43graft, 44pericycle, 48

Cane fruit, 2Canker:

apple, 126bacterial, 130

Canopy, 65Capillary:

action, 48benches, 181matting, 181, 211rise, 173, 181, 220

Capping, 166, 168Capsid:

black kneed, 139common green, 108, 139

Captan, 125Carbendrazim, 124, 145Carbohydrates, 53Carbon, 53, 186, 188Carbon cycle, 186–187Carbon dioxide, 48, 54, 56, 57,

59, 60, 186, 187, 199enrichment, 54, 56, 187pollution, 59

Carbonic acid, 153, 199Carbon:

nitrogen ratio, 188, 192Carnation rust, 125Carotenoid, 79Carrot fly, 111Casparian strip, 47, 48Caterpillar, 102, 109Cation exchange, 158, 189Cations, 158, 196Cellulose, 40, 42, 53Cell(s), 40–41, 80–81

differentiation, 43division, 43, 71, 81expansion, 43membrane, 40, 46system, 221wall, 40, 47

Centipedes, 116Certified plants, 134Chalk, 154, 162, 166, 169, 173,

175, 182, 194, 198Chelates, 201Chemical control, 142–149

contact, 144fumigants, 144groups, 146inorganic, 145organic, 145residual, 144safety, 148sterilization, 136stomach, 144systemic, 144, 145

Cheshunt mixture, 128Chickweed, 87, 89, 90–91, 107Chilean potassium nitrate, 200Chilling injury, 51, 56Chimaera, 85Chlorophyll, 51, 79Chloroplasts, 41, 57, 60Chromosomes, 40, 80–81

Chlorosis, 50, 51, 57, 205Chlorpropham, 144Chlorpyrifos, 144, 146Chrysalis, 109Chrysanthemum stunt viroid,

70, 133Class, 28, 29Classification, 28–36Clay, 143, 152, 154, 158–159,

160, 161, 162, 163, 169,173, 189, 190, 191, 194,196, 203

expanded, 219, 220pots, 217

Climacteric, 79Climate, 13–27, 189–190

coastal, 21continental, 17maritime, 17origins, 13–18world, 21

Climax species, 5Clods, 164, 166, 175Clone, 69Closed systems, 219Clubroot, 47, 127, 137Coconut waste, 214Colchicine, 85Cold frame, 67Cold period, 68Coir, 213, 214Collenchyma, 42Colloidal systems, 160Colloids, 159, 160, 162, 189,

197Colluvial, 156Combine drilling, 200Communities, 3–9

single species, 3Compaction, 164, 176Companion planting, 8Competition, 4–6, 87Compositae, 30Composting, 191,Composts, 191, 194, 212–217

formulations, 215–216GCRI, 215, 216general purpose, 215, 216John Innes, 212loam, 212–213loamless, 213mixing, 216–217seed, 212, 215, 216soilless, 213

Concentration gradient, 51

Concrete, 211Conduction, 14Conductivity, 47, 207, 208

meter, 208Conifer, 29

root rot, 128Conservation, 9–11, 86Consistency, 175Consumers, 6, 184Containers, 210–211, 217Convection, 14Control:

biological, 106, 139–142chemical, 142–147cultural, 96, 136–139integrated, 147–148legislative, 135–136preventative, 124, 127, 132,

133supervised, 148

Copper, 51, 126, 128, 197,205

Cordon, 73Corers, 208Cork, 44Corms, 68Corolla, 76Cortex, 42, 47COSHH, 149Cotyledons, 62, 64, 67, 92, 95Couch grass, 88, 93–94, 143Countryside management, 1Crane fly, 112Crawlers, 78, 107Creeping thistle, 92–93Critical period, 74, 124Crop:

removal, 197residues, 129

Cross pollination, 31Crown gall, 86, 130Cruciferae, 30, 31Crumbs, 164, 166, 198Cucumber mosaic virus, 87,

91, 132Cultivars, 31Cultivation, 64, 89, 136, 165,

166–167, 190Cultural control, 136–139Cut flowers, 77Cuticle, 49, 57, 10, 121Cuttings, 69–70, 133

hardwood, 69heeled, 70leaf, 70

Index 223


Cuttings, (continued)root, 70semi-ripe, 70soft-wood, 70stem, 69

Cutworms, 110Cyanide, 98Cyclic lighting, 74Cypermethrin, 145Cyst, 117, 118Cytosine, 80Cytoplasm, 40Cytokinins, 62, 64, 71, 72, 76

2, 4-D, 43, 94, 143Damping down, 49Daminazide, 73Damping-off, 60, 121, 127Daylength, 74, 104Day-neutral, 74Dazomet, 127, 136, 146DDT, 10, 149Dead-heading, 78Debris, 132Decay, 184Deciduous, 34Decomposers, 6, 184Decomposition, 186, 188Deep bed system, 4Deer, 99Deficiencies, 50–51, 204–205

boron, 205carbon dioxide, 49copper, 50, 205induced, 205iron, 50, 205manganese, 50, 205mineral, 50–51, 204–205molybdenum, 5, 205simple, 204trace elements, 50–51, 205zinc, 50, 205

