Horticulture in Context

.The many facets of horticulture have much in common, each being concerned with the growing of plants. Despite the wide range of the industry, embracing as it does activities from the preparation of a cricket square to the production of uniformly sized cucumbers, there are common principles which guide the successful management of the plants involved.

This section puts the industry, the plant, the plant communities and ecology into perspective, and considers the aspects of conservation and organic growing and looks forward to the more detailed explanations of horticultural practice in the following sections.]]

Horticulture may be described as the practice of growing plants in a relatively intensive manner.

This contrasts with agriculture, which, in most western European countries, relies on a high level of machinery use over an extensive area of land, consequently involving few people in the production process. However, the boundary between the two is far from clear, especially when considering large-scale vegetable production. Horticulture often involves the manipulation of plant material, e.g. by propagation, by changing the above ground environment, or by changing the root environment. There is a fundamental difference between production horticulture, whether producing plants themselves or plant products, and service horticulture, i.e. the development and upkeep of gardens and landscape for their amenity, cultural and recreational values. Increasingly, horticulture can be seen to be involved with social well-being and welfare through the impact of plants for human physical and mental health. It encompasses environmental protection and conservation through large- and small-scale landscape design and management.

Where the tending of plants for leisure moves from being horticulture to countryside management is another moot point. In contrast, the change associated with replacing plants with alter native materials, as in the creation of artificial playing surfaces, tests what is meant by horticulture in a quite different way.

This book concerns itself with the principles underlying the growing of plants in the following sectors of horticulture:

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

• Landscaping, garden construction and maintenance which involves the skills of construction together with the development of planted areas (soft landscaping). Closely associated with this sector is grounds maintenance, the maintenance of trees and woodlands (arboriculture and tree surgery), specialist features within the garden such as walls and patios (hard landscaping) and the use of water (aquatic gardening).

• Interior landscaping is the provision of semi permanent plant arrangements inside conservatories, offices and many public buildings, and involves the skills of careful plant selection and maintenance.

• Protected cropping enables plant material to be supplied outside its normal availability, e.g. chrysanthemums all the year round, tomatoes to a high specification over an extended season, and cucumbers from an area where the climate is not otherwise suitable. Plant propagation, providing seedlings and cuttings, serves outdoor growing as well as the greenhouse industry.

Protected culture, mainly using low or walk-in polythene tunnels, is increasingly import-ant in the production of vegetables, salads, bedding plants and flowers.

• Nursery stock is concerned with the production of soil-grown or container-grown shrubs and trees. Young stock of fruit may also be established 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 of plants in gardens including both public and private gardens and may reflect many aspects of the areas of horticulture described. It often embraces both the decorative and productive aspects of horticulture.

• Garden centers provide plants for sale to the public, which involves handling plants, maintaining them and providing horticultural advice.

A few have some production on site, but stock is usually bought in.

THE PLANT

There is a feature common to all the above aspects of horticulture: the grower or gardener benefits from knowing about the factors that may increase or 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 the ideal performance of the plant in particular circumstances. In most cases this will mean optimum growth, as in the case of a salad crop such as lettuce where a fast turnover of the crop with once over harvesting that grades out well. However, the aim may equally be restricted growth, as in the production of dwarf chrysanthemum pot plants or in the case of a lawn that would otherwise require frequent cutting. The main factors to be considered are summarized in ill. 1.1, which shows where in this book each is discussed.

It must be stressed that the incorrect functioning of any one factor may result in undesired plant performance. It should also be understood that factors such as the soil conditions, which affect the underground parts of the plant, are just as import ant as those such as light, which affect the aerial parts. The nature of soil is dealt with in Section 13.

Increasingly, plants are grown in alternatives to soil such as composts and rockwool and these are reviewed later.

Weather generally plays an important part in horticulture. It is not surprising that those involved in growing plants have such a keen interest in weather forecasting in order to establish whether conditions are suitable to work in or because of the direct effect of temperature, water and light on the growth of plants. The climate is dealt with in Section 2, which also 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 in nature. However, specimen plants such as leeks, marrows and potatoes, lovingly reared by enthusiasts looking for prizes in local shows, grow to enormous sizes when freed from competition. In landscaping, specimen plants are placed away from the influence of others so that they not only stand out and act as a point of focus, but also can attain perfection of form. A pot plant such as a fuchsia is isolated in its container, but the influence of other plants, and the consequent effect on its growth, depend on spacing. Generally, plants are to be found in groups, or communities.

