Practical Science for Gardenersby Mary Pratt
Informative and entertaining, this book will stimulate experimentation and encourage gardeners to review and improve their current gardening practices. Once gardeners learn how plants are constructed, it is easier to envision how they'll grow and flourish. For the enthusiastic beginner or the master gardener, Practical Science for Gardeners unravels the/i>… See more details below
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Informative and entertaining, this book will stimulate experimentation and encourage gardeners to review and improve their current gardening practices. Once gardeners learn how plants are constructed, it is easier to envision how they'll grow and flourish. For the enthusiastic beginner or the master gardener, Practical Science for Gardeners unravels the mysterious inner life of plants.
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Read an Excerpt
Nature's arms race and how to negotiate a peace deal
Neither 'mother nature' nor 'nature red in tooth and claw' are really appropriate epithets for the non-human world. 'Nature's arms race', in which gardeners and farmers become intimately involved, is nearer to the mark. I suppose motherly qualities come to mind because all our nutritional needs have ultimately been met from wild plants and animals. But nature is not in the least motherly to its own offspring. Out there, be it in wilderness or garden, it's 'each one for itself'. This, of course, leads to the 'red in tooth and claw' concept but, curiously, things are not as bad as they may appear.
Ecological research has shown that individual species tend to occupy their own little niches, their particular way of 'making a living', to which they are well adapted, and in which they can avoid direct competition with other species for scarce resources. For example, several species of bird live on coastal mudflats, each endowed with a particular length of beak adapted to feeding on creatures found at different depths in the mud. They can thus all live happily together without having to squabble over their next meal. Admittedly the creatures living in the mud are being preyed upon but, at least, the birds aren't fighting each other for access to food.
Plants, like animals, occupy their own special niches — adapted to particular conditions. They are ostensibly docile and peaceable, quietly minding their own business. They just get on with making their own food, and are fortunate in their ability to survive nibbling by herbivorous animals. But many of them are remarkably good at fending off potential enemies — you'll have noticed that there are certain plants which slugs won't touch. Some of them are not at all innocent, entering the fray with a vengeance. There are some pretty nasty weapons in their armoury, stinging cells, spines, alkaloid poisons to name but a few. So, if 'red in tooth and claw' conjures up snarling carnivores, nature could equally well be described as 'green in thorn and killer chemicals'! There is thus a constant arms race going on — organisms needing to feed, but also having to avoid being preyed upon or outwitted in competition. We, as gardeners, are very much part of that arms race because we are in the business of manipulating things to suit our needs.
It makes sense to think of the living world, the biosphere, as an intricate and miraculously self-sustaining network. The expression 'the balance of nature' is sometimes used in connection with this network, but it's not necessarily a balance in the sense of a see-saw which is poised, equally weighted at either end. It is, rather, a constantly fluctuating balance — a see-saw being played on and moving up and down. Sometimes, due to some cataclysmic event or to human mismanagement, the fluctuations can get out of hand, drastic crashes occur (and the players may fall off!). Humans have not treated the biosphere well, so gardeners, being intimately involved in the network, have a particular responsibility to be constructive and careful in their management of it. This chapter is about being involved in the arms race but achieving a sensible balance without being too destructive.
Because plants and animals, and their relationships with their environment, are so complex, the science of the topics in this chapter is by no means precise. What follows is, I hope, a guide through a somewhat tortuous maze.
Ecologists — who study living things in relation to their environment — have tried to work out the way in which species fit into their niches and to unravel the complex interactions between them. Feeding relationships are important here. Who feeds on what can be determined by direct observation, or by analysing the contents of a predator's digestive system. A picture emerges of networks of feeding relationships, referred to as food webs. An example of a garden food web might be depicted as in Figure 5-1. It must be emphasized that most diagrams like this are gross over-simplifications. There are more creatures in our gardens than we might suppose, and finding out all their feeding habits would be a challenge indeed.