DEFRA, 135, 145, 148Dehiscent seed, 63Deltamethrin, 107Denitrification, 188Depressions, 17Derris, 145Detergents, 215Detoxification, 185Detritovores, 6Dichlobenil, 144, 146Dichlofluanid, 123Dichloran, 146Dichloropropene, 133

Dicotyledonae, 30, 41Diffusion, 46, 49Diflubenzuron, 145Difenacoum, 98Digging, 166Dihybrid cross, 83Dimethoate, 144Dinocap, 124Diploid, 81Dioecious, 76, 93Dips, 138Diquat, 124Disbudding, 72Disease resistance, 51, 84Diseases, 121–134Disease tolerance, 126Ditches, 176, 177, 178Divisions, 28, 29DNA, 40, 80, 85, 86, 131Dock, 89, 95Dominance, 81, 82, 83

apical, 93Dormancy, 64, 63–65, 88, 92Dormant season, 144Dosage, 146Downy mildew, 85, 122–123Drainage, 87, 175–178, 197,

220depth, 177French, 177mole, 167, 177outlets, 178pipes, 177plastic, 177poor, 176symptoms, 176–177systems, 177

Drey, 99Drip irrigation, 181Dry fruit, 63Duroplast foam, 219Dusts, 146Dutch elm disease, 126Dwarfing rootstock, 72

Earthworms, 35, 185Earwigs, 109Ecdysis, 101Ecology, 4Ecosystem, 5EDDHA, 201EDTA, 201Eelworm, 117–120

chrysanthemum, 119root knot, 119

stem and bulb, 118wool, 119

Embryo, 62, 64, 77Emulsifiable concentrate, 146Emulsifier, 146Emulsion, 160Encarsia formosa, 107,

141–142Endodermis, 47, 48Endoplasmic reticulum, 41Endosperm, 62, 77Energy, 13, 59, 60, 68, 152

electro-magnetic, 13flow, 5

Enrichment CO2, 56, 187Enzymes, 41, 51, 56Ephemerals, 34, 88Epidermis, 41, 47, 57Epigeal, 66Epiphytes, 38Epsom salt, 194Erosion, 152–153, 154, 169,

191Ethylene, 62, 77, 79, 152Etiolation, 66Etridiazole, 128European Community

Regulations, 12, 194Eutrophication, 10Evergreen, 34Evaporation, 173–174, 180,

182Evaporimeter, 181Evapo-transpiration, 174–175Expanded aggregates (Leca

and Hortag), 213, 220Expanded polystyrene, 215

F1 generation, 81, 82F2 generation, 82, 83

hybrid, 78, 84Family, 29Farmyard manure, 191, 192,

194,Fasciation, 43Fatty acid detergent, 107Felspar, 153Fencing, 97Fenland, 190FEPA, 10, 148, 149Ferns, 29, 88Ferrous sulphate, 96Fertility, 11, 137, 192, 194Fertilization, 62, 77cross, 93

Fertilizers, 199–202, 205–208application, 200compound, 200controlled release, 201foliar, 202formulations, 201granules, 201hygroscopic, 201inorganic, 200liquid, 201organic, 200prilled, 201programmes, 205–208recommendations, 207requirements, 192, 200, 206quick release, 11, 201slow release, 191, 201straight, 199

Field capacity, 117, 169,172–173, 175

Field emergence, 65Fillers, 146Fines, 157, 210, 221Fireblight, 129–130Flies, 111–112Fly-agaric toadstool, 9Flooded benches, 181Flooding, 183, 208Flora, 31, 88

formula, 31Florigen, 76Flower, 38, 73–78

colour, 78development, 76initiation, 74, 75structure, 76

Flow meters, 183Fluoride damage, 59Foam, 160Fogging machines, 147Foliage-acting, 143Foliar feeding, 201, 202Food and Environment

Protection Act (FEPA),6, 123

Food chains, 6–7, 53, 149Food webs, 6–7, 53, 184Formalin, 127Formulations, 146, 201

baits, 146compost, 215dusts, 146fertilizer, 201granules, 146liquids, 146

224 Index


quick acting, 201seed dressings, 146smokes, 146wettable powders, 146

Friable, 167, 175range, 175

Frits, 201Frost, 19, 165Frost action, 165Fruit, 63, 77, 78–79

cane, 2dehiscent, 78dry, 78indehiscent, 78ripening, 79set, 78soft, 2succulent, 78top, 2