PLANT COMMUNITIES

Neighboring plants can have a significant effect on each other since there is competition for factors such as root space, nutrient supply and light. As in natural plant communities, some of the effects can be beneficial whilst others are detrimental to the achievement of horticultural objectives.

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

Microclimate Light, Harmful substances.

Single species communities

When a plant community is made up of one species it is referred to as a monoculture. On a football field there may be only ryegrass (Lolium species) with all plants closely spaced just a few millimeters apart. Each plant species, whether growing in the wild or in the garden, may be considered in terms of its own characteristic spacing distance (or plant density).

In a decorative border, the bedding plant Alyssum will be spaced at 15 cm intervals, whereas a Pelargonium plant may require 45 cm between plants. For decorative effect, the larger plants are normally placed towards the back of the border and at a wider spacing.

In a field of potatoes, the plant spacing will be closer within the row (40 cm) than between the rows (70 cm) so that suitable soil ridges can be produced to encourage tuber production, and machinery can pass unhindered along the row.

In nursery stock production, small trees are often planted in a square formation with a spacing ideal for the plant species, e.g. the conifer Chamaecyparis at 1.5m.The recent trend in producing commercial top fruit, e.g. apples, is towards small trees (using dwarf rootstocks) in order to produce manageable plants with easily harvested fruit. This has resulted in spacing reduced from 6 to 4m.

A correct plant-spacing distance is that most likely to provide the requirements shown in ill. 1.1 at their optimum level. Too much com petition for soil space by the roots of adjacent plants, or for light by their leaves, would quickly lead to reduced growth. Three ways of overcoming this problem may be seen in the horticulturist's activities of transplanting seedlings from trays into pots, increasing the spacing of pot plants in green houses, and hoeing out a proportion of young vegetable seedlings from a densely sown row. An interesting horticultural practice, which reduces root competition, is the deep-bed system, in which a 1m depth of well-structured and fertilized soil enables deep root penetration. However, growers often deliberately grow plants closer to restrict growth in order to produce the correct size and the desired uniformity as in the growing of carrots for the processing companies.

Whilst spacing is a vital aspect of plant growth, it should be realized that the grower might need to adjust the physical environment in one of many other specific ways in order to favor a chosen plant species. This may involve the selection of the correct light intensity; a rose, for example, whether in the garden, greenhouse or conservatory, will respond best to high-light levels, while a fern will grow better in low light.

Another factor may be the artificial alteration of day length, as in the use of 'black-outs' and cyclic lighting in the commercial production of chrysanthemums to induce flowering. Correct soil acidity (pH) is a vital aspect of good growing: heathers prefer high acidity, whilst saxifrages grow more actively in non-acid (alkaline) soils. Soil texture, e.g. on golf greens may need to be adjusted to a loamy sand type at the time of green preparation in order to reduce compaction and maintain drainage.

Each crop species has particular requirements, and it requires the skill of the horticulturist to bring all these together. In greenhouse production, sophisticated control equipment may monitor air and root-medium conditions every few minutes, in order to provide the ideal day and night requirements.

Competition between species

The subject of 'ecology' deals with the interrelationship of plant (and animal) species and their environment. Below are described some of the eco logical terms and concepts which most commonly apply to the natural environment, where human interference 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 a plant or animal lives. For a water lily, its only habitat is a pond or a slow-moving river. In contrast, a species such as a blackberry may be found in more than one habitat, e.g. heathland, woodland and in hedges. The common rat, often associated with humans, is seen in various habitats (e.g. farms, sewers, hedgerows and food stores).

On a smaller scale, the term microhabitat is used to pinpoint a particular part of a plant or soil where a particular plant, or a small organism occurs.

The glasshouse whitefly occupies the under-leaf microhabitat of a Fuchsia plant. The wilt fungus Verticillium alboatrum lives in the xylem micro habitat of plants.