Weeds, pests and diseases
In the food web in Figure 5-1 there are plants to be cherished, a plant which is not welcome, animals which can be tolerated, animals which are a nuisance, and animals which are positively beneficial. This is just a very small selection of the kinds of creatures and their feeding relationships which might exist in a garden. There is also a myriad of micro-organisms, busy being parasites, often causing nasty diseases; some, on the other hand, are going about their business harmlessly, and some are being really helpful. Organisms which we call pests or weeds are simply those which militate against our efforts to produce beautiful flowers, immaculate foliage, and copious, unblemished vegetables. In Figure 5-1 they are the cabbage white butterfly caterpillar, the tortrix moth caterpillar, the aphid, the snail and the dandelion.
There is no point in listing the numerous different kinds of nuisances and ways of identifying them. There are excellent books available and you will find some suggestions in the Further Reading list. If they are not obviously available on your library shelves, or in the local garden centre, remember that libraries will order things for a small fee.
Preserving the cherished, the tolerable and the beneficial, whilst keeping at bay the undesirables
There is a school of thought which insists that it must all be done 'organically', relying on natural predators and parasites, using hand or mechanical removal of weeds or other nasties, and spraying only with substances which occur naturally or are approved by the Soil Association. Others think this is a load of nonsense — a visit to the chemicals section of a garden centre demonstrates that there is a vast army of gardeners who spray all manner of man-made concoctions on their plants. Scientific evidence would suggest, though, that — as with soil management — the most sensible approach is to keep an open mind and adopt a combination of both.
The next few sections contain guidelines towards achieving a compromise — the use of a combination of biological and man-made chemical agents. This kind of approach is technically called Integrated Pest Management (IPM). In general it makes sense to use the predators and parasites in natural food webs to keep things under control. This is particularly so in a garden where the scale of operation is fairly small. They are not, however, likely to eradicate a pest species completely, for reasons I shall explain. If things are really troublesome, modern science offers well-tested alternatives, though man-made pesticides are not without their problems — not least the fact that pests become resistant to them. Some people worry about the human health and safety aspects of pesticides but the Food and Environment Protection Act, 1985, the Pesticides Safety Directorate, and European legislation are there to safeguard us. In the USA there is the Federal Environmental Pesticide Control Act of 1972. All products legally permitted for use in agriculture and horticulture have been thoroughly tested for safety. It is therefore technically illegal to attack the aphids on your roses by spraying them with diluted washing-up liquid; detergents haven't been tested with their gardening safety in mind!
One of the themes of this book, though, is that science isn't perfect — scientists do the best they can. It's worth remembering that just as, in medicine, there may be changes of treatment in the light of new discoveries, so scientists working on pesticides may change their tune in view of new evidence. What was once thought safe may be deemed no longer so, and products are taken off the market (as has happened recently under European rules). The range of pesticides available to gardeners is now very limited. It's my view that the danger which this small selection poses to humans, domestic animals and the environment is utterly insignificant compared with other risks of modern life.
Controlling animal pests without artificial chemicals
This section covers those methods which are commonly termed biological, physical and cultural — those which rely on the use of other living organisms, traps and barriers, or particular cultivation practices.
Predators and parasites
There are many examples of predators or parasites which make their living by consuming undesirables in the garden. Blue tits, ladybirds and lacewing larvae eat aphids. There are tiny parasitic wasps which have the unpleasant habit of laying their eggs inside the larvae of moths or flies, so that the grubs consume their host from the inside — Encarsia, used in glasshouses to control white fly, is an example. But there are snags about counting on these agents. One factor is the relationship between predator and prey, a topic which has been much studied by ecologists.
Interactions between predators and prey
The relationship between a single predator and its prey often follows a particular pattern. At first the predator has an abundance of prey and is able to reproduce so successfully that its numbers increase in parallel with prey numbers. But there comes a point where predation begins to reduce the numbers of prey, there is less food available for the predator, and its numbers begin to decline. After a phase of reduction in predator numbers the prey species begins to recover and the pattern repeats itself.