Fumigant, 106, 107, 114, 115,133

Fungi, 49, 76, 121–129, 140, 186biology, 9, 121–123classification, 35

Fungicide, 145protectant, 125, 145systemic, 126, 145

Fungus gnat, 112Fusarium:

patch, 128wilt, 129, 138

FYM, 191, 192, 194

Gall(s):big bud, 115mites, 115pineapple, 107

Garden:centres, 2construction, 1maintenance, 1

Gamete, 81Garden design:

decorative, 39infill, 39‘pretty’, 39skeletal, 39special, 39

Gaseous exchange, 152Gassing, 97Gause, 4Gel, 160Gene banks, 11, 86, 139Genes, 80, 81, 82, 83, 84Genetic code, 80

Genetic modification, 86,131

Genetics, 80–86Genotype, 81Genus, 29, 30Geotropism, 66Germination, 61, 64–65

percentage, 65temperatures, 65

Gibberellins, 62, 64, 76, 78 79Gleying, 176Gleys, 156Glucose, 54, 59GM, 86, 131Glyphosate, 93, 143Golf greens, 220Golgi apparatus, 41Grafting, 44, 71

tape, 72Granite, 152, 153Granules, 146, 201Gravel, 128, 156, 160, 213, 220Gravity, 37Gravitational water, 175Grease bands, 110, 138Greenfly, 104Green capsid, 108Greenhouse industry, 1Green plants, 186, 187Grey mould, 121, 126Grit, 160Groundsel, 87, 88, 89, 91, 111,

138Grounds maintenance, 1Groundwater, 173Growing media, 210–211

analysis, 50, 205–206, 207Growing room, 67Growing season, 18–21Growth:

adult, 67juvenile, 67–68regulators, 62retardation, 73secondary, 43

Guanine, 80Guard cells, 49Guttation, 48Gypsum, 194, 204

Haber process, 188Habitat, 4Hand texturing, 163Haploid, 81Hardening off, 67

Hardiness, 34Harrowing, 167Headwall, 177Health and Safety

Regulations, 149Heartwood, 44, 111Heat capacity, 17Heat treatment, 132, 133, 137Heat stores, 23Hedgerows, 10Heels, 70Hemiptera, 104–108Herbicides, 87, 90, 142–144,

146–147, 149concentration, 95, 142contact, 91, 92, 94, 143foliage-acting, 90, 91, 143mixtures, 144post-emergent, 143, 144pre-emergent, 91, 143, 144pre-sowing, 143, 144residual, 143, 144residues, 192safety, 148–149selective, 92, 142soil acting, 91, 93, 143, 144,

169translocated, 93, 143

Heredity, 80–86Hermaphodite, 76, 100Hilum, 62, 64High pressure, 18Hoeing, 91, 92, 136, 182Homozygous, 83Honey fungus, 128Honeydew, 105, 107, 108Hoof and horn, 200Hormones, 71, 76Horsetails, 29, 89Hortag, 220Hoses, water, 181Hot water treatment, 119Hoverfly, 8, 140Humidity, 24, 25–26, 48, 138

relative, 25Humification, 186, 189Humus, 187, 189, 190, 191, 198Hybrids, 31

intergeneric, 31interspecific, 31

Hybrid vigour, 84Hydrogen ions, 196Hydrogen sulphide, 188Hydroponics, 183, 213, 217Hydrotropism, 67

Hydroxyl ions, 196Hygienic growing, 69, 138Hygroscopic, 201Hypocotyl, 62, 64Hypogeal, 66Hyphae, 9, 121

Igneous rocks, 153Illuminance, 55Imbibition, 65Inbred parent lines, 84Inbreeding, 84Indehiscent seeds, 63, 78Indole acetic acid (IAA), 61Induced deficiencies, 205Induced dormancy, 65Infected plant material, 139Infection, 123Infiltration rates, 171Inflorescence, 38, 76Inheritance, 81–84Inhibitors, 64Initiation, 71, 74Inoculation, 132Insecticide(s), 78, 144–145

contact, 108, 144fumigant, 144residual, 110, 112, 144stomach, 144systemic, 108, 111, 144

Insects, 101–114beneficial, 8biology, 101–102life cycles, 102–104, 106

Inspection, 137Instars, 101Intergeneric hybrid, 31Integrated control, 147–148Integrated environmental

control, 56Integrated pest management,

147–148Interior landscaping, 2International Code of

Botanical Nomenclature,28

Internodes, 37, 38Interspecific hybrid, 31Invertebrate, 100Ions, 51Ion antagonism, 204, 205Ion exchange resins, 201, 220Iprodione, 126, 129, 146Iron, 51, 57, 197, 205

deficiency, 51

Index 225


Iron oxide, 153, 158, 176Irrigation, 90, 178–181, 220,

221drip, 181guide, 179overhead, 183plan, 178–179sub, 221trickle, 181

John Innes composts, 212Juvenility, 67–68

Kaolinite, 153, 158Kieserite, 200, 204

Lacewings, 139Ladybird, 8, 108, 140Lamina, 38Lamps, 55–56Landscaping, 1

hard landscaping, 1soft landscaping, 1

Larch, 107Larva, 102, 104Latent heat, 23Latitude, 14Law of limiting factors, 54Layering, 68LD, 50, 148Leaching, 202Leafhopper, 108Leaf miner, 111Leaf mould, 192, 213Leaf:, 57–59

blade, 38canopy, 174colour, 59compound, 32cooling, 49cuttings, 70fall, 79form, 32, 58–59lamina, 38litter, 154, 155margins, 32mould, 192–193petiole, 38retention, 68scars, 45, 79shape, 32simple, 32texture, 58‘true’, 67veins, 32, 49, 57