Niche. Given the dynamic way that plants and animals grow in size and numbers, and compete against each other, it is not surprising to find that each species of plant or animal has an ideal location for its best growth and survival (this location is called its niche). The term 'niche' carries with it an idea of the specialization that a species may exhibit within a community of other plants and animals. A niche involves, for plants, such factors as temperature, light intensity, humidity, pH, nutrient levels, etc. For animals such as pests and their predators, there are also factors such as preferred food and 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 the factors mentioned above, but it is useful in emphasizing specialization within a habitat. The biologist, Gause, showed that no two species can exist together if they occupy the same niche. One species will, sooner or later, start to dominate.

For the horticulturalist, here is the important concept that for each species planted in the ground, there is an ideal combination of factors to be considered if the plant is to grow well. Although this concept is an important one, it should not be taken to an extreme. Most plants tolerate a range of conditions, 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 of organisms than that of a habitat.As with the term habitat, the term biome is biological in emphasis, concentrating on the species present.This is in contrast to the wider ecosystem concept described below. Commonly recognized biomes would be 'temperate woodland', 'tropical rainforest', 'desert', 'alpine' and 'steppe'. About 35 types of biomes are recognized worldwide, the classification being based largely on climate; on whether they are land based or water based; on geology and soil; and on altitude above sea level. Each example of a biome will have within it many habitats. Different biomes may be characterized by markedly different potential for annual growth. For example, a square meter of temperate-forest biome may produce about 10 times the growth of an alpine biome.

Ecosystem. This term brings emphasis to both the community of living organisms and to their non-living environment. Examples of ecosystems are a wood, a meadow, a chalk hillside, a shoreline and a pond. Implicit within this term (unlike the terms habitat, niche and biome) is the idea of a whole integrated system, involving both the living (biotic) plant and animal species, and the non-living (abiotic) units such as soil and climate, all reacting together within the ecosystem. Ecosystems can be described in terms of their energy flow, showing how much light is stored (or lost) within the system as plant products such as starch (in the plant) or as organic matter (in the soil). Also, the several other systems such as carbon, nitrogen and sulphur cycles, and water conservation may also be presented as features of the ecosystem in question.

Succession. Communities of plants and animals change with time. The species composition will change as will the number of individuals within each species. This process of change is known as 'succession'. Two types of succession are recognized. The first one, known as primary succession is seen in a situation of uncolonized ground. Sand dunes, disused quarries and landslide locations are good examples. This process runs in parallel with the formation of soils. It can be seen that plant and animal species from outside the new habitat will be the ones involved in colonization.

The second (and more common example in Britain) is secondary succession, where a bare habitat is formed after vegetation has been burnt, or chopped down, or covered over with a flood silt deposit. In this situation, there will often be plant seeds and animals which survive under the barren surface to begin colonization again, by bringing top soil to the surface, or at least some of its associated beneficial bacteria and other micro-organisms.

The first species to establish are aptly called the 'pioneer community'. In felled woodland, these may well be mosses, lichens, ferns and fungi. In contrast, a drained pond will probably have Sphagnum moss, reeds and rushes, more at home in this wetter habitat.

The second succession stage will see plants such as grasses, foxgloves and willow herb taking over in the ex-woodland area. Grasses and sedges are the most common examples seen in the drained pond. Such early colonizing species are sometimes referred to as opportunistic. They often have similar characteristics to horticultural weeds, viz. extended seed germination period, rapid plant establishment, short time to maturity and considerable seed production. They quickly cover over the previously bare ground.

The third succession stage involves larger plants, which, over a period of about 5 years, gradually reduce the opportunists' dominance. Honeysuckle, elder, and bramble are often species that appear in ex-woodland, whilst willows and alder occupy a similar position in the drained pond. The term competitive is applied to such species.

The fourth stage introduces tree species that have the potential to achieve considerable heights.

It may well happen that both the ex-woodland and the drained pond situation end up with the same tree species such as birch, oak and beech. These are described as climax species, and will dominate the habitat for a long time so long as it remains undisturbed, by natural or human forces. Within the climax community, there often remain some specimens of the preceding succession stages, but they are now held in check by the ever-larger trees.