Scientific evidence for this effect comes from situations where the ecosystem is a simple one. An example is a Canadian study, published as long ago as 1937, which collected and collated hunting and trapping records of the numbers of lynx (predator) and snowshoe hare (prey) caught over a period of almost 100 years. The results show a striking picture of approximately 10-year cycles; as numbers of hare increased so, after a brief delay, did the numbers of lynx, but there always came a point where hare numbers started to decrease followed by a crash in lynx numbers. Then hares recovered and so did the lynx. I won't bother you with the graph — but it shows a very pretty and obvious 'boom-and-bust' pattern, with the lines for lynx and hare running up and down in parallel with a slight lag of lynx behind hare. Another more recently studied example is that of lemmings and their predators on the arctic tundra of northern Europe. Here lemmings are the main small herbivores, and the carnivores which prey on them are arctic fox, snowy owl, long-tailed skua and stoat. Lemming numbers have long been known to fluctuate markedly, leading to various myths, including mass suicide, and hypotheses to do with weather and availability of food plants. Over a 15-year period these animals have been closely studied, and the 'boom-and-bust' story has been shown to hold good for them too. As lemming numbers increase so do the numbers of predators, reaching a point where the pressure on the lemmings is such that their numbers start to decrease. With not enough food around the foxes, owls and skuas apparently move off to search for other prey species which, in the arctic, are not numerous, and the stoats, which feed only on lemmings, die off. The lemmings then begin to do better and the cycle starts again.
In a garden similar cycles may occur. An example is the potential interaction between aphids and their predators. Ladybird larvae might come across an abundant supply of food on your rose bushes. A veritable feast ensues and the ladybirds do so well that they multiply prodigiously. Then there comes a point when aphids are beginning to be in short supply and the ladybirds can no longer support so many offspring. But as soon as predator numbers die down, the aphids begin to have a field day again and return to their former abundance. So you are back to square one.
The story is simple. To be a successful parasite you mustn't kill your source of food straight away. A parasite is an organism which feeds from a living plant or animal — its host. Much of the time the thieving beast sits happily consuming nice nutritious juices from its host's body, occasionally producing vast numbers of offspring, eggs, seeds or spores, which are specially designed to invade a new host. The crucial thing is that a successful parasite doesn't immediately finish off its host, thereby running the risk of being thrown out of its cushy lodgings before having had a chance to reproduce or to emerge as an adult. The host is simply weakened. So, while parasites may be a useful way of reducing the virulence of a pest species, and perhaps reducing its capacity to multiply, they will rarely wipe out the pest altogether.
Predators and parasites in the garden
By all means encourage predatory species — ladybirds, lacewings, ground beetles, insectivorous birds — but don't expect them to do more than periodically keep the numbers of pests at moderate levels. Lacewings can be encouraged by providing them with overwintering quarters — such 'lacewing hotels' can be purchased now through gardening catalogues. Ladybirds often hibernate in hollow stems, so leave tidying away some of the stems of large herbaceous perennials until warm weather in the spring. Nematode worms which parasitize slugs can be purchased by mail order from gardening catalogues, particularly from firms specializing in 'organic' gardening.
Greenhouses and conservatories
The Victorians used to introduce frogs into their greenhouses and encourage them to stay there. Why not try it? Use such parasitic organisms as are on offer for use in glasshouses (obtainable from catalogues) but don't expect too much of them. Examples are the parasitic wasp, Encarsia, for controlling greenhouse white fly, nematode worms which transmit fatal bacterial diseases to vine weevil grubs, and the toxin-producing bacterium Bacillus thuringiensis for moth and butterfly caterpillars. Don't sweep away spiders' webs; spiders are the tigers of your jungle and will gobble up any pests which happen to land in their well-laid traps.
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