Leatherjacket, 112Leca, 220Leopard moth, 110Legislation, 65, 135Legumes, 188, 194Leguminosae, 30Lenticels, 45, 129Leys, 193Lichens, 36, 154Light, 55–56, 65, 79

far red, 74intensity, 55, 74illuminance, 55red, 74wavelength, 55, 56, 74

Lighting, 55–56supplementary, 55total replacement, 55–56

Lignin, 42, 186, 187, 189Lignite, 213, 215Lime:

application, 199burnt, 199caustic, 199cob, 199fineness, 198free, 196hating, 198hydrated, 199lime induced chlorosis, 205loving, 198quick, 199requirement, 198, 207slaked, 199

Limestone, 194, 199dolomitic, 199magnesium, 199

Liming:effects, 197–199materials, 198–199

Linnaeus, 30Liquid feeding, 201Liverworts, 29, 96Load, 153Load-bearing, 168, 169,

175Loams, 161, 162, 163, 173,

212–213Loess, 156Long-day plants, 74–75Lumbricus terrestris, 185Lux levels, 55, 74

Machinery, 87Macropores, 163, 173, 175

Magnesium, 51, 57, 197, 203,204

limestone, 199sulphate, 200

Maiden tree, 73Malathion, 144Mancozeb, 124, 125, 145Mandibles, 101Manganese, 51, 176, 197, 205Manures, 191, 192, 199Margins, 32Marker-assisted breeding, 86Marling, 161MCPA, 93Mealy bug, 107Meat and bone meal, 200Mechanical:

analysis, 161transmission, 131

Medullary rays, 45Meiosis, 81Mellow soil, 165Mendel, 82, 83Meristems, 51, 59, 70, 133

apical, 43ground, 43grass, 43

Mesophyll:palisade, 57spongey, 57

Mesopores, 163Metaldehyde, 100Metamorphic rocks, 154Metamorphosis, 102Metham sodium, 136Methiocarb, 100, 146Methyl bromide, 136Micas, 153Microclimate, 22–24Microelements, 50, 204, 205Microhabitat, 4Micro-organisms, 152, 185,

186, 187, 188, 189Micropores, 163Micropyle, 62Mildew:

downy, 122–123powdery, 124

Millipedes, 116Minerals, 50–52, 57

deficiency, 50–51, 204–205essential, 50uptake, 47, 51–52

Mineralization, 186Minimum cultivation, 166

Misting, 70Mites, 114–116

blackcurrant gall, 115big bud, 115cyclamen, 115predator, 141red spider, 114–115, 141tarsonemid, 115

Mitochondria, 41, 60Mitosis, 81, 85Modules, growing, 217Moisture holding capacity,

173Mole, 99Mole drainage, 167, 177Molybdenum, 51, 197, 205Monoammonium phosphate,

200Monocotyledonae, 30, 42, 71Monoculture, 3, 10Monoecious, 76Monohybrid cross, 82Montmorillinite, 153Mosaic, 131Mosses, 29, 88, 95–96, 154Moths, 109–111Mould, 121Mowing, 43, 92Mulches, 174, 193–194Mutations, 69, 85Mycelium, 121, 124, 128Mycoplasma, 131Mycorrhiza, 9, 122, 186, 203Myxamatosis, 98

Necrosis, 51Nectria, 126Nematodes, 35, 117–120, 133,

185chrysanthemum, 119dagger, 120migratory, 120needle, 120potato cyst, 117–118root knot, 119spear, 117stem and bulb, 118

Nematicides, 142, 145Neutralizing value, 198NFT, 218–219Nicotine, 145Niche, 4Night-break, 74Nitrates, 9, 10, 90, 187, 188,

202, 216

226 Index


Nitric acid, 219Nitrites, 187, 188Nitrobacter, 188Nitrogen, 50, 187, 188, 197,

202cycle, 187–188fixation, 8, 188

Nitrosomonas, 188Nodes, 37, 38, 94Nomenclature, 28Nozzles:

cone, 147fan, 147

Notifiable, 135Nucleic acids, 50Nucleoproteins, 53Nucleus, 40, 77, 80Nursery stock, 2Neutralizing value, 198Nutrient(s), 151, 211

availability, 11, 169, 197,205, 207

cycling, 186–188functions, 50–51deficiencies, 50–51major, 50minor, 50non-essential, 50plant, 202–205requirements, 50, 206, 207sources, 194