This short discussion of succession has emphasized the plant members of the community. As succession progresses along the four stages described, there is usually an increase in biodiversity, i.e. increase in numbers of plant species. It should also be borne in mind that, for every plant species there will be several animal species dependent on it for food, and thus succession brings biodiversity in the plant, animal, fungal and bacterial realms.

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

Food chains

At any one stage along the succession sequence in a habitat, there will be a particular combination of living things (organisms) associated with the plant community. In a crop situation, e.g. strawberries, the crop plant itself is the main source of energy for the other organisms, and is referred to, along with any weeds present, as the primary producer in that habitat. Any pest (e.g. aphid) or disease (e.g. mildew) feeding on the strawberries is termed a primary consumer, whilst a ladybird eating the aphid is called a secondary consumer. A habitat may include also tertiary, quaternary consumers, etc. Any combination of species such as the above is referred to as a food chain and each stage within a food chain is called a trophic level:

e.g. strawberry --> aphids --> ladybird

In the pond habitat, a comparable food chain would be:

green algae --> Daphnia crustacean --> minnow fish

Within any plant community, there will be com parable food chains to the one described above. It is normally observed that in a monoculture such as strawberry, there will be a relatively short period of time (up to 5 years) for a complex food chain to develop. However, in a long-term stable habitat such as oak woodland or a mature garden-growing perennial, there will be many plant (primary producer) species, allowing many food chains to occur.

Furthermore, primary consumer species, e.g. cater pillars and pigeons, may be eating from several different plant types, whilst secondary consumers such as predatory beetles and tits will be devouring a range of primary consumers on several plant species. In this way, a more complex, interconnected community is developed, called a food web.

An interesting feature of succession is that, as time passes, the habitat acquires a greater diversity of species, and more complex food webs, including the important rotting organisms such as fungi which break down ageing and fallen trees. Effective countryside management particularly utilizes these food webs, and succession principles when striking a balance between the production of species diversity and the maintenance of an acceptably orderly managed area.

Decomposers

At this point, the whole group of organisms involved in the recycling of dead organic matter (called decomposers or detritovores) should be mentioned in relation to the food-web concept. The organic matter (see also Section 15) derived from dead plants and animals of all kinds is digested by a succession of species: large animals by crows, large trees by bracket fungi, small insects by ants, roots and fallen leaves by earthworms, mammal and bird feces by dung beetles, etc. Subsequently, progressively small organic particles are consumed by millipedes, springtails, mites, nematodes, fungi and bacteria to eventually create the organic molecules of humus that are so vital a source of nutrients, and a means of soil stability in most plant-growth situations. It can thus be seen that although decomposers do not normally link directly to the food web; they are often eaten by secondary consumers. They also are extremely important in supplying inorganic nutrients to the primary producer plant community.

Sustainable development

From the content of preceding paragraphs, it can be seen that the provision of as extensive a system of varied habitats, each with its complex food web, in as many locations as possible, is increasingly being considered desirable in a nation's environment provision. In this way, a wide variety of species numbers (biodiversity) is maintained, habitats are more attractive and species of potential use to mankind are preserved. In addition, a society that bequeaths its natural habitats and ecosystems to future generations in an acceptably varied, useful and pleasant condition, is contributing to the sustainable development of that nation.

Biomass

At any one time in a habitat, the amount of living plant and animal tissue (biomass) can be measured or estimated. In production horticulture, it is clearly desirable to have as close to 100 per cent of this biomass in the form of the primary producer (crop), with as little primary consumer (pest or disease) as possible present. On the other hand, in a natural woodland habitat, the primary producer would represent approximately 85 per cent of the biomass, the primary consumer 3 percent, the secondary consumer 0.1 per cent and the decomposers 12 percent. This weight relationship between different trophic levels in a habitat (particularly the first three) is often summarized in graphical form as the 'pyramid of species'.

A further concept relevant to the plant-animal relationships relates to energy. The process of photosynthesis enables the plant to retain, as chemical energy, approximately 1 per cent of the sun's radiant energy falling on the particular leaf's surface. As the plant is consumed by primary consumers, approximately 90 per cent of the leaf energy is lost from the biomass, either by respiration in the primary consumer, by heat radiation from the primary consumer's body or as dead organic matter 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 biomass as against its total organic matter are an important feature. This balance can be markedly affected by physical factors such as soil type, by climatic factors such as temperature, rainfall and humidity, and can also be affected by the management system operating in that habitat (or ecosystem). For example, a temperate woodland on 'heavy' soil with 750mm annual rainfall will maintain a relatively large soil organic matter content, permitting good nutrient retention, good water retention and resisting soil erosion even under extreme weather conditions.