Nutrient film technique(NFT), 47, 119, 218–219

Nymph, 101

Oedema, 48Offsets, 68Ontario units, 19Oospores, 127–128Opening solutions, 76Open systems, 219Opportunist, 5Order, 28, 29Organic:

acids, 197additions, 191benefits, 191bulky, 190, 191estimating, 198growing, 11–12, 147, 194levels, 189–191manures, 191, 192, 199matter, 6, 7, 114, 198movement, 11production, 148, 194

residues, 191soils, 184–194, 190

Organelle, 40Organisms, 184–186, 191Organs, 40Osmosis, 46–47, 48, 51, 65, 208Outbreeding, 84Overhead watering, 183Overwintering, 62Ovaries, 76, 78Ovicide, 144Ovules, 76Oxidation, 153Oxygen, 54, 57, 59, 60, 65, 152,

210, 218, 219

Palisade mesophyll, 57Pans, 160, 165, 166, 167

iron, 165natural, 160, 165rotovation, 167stone, 160

Paraffin, 167Paraquat, 91, 143Parasite, 139, 141Parenchyma, 40, 42, 131Passage cells, 48Parthenocarpy, 78Parthenogenesis, 104Partial sterilization, 185Particle size classes, 157Peach potato aphid, 104, 131Peat, 138, 143, 144, 162, 169,

213alternatives, 213–214bags, 207horticultural, 192modules, 217sedge, 190, 192soils, 169sphagnum, 190, 192

Pedicel, 38Pedigree breeding, 84Peds, 164Pellets, 100Perennials, 30, 92

herbaceous, 34weeds, 92–95woody, 34

Pericycle, 48Pirimicarb, 144Perithecia, 126Perlite, 213, 214, 219Permanent wilting point, 174Permeable fill, 177

Permeability, low, 177Pesticides, 11, 142–149

detoxification, 185phytotoxicity, 149protective clothing, 149resistance, 145–146safety, 148–149wildlife, 149

Pests, 8, 87, 97–120Petalody, 76Petals, 76Petiole, 37, 58pH, 137, 173–176, 196, 198,

207control, 195–196decreasing, 199scale, 172

Phenotype, 81, 82Pheremone traps, 138, 148Phloem, 42, 44, 47, 48, 57, 58,

87, 127, 131Phosphate(s), 9, 202–203

fixation, 203rock, 203slow acting, 203water soluble, 203

Phosphoric acid, 200, 219Phosphorus, 50, 197, 202–203Photoperiodism, 74–75Photosynthesis, 41, 46, 48,

53–58, 66, 186, 187requirements for, 54–57

Phototropism, 66Phyllosphere, 9Phytochrome, 74Phytophthora, 123, 127, 128Phytosanitary certificate,

135Phytoseiulus persimilis, 141Phytotoxicity, 149Pigments, 79Pine needles, 193Pirimicarb, 144Pith, 42Plant:

ageing, 79breeding, 80–86certified material, 133communities, 2–8containers, 217–218day neutral, 74density, 3development, 61–79flowering, 30, 73–78form, 37–40

growth, 53–60growth rate, 39growth regulators, 62hormones, 61–62insectivorous, 38kingdom, 29life cycle, 62, 67long day, 74nutrition, 195–209origins, 32roots, 184, 186short day, 74size, 39spacing, 3–4stress, 67tissue analysis, 205tolerance, 198vegetative, 67–72

Plant Health Act, 135Planting, 115, 137Plantlets, 71Plant Varieties and Seeds

Act, 86Plasmodesmata, 40, 43Plasmodium, 127Plasmolysis, 47, 49Plastic foams, 215Plastid, 41Ploughing, 93, 136, 166Plough pans, 166Plum pox, 132–133Plumule, 62, 64, 66Poisonous plants, 87Pollen, 76, 78

tube, 77Pollination, 77

cross, 77, 84insect, 76, 77self, 77, 84wind, 76, 77

Pollution, 59Polymers, 215Polyploids, 85Polystyrene, 213, 214, 215Polyurethane foam, 219Ponding, 171Pores, 152, 163, 173, 210Porosity, 163, 210Potash, 203, 204Potassium, 51, 197, 203–204

chloride, 200, 203nitrate, 64, 200, 203sulphate, 194, 200

Potato, 3blight, 123–124

Index 227


Potential transpiration, 174,180

Potting-on, 67Power requirement, 168Powdery mildew, 124Precipitation, 25Predator, 99, 115, 116, 139Pressure, 18Preventative control, 97, 126Pricking-off, 67Prickles, 38Prills, 201Primary producer, 6, 184Primary consumer, 6, 184Principle of Independent

Assortment, 83Principle of Segregation, 83Procambium, 43Producers, 184Production horticulture, 1Professional gardening, 2Propachlor, 91, 144Propagule, 68, 69Propane, 187Protected cropping, 2Protected culture, 2, 208Protectant, 125Protective clothing, 149Proteins, 50, 53, 99, 187, 202Protoderm, 43Protoplasm, 40, 46, 58Pruning, 72–73, 115, 126, 139Pteridophyta, 29Pumice, 214Pupa, 102, 107, 109Pure colour, 81Pyrethrum, 145Pythium, 127, 219

Quartz, 153, 158

Rabbit, 97–98Radiation, 13, 14–15, 49, 174Radicle, 62, 64Rainfall, 17, 87, 171, 180, 182,

197, 206Raingauge, 25, 171Rain shadow, 17Raking, 167Rat, brown, 98Raw soil, 165Receptacle, 76Recessive, 81, 82, 83Recombinant DNA

Technology, 85–86

Recycling, 154, 186, 187, 188,191

Red spider mite, 114–115, 141Removal of infected material,

139Rendzinas, 157Repellent, 97, 138Representative samples, 207Residual chemicals, 118, 143,

144Resistance:

fungal, 124pest, 141, 142, 145plant, 118, 120, 124, 132, 139

Respiration, 45, 52, 55, 59–60,63, 64

anaerobic, 60, 65Respirator, 149Response periods, 178, 179Reversion virus, 115, 133Ribosome, 41Rhizomorph, 122, 128Rhizobia/ium, 8, 186, 188, 191Rhizomes, 58, 68, 93Rhizosphere, 8–9, 186, 203Ring-barking, 97Ringing, 44RNA, 40Rocks, 153–154Rock phoshate, 194Rockwool, 213, 214, 219Root(s), 46–48, 51–52, 176,

184, 186activity, 198adventitious, 68ageing, 48cap, 47cortex, 47diseases, 127environment, 169fibrous, 37, 38hairs, 47, 127, 208lateral, 93pressure, 48pruning, 73requirements, 151–152stock, 44, 71, 130structure, 47–48taproot, 37, 38, 68, 93, 95tip, 51

Rotary:cultivators, 93, 136, 167diggers, 167

Rotation, 118, 119, 127, 129,137

Rumex, 95–96Runners, 68Run-off, 171, 183Rust:

carnation, 125

Safety, 148–149Salt concentration, 47, 181,

208Sampling, 207Sand, 157–158, 162, 163,

167–169, 177, 213, 221Sandslitting, 177Saprophyte, 121, 127, 129, 186Sapwood, 44Sawdust, 194Sawfly, 113Scale insect, 108Scarers, 99Scarification, 64Sciarid fly, 112Scion, 44, 71, 130Sclerenchyma, 42Sclerotium, 122, 126, 129Scorch, 208Seaweed, 194, 213Secondary consumer, 6, 184Secondary growth

(thickening), 42, 45Sedimentary rocks, 153–154Seed-producing plants, 29Seed, 62–65, 119, 131

Acts, 65compost, 67coat, 64dehiscent, 63dispersal, 62–63, 90dormancy, 63–64, 76, 91dressing, 113, 146dry, 78germination, 64–65, 88, 91indehiscent, 63over-wintering, 62quality, 65, 87, 137sowing, 67structure, 64succulent, 78viable, 64water supply, 64–65, 67

Seedbed, 137Seedling, 50, 65–67, 127

development, 66Selectivity, 92, 142Self-fertile, 90, 92Selfing, 84

Self-pollination, 77Self-sterile, 92Semi-permeable (partially

permeable) membrane,47

Senecio vulgaris, 91Senescence, 76Sepals, 76Service horticulture, 1Settling velocities, 162Sexual reproduction, 77Sewage sludge, 193, 213

air dried, 193Shoddy, 200Shot-hole effect, 130Shrub, 34, 45Silt, 157, 160, 161, 162, 168,

175, 190Silt traps, 176, 177, 178Slugs, 100–101, 185Slurry, 194Smokes, 146Snails, 36, 100, 185Snow mould, 128Sodium, 153

thiosulphate, 77Soft fruit, 2Soft rot, 130Soil:

acidity, 196–197aggregates, 164, 166, 171,

189, 184alluvial/ium, 156analysis, 207atmosphere, 152augers, 208beds, 166brown earths, 156calcareous, 162capping, 165cation exchange, 158, 159clay, 158–159colloids, 162colluvial, 156colours, 176components, 152, 157–160composition, 152–154conductivity, 208consistency, 150–151corers, 208development, 154–156,

164–166fertility, 137, 207formation, 152–154‘hungry’, 190

228 Index


load-bearing, 168, 175management, 1, 167–169microorganisms, 152, 184,

188mineral, 152, 157moisture deficit, 180–181organic matter, 164–194organic, 190organisms, 184–186, 198pans, 160, 165, 166, 167particle size, 157peat, 169, 190pH, 185, 195–196pore space, 152, 163, 173,

210power requirements, 168production horticulture, 1profile, 154sandy, 162, 167–168saturated, 171–172sedentary, 155settling velocities, 162silt, 160solution, 152, 208stability, 166, 198sterilization, 136structure/s, 162–166, 168,

185, 191, 198structural stability, 166, 176subsoil, 155temperature, 24, 93, 185,

192texture, 160–162topsoil, 155, 185transported, 155–156types, 87, 156–157, 167–169,