For these reasons, the habitat is seen to be relatively stable. On the other hand, a tropical forest on a 'light' soil with 3000mm rainfall will have a much smaller soil organic matter reserve, with most of its carbon compounds being used in the living plants and animal tissues. As a consequence, nutrient and moisture retention and resistance to soil erosion are usually low; serious habitat loss can result when wind damage or human interference occurs. For temperate horticulturists, the main lesson to keep in mind is that high levels of soil organic matter are usually highly desirable, especially in sandy soils that readily lose organic matter.

Garden considerations

When contemplating the distribution of our favorite species in the garden (ranging from tiny annuals to huge trees), a thought may be given to their position in the succession process back in the natural habitat of their country of origin. Some will be species commonly seen to colonize bare habitats. Most garden species will fall into the middle stages of succession. A few, whether they be trees, climbers or low-light-requirement annuals or perennials will be species of the climax succession.

The garden border contains plant species, which compete aggressively in their native habitat. The artificial interplanting of species from different parts of the world (the situation found in almost all gardens), may give rise to unexpected results as this competition continues year after year. Such experiences are part of the joys, and the heartaches of gardening.

Companion planting

An increasingly common practice in some areas of horticulture (usually in small-scale situations) is the deliberate establishment of two or more plant species in close proximity with the intention of deriving some cultural benefit from their association. Such a situation may seem at first sight to encourage competition rather than mutual benefit. Many supporters of companion planting reply that plant and animal species, in the natural world show more evidence of mutual cooperation than of competition.

Some experimental results have given support to the practice, but most evidence remains anecdotal. It should be stated, however, that whilst most commercial producers in western Europe grow blocks of a single species, in many other parts of the world two or three different species are inter planted as a regular practice.

Several biological mechanisms are quoted in support of companion planting:

• Nitrogen fixation. Legumes such as beans convert atmospheric nitrogen to useful plant nitrogenous substances by means of Rhizobium bacteria in their root nodules.

Beans interplanted with maize are claimed to improve maize's growth by increasing its nitrogen uptake.

• Pest suppression. Some plant species are claimed to deter pests and diseases. Some examples are listed. Onions, sage and rosemary release chemicals that mask the carrot crop's odor thus deterring the most serious pest, carrot ?y from infesting the carrot crop. African marigolds (Tagetes) deter glasshouse whitefly and soil borne nematodes by means of the chemical, thiophene. Wormwood (Artemisia) releases methyl jasmonate as vapor that reduces cater pillar feeding, and stimulates plants to resist diseases such as rusts. Chives and garlic reduce aphid attacks.

• Beneficial habitats. Some plant species present a useful refuge for beneficial insects, such as ladybirds, lacewings and hoverflies. In this way, companion planting may preserve a sufficient level of these predators and parasites to effectively counter pest infestations. The following examples may be given: carrots attract lace wings; yarrow (Achillea), ladybirds; goldenrod (Solidago), small parasitic wasps; poached-egg plant (Limnanthes) attracts hover flies. In addition, some plant species can be considered as traps for important pests. Aphids are attracted to nasturtiums, flea beetles to radishes thus keeping the pests away from a plant such as cabbage.

• Spatial aspects. A pest or disease, specific to a plant species will spread more slowly if the distance between individual plants is increased.

Companion planting achieves this goal. For example, potatoes interplanted with cabbages will be less likely to suffer from potato blight disease. The cabbages similarly would be less likely to be attacked by aphid.

ORGANISMS OCCURRING ON PLANT SURFACES

A further aspect of species interaction can be seen at the microscopic level in the soil and on the aerial parts of plants. The plant surfaces of leaves, stems and roots present an environment for beneficial and for damaging organisms. The latter group are described in the next section.

Research has indicated the complex microbial composition of plant surfaces and their importance to successful plant growth. The term rhizosphere is used to describe the environment for bacteria, fungi, mites and nematodes situated around the root, whilst the comparable term phyllosphere applies to the environment on the leaf and stem.