190water, 171–183water balance, 180waterlogged, 60, 176well-drained, 176workability, 168, 175, 189

Solarimeter, 181Sooty mould, 105, 107Sorting, 154, 155Spacing, 3Spading machines, 167Species, 29, 30

competition, 4competitive, 5

Specimen plants, 2Speedwell, 89, 92Spent mushroom compost,

194, 213Spiders, 140

Spermatophyta, 29Spikelet, 38Spines, 38Spiracles, 101, 140Spore, 122

asexual, 121, 123, 125, 128case, 121, 124sexual, 121, 128

Sport, 85, 132surfaces, 220–221

Spray warning, 148Spray lines, 181Spreaders, 109, 116, 125, 146Springs, 173Springtails, 113Spruce, 107Spruce-larch adelgid, 107Squirrels, 98–99Stability, 191, 210Stale seedbed, 137Stamens, 76Starch, 53Steam sterilization, 127, 129Stele, 48Stem, 41–42

cuttings, 69growth, 43–45structure, 41–42

Sterilant, 76Sterilization, 185, 212,

chemical, 120, 128, 129, 136equipment, 216partial, 91, 116, 120, 129,

136, 185, 216Stevenson’s screen, 24Stigma, 76Stipules, 58Stock, 44, 71, 130, 132, 133Stock plants, 68, 128Stolons, 68, 94Stomata, 46, 48, 49, 57, 104,

119, 122, 126, 130Stone meal, 194Stones, 160Stopping, 72Storage, 60, 68

conditions, 60Stratification, 64Straw, 188, 192, 213, 215Strychnine, 99Stumps, 128Style, 76Stylets, 107Subsoil, 155Subsoilers, 166, 167, 168

Succession, 5Succulent fruit, 78Suckers, 69Sugars, 53, 59, 79Sulphate of potash, 194Sulphates, 188Sulphur, 51, 145, 188, 194, 199,

204cycle, 204dioxide, 59, 125

Superphosphate, 200, 203Surface area, 158Surface broadcast, 200Surface run-off, 171Surface tension, 172Suspension, 162Sustainable development, 7Symbionts, 9, 122, 186Symphilids, 116Symptoms, 123, 131Systemic, 106, 107, 126, 145Systematics, 28

Taxonomy, 28Temperature, 49, 56, 60, 64,

65, 75, 79, 114, 124, 129,136, 161, 193, 210

measurement, 24Tendrils, 38, 58Tensiometers, 181, 221Testa, 62, 63, 64, 77, 131Textural triangle, 162Texture, 160–162Texturing, 162Thatch, 190Thermometers, 24Thermophile, 56Thiophanate-methyl, 129Thiram:

soak, 137Thistle, 88–89Thorax, 101Thorns, 38Thymine, 80Thrips, 108, 115Tidiness, 88Tiles, 177Till, 154Tiller, 38, 43Tilth, 166Timeliness, 143, 175Tines, 175Tip-burn, 48Tissue, 40Tissue culture, 70–71, 138

Toadstools, 128Tomato:

mosaic virus, 132blossom end rot, 51

Top-dressing, 201Top fruit, 2Topography, 22, 154Topsoil, 155, 185, 213Town refuse, 191Trace elements, 169, 194,

204–205, 220Transported soils, 155–156Translocation, 58, 93, 94, 95,

117Transmission, 131Transpiration, 48–50, 171, 174,

180potential, 174, 180pull, 48, 49rate, 49, 174

Transplanting, 4Traps, 9, 97, 99, 100, 138Tree, 34, 45

crops, 139stumps, 128surgery, 1

Trichoderma viride, 140Trickle lines, 181Triple superphosphate, 200Triploid, 85Tropism, 66Tubers, 58, 68, 89Tulip break, 132Tulip topple, 51Turf culture, 1Turgor, 46, 174Turnip brown heart, 51Twitch, 93

Umbelliferae, 30Umbels, 30, 32Urea, 200, 202Urea-formaldehyde, 202, 216Urine, 194

Vacuole, 4, 40, 46Variation, 69, 80Variegation, 57, 59Varieties, 30Vascular bundles, 42, 57Vascular wilt, 129Vector, 131, 132Vegetable production, 1Vegetative, 68–72, 88, 131, 202

propagation, 68–72, 88, 137

Index 229


230 Index

Vermicompost, 213Vermiculite, 158, 194, 213, 214Vermin trap, 177Vernalization, 75Verticillium wilt, 4, 91, 129Viability, 64Vine weevil, 113Viroids, 131, 133Viruses, 36, 105, 131–134, 138,

140arabis mosaic, 120biology, 131chrysanthemum stunt, 131,

133cucumber mosaic, 131, 132green-petal, 108leaf mosaic, 131plum pox, 132reversion, 133testing, 131, 137tobacco rattle, 131tomato mosaic, 85, 131tomato spotted wilt, 109tulip break, 131

Vitamins, 70Vivipary, 104

Warfarin, 99Warp, 213Warrens, 97Wasp, 107, 140

Water, 46–50, 56, 59, 128, 151,171–183, 173

application, 181, 183available, 175, 178, 180butts, 67, 127conservation, 182–183culture, 217currents, 16films, 171, 172functions, 46gravitational, 171hard, 181hot water treatment, 119loss, 60movement, 46, 48photosynthetic, 56quality, 128, 181–182recycling, 182uptake, 46vapour, 46, 48warm water treatment,

138Water-holding capacity, 173,

191Watering can, 181Waterlogging, 176Watertables, 172, 173

perched, 173, 220Weather systems, 14, 16Weather data, 24, 174Weather measurement, 24–27

Weathering, 152–153biological, 153chemical, 153physical, 153

Weed(s), 87–96, 119, 132annual, 90–92biology, 88broad-leaved, 143competition, 87control, 90damage, 87ephemeral, 88growth habit, 88identification, 88perennial, 92–95seedlings, 89

Weed’s Act, 135Weil’s disease, 98Western flower thrips, 109Wettable powder, 146Wetting and drying cycle,

164Wetter/spreader, 109, 116,

125, 146, 215WHC, 173Whiptail (brassicas), 51Whitefly, 87, 107, 141Wildlife, 10, 149Wilting, 40, 57, 127, 174temporary, 174Wilt fungi, 91, 129, 139

Wind, 14, 16direction, 27erosion, 169, 191rocking, 152speed, 26

Windbreaks, 68Winter moths, 110Wire fencing, 97Wireworm, 113Wood, 44

heart, 44sap, 44

Wood ash, 194Wood fibre, 213Woodlouse, 116Woodpigeon, 99Woodwastes, 213Workability, 168, 175, 189Worm casts, 185

Xanthophyll, 79Xerophytes, 37Xylem, 42, 43, 44, 47, 48, 57,

127, 129

Yarrow, 89, 94

Zero tillage, 166Zinc, 51, 197, 205Ziram, 100Zygote, 77


Plate 1 Flower border showing the use of flower colour. The light blue of Brunnera macrophylla harmonises with thedarker blue Anchusa azurea and, in the background, the yellow-flowered Asphodeline lutea.

Plate 2 Contrasts in leaf shape and form: the linear leaves of Hemerocallis lilioasphodelus contrast with the palmatelobed leaves of Rodgersia podophylla and, in the background, the pinnate leaves of the fern Osmunda regalis.

Plates.qxd 6~4~04 2:30 PM Page 1

Plate 3 Protective adaptions. Leaf prickles of holly (left): thorns of Pyracantha (left centre): stem prickles of rose (rightcentre): and thorns of Eleagnus. Note also the variegated leaves of the holly and Eleagnus.

Plate 4 Spines of cactus. Plate 5 Foliage colour and contrast: The white foliage ofCornus contraversa ‘Variegata’, and yellow foliage of theChaemaecyparis ‘Nana Aurea’ contrast with the dark-purple leaved tree (Fagus sylvatica Atropurpurea Group).


Plate 6 Blackspot of rose.

Plate 7 Juvenility. Leaf retention in the lower juvenilebranches of beech tree and beech hedging; compare withthe bare older branches.

Plate 8 Juvenility. The juvenile growth of ivy (left)showing adventitious roots on the stem and the lobedleaves compared with the adult leaf shape (right). Notealso the variegated leaf pattern.


Plate 9 Potassium deficiency symptoms. Plate 10 Magnesium deficiency symptoms.

Plate 11 Chrysanthemum powdery mildew. Plate 12 Rose powdery mildew.


d e f

a b c

g h i

Plate 13 (a–g) Barks: decorative patterns and colours exhibited in a range of trees; (h) Apple canker; (i) Armillaria.Honey fungus rhizomorphs (bootlaces) on infected wood. Inset – a clump of characteristic honey-coloured toadstoolson an infected trunk.


Plate 14 Effect of aspect; note the difference between thenorth-facing slope (right) with snow still lying after it hasmelted on the south-facing slope (left).

Plate 15 Agrometerological station showing rain gauge(left), Stevenson’s screen (centre) and anemometer (right).

Plates 16 Stevenson’s screen showing Max MinThermometer (base) and Wet and Dry Thermometer (centre).

Plate 17 Whirling hygrometer with calculator.


Plate 18 NFT lettuce with close up showing gullies and nutrient solution delivery.

Plate 19 Rocks. Granite: pink (left); silver (top); sand-stone (right), shade (bottom).

Plate 20 Growing media. Top to bottom: rockwood,perlite, vermiculite, Leca.


(a) (b) (c) (d)

Plate 21 Soil capping.

Plate 22 Soil types (a) Podsol; (b) Gley; (c) Brownearth; (d) Rendzina.

Plate 23 Soil prism. Note rusty mottle.


Principles of Horticulture 4th ed. by C.R.Adams.pdf

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