Rhizosphere

The term 'rhizosphere' refers to the environment closely adjacent to roots. It is relatively stable compared to the leaf environment, particularly in terms of temperature and humidity. The components of this zone (soil mineral particles, water, gases, organic matter and micro-organisms) inter act to influence root activity. The area of root near the tip produces numerous root hairs, which are important for absorption of water and nutrients.

In the area behind this tip zone, however, there is often a complex association of micro-organisms whose contribution to root activity may be equal to that of the root hairs. The most striking members of this community are the fungi that develop close associations with the root tissues, often invading the root, but not damaging it. These fungi are known as mycorrhizae. The tiny strands spread out into the surrounding soil and act as an additional absorption system for the plant.

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

Mycorrhizae have been shown in many species to have important roles in absorbing water and nutrients such as phosphate and nitrate. Phosphate is the least mobile major nutrient in the soil, and the plant must 'reach out' to find it.

Mycorrhizae help plants to do this. An extreme case is the heather (Erica) genus which is often found growing in acidic soils, where phosphate is insoluble, where root growth is limited, and where there are very few bacteria involved in the break down of organic matter. Under these conditions, there is a special need for mycorrhizae. While the mycorrhizae are responsible for assisting the root's function, there is a reciprocal process whereby the plant, in return, provides the fungus with sugars and other organic substances favorable for its growth. This plant-fungus association is a good example of symbiosis.

The use of sterile, compost-based growing media for propagation may deny the young plants a source of mycorrhizal fungi. Consequently, specially prepared cultures of these fungi are some times introduced into growing media to stimulate plant growth, e.g in the production of young conifers. The particular requirements of the mycorrhizae have to be met where they are important for the plants success, e.g. many orchids are grown in translucent pots in order to provide for the light requirement of the species of fungus concerned.

Phyllosphere

Phyllosphere bacteria on the leaf may be 'casual' or 'resident'. Casual species, e.g. Bacillus mainly arrive from soil, roots and water, and are more common on the leaves close to the ground. These species are capable of rapid increase under favorable conditions, but then may decline. Resident species, e.g. Pseudomonas, may be weakly parasitic on plants, but more commonly persist (often for considerable periods) without causing damage, and on a wide variety of plants.

There is increasing evidence that phyllosphere bacteria may reduce the infection of diseases such as powdery mildews, Botrytis diseases on lettuce and onion, and turfgrass diseases. Practical disease control strategies by phyllosphere organisms have not been developed, but there remains the general principle that a healthy, well-nourished plant will be more likely to have organisms on the leaf surface available to reduce fungal infection.

CONSERVATION

The ecological aspects of horticulture have been highlighted in recent years by the conservation movement. One aim is to promote the growing of crops and maintaining of wildlife areas in such a way that the natural diversity of wild species of both plants and animals is maintained alongside crop production, with a minimum input of fertilizers and pesticides. Major public concern has focused on the effects of intensive production (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, providing an attractive display during several months of the year. The conversion of productive land into wild flower meadow requires lowered soil fertility (in order to favor wild species establishment and competition), a choice of grass seed species with low opportunistic properties, and a mixture of selected wild flower seed. The maintenance of the wild flower meadow may involve harvesting the area in July, having allowed time for natural flower seed dispersal. After a few years, butterflies and other insects become established as part of the wild flower habitat.

The horticulturist has three notable aspects of conservation to consider. Firstly, there must be no willful abuse of the environment in horticultural practice. Nitrogen fertilizer used in excess has been shown, especially in porous soil areas, to be washed into streams, since the soil has little ability to hold on to this nutrient. The presence of nitrogen in watercourses encourages abnormal multiplication of micro-organisms (mainly algae).On decaying, these remove oxygen sources needed by other stream life; particularly fish (a process called 'eutrophication').

Secondly, another aspect of good practice increasingly expected of horticulturists is the intelligent use of pesticides. This involves a selection of those materials least toxic to man and beneficial animals, and particularly excludes those materials that 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 properties should be noted. Firstly, it is long lived (residual) in the soil. Secondly, it is absorbed in the bodies of most organisms with which it comes into contact, being retained in the fatty storage tissues. Thirdly, it increases in concentration approximately 10 times as it passes to the next member of the food chain. As a consequence of its chemical properties, DDT was seen to achieve high concentrations in the bodies of secondary (and tertiary) consumers, such as hawks, influencing the reproductive rate and hence causing a rapid decline in their num bers in the 1960s.This experience rang alarm bells for society in general, and DDT was eventually banned in most of Europe.

The irresponsible action of allowing pesticide spray to drift onto adjacent crops, woodland or rivers has decreased considerably in recent years.

This has in part been due to the Food and Environment Protection Act (FEPA) 1985, which has helped raise the horticulturist's awareness of conservation.

A third aspect of conservation to consider is the deliberate selection of trees, features and areas which promote a wider range of appropriate species in a controlled manner. A golf course man ager may set aside special areas with wild flowers adjacent to the fairway, preserve wet areas and plant native trees. Planting bush species such as hawthorn, field maple and spindle together in a hedgerow provides variety and supports a mixed population of insects for cultural control of pests.

Tit and bat boxes in private gardens, an increasingly common sight, provide attractive homes for species that help in pest control. Continuous hedgerows will provide safe passage for mammals.

Strips of grassland maintained around the edges of fields form a habitat for small mammal species as food for predatory birds such as owls. Gardeners can select plants for the deliberate encouragement of desirable species (nettles and Buddleia for butterflies; Rugosa roses and Cotoneaster for winter feeding of seed-eating birds; poached-egg plants for hoverflies).

It is emphasized that the development and maintenance of conservation areas require continuous management and consistent effort to maintain the desired balance of species and required appearance of the area. As with gardens and orchards, any lapse in attention will result in invasion by unwanted weeds and trees.

In a wider sense, the conservation movement is addressing itself to the loss of certain habitats and the consequent disappearance of endangered species such as orchids from their native areas.

Horticulturists are involved indirectly because some of the peat used in growing media is taken from lowland bogs much valued for their rich variety of vegetation. Considerable efforts have been made to find alternatives to peat in horticulture and protect the wetland habits of the British Isles.

Conservationists also draw attention to the thoughtless neglect and eradication of wild-ancestor strains of present-day crops; the gene-bank on which future plant breeders can draw for further improvement of plant species. There is also concern about the extinction of plants especially those on the margins of deserts that are particularly vulnerable if global warming leads to reduced water supplies. In situ conservation mainly applies to wild species related to crop plants and involves the creation of natural reserves to protect habitats such as wild apple orchards and there is particular interest in 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.

ORGANIC GROWING

The organic movement broadly believes that crops and ornamental plants should be produced with as little disturbance as possible to the balance of microscopic and larger organisms present in the soil, and also in the above-soil zone. This stance can be seen as closely allied to the conservation position, but with the difference that the emphasis here is on the balance of micro-organisms. Organic growers maintain soil fertility by the incorporation of animal manures, or green manure crops such as grass-clover leys. The claim is made that crops receive a steady, balanced release of nutrients through their roots; in a soil where earthworm activity recycles organic matter deep down, the resulting deep root penetration allows an effective uptake of water and nutrient reserves.

The use of most pesticides and quick-release fertilizers is said to be the main cause of species imbalance, and formal approval for licensed organic production may require soil to have been free from these two groups of chemicals for at least 2 years. Control of pests and diseases is achieved by a combination of resistant cultivars and 'safe' pesticides derived from plant extracts, by careful rotation of plant species, and by the use of naturally occurring predators and parasites.

Weeds are controlled by mechanical and heat producing weed-controlling equipment, and by the use of mulches. The balanced nutrition of the crop is said to induce greater resistance to pests and diseases, and the taste of organically grown food is claimed to be superior to that of conventionally grown produce.

The organic production of food and non-edible crops at present represents about 5 per cent of the European market. The European Community Regulations (1991) on the 'organic production of agricultural products' specify the substances that may be used as 'plant-protection products, deter gents, fertilizers or soil conditioners'. 'Conventional horticulture' is, thus, still by far the major method of production and this is reflected in this book. However, it should be realized that much of the subsistence cropping and animal production in the Third World could be considered 'organic'.

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