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COMPANION PLANTING

This is a technique practiced by many organic gardeners. The theory behind companion planting is that plants have specific likes and dislikes concerning their close companions in the garden and will do better if planted in close proximity to the correct plant. Similarly, by planting a particular species in the garden you can reduce the number of weeds or attract certain pest predators.

Many of the recommendations for companion planting are based on folklore and, as with many of these tales, there is some truth in them. There is a well-known theory that, because carrot-fly are attracted by smell, they can be prevented by planting carrots between rows of onions, so the smell of the carrots is disguised. On the other hand, the cabbage white butterfly is attracted to its host plant by smell and it can be fooled by planting the highly aromatic French marigold (Tagetes) between the rows of cabbages. Many scientifically controlled trials have been conducted which indicate a reduction in attack when this was done. Many gardeners have also reported similar results with eelworms, soil pests that attack potatoes in particular, where French marigolds are grown. Scientific research has confirmed that this is indeed due to a secretion from the roots of the marigolds. French marigolds are also said to help kill weeds, in particular couch grass.

There is absolutely no doubt that marigolds (Tagetes and Calendula), planted near tomatoes or roses, for example, greatly reduce the frequency of attack by aphids - the most persistent of all garden pests. This is simply because they attract hover-flies whose larvae devour aphids by the thousand. Hover-flies are the most valuable pest predators in the garden and there are different species all over the world. Before laying her eggs, the female needs protein, which she gets from pollen. She then lays individual eggs on colonies of aphids so that the larvae have a readily available source of food when they hatch. The hover-fly has a short feeding tube so needs to feed from an open-structured flower where the pollen is easily accessible. Therefore by planting marigolds (Tagetes and Calendula), poppies (Papaver sp.), nasturtiums (Tropaeolum) or dwarf morning glory (Convolvulus tricolor) between plants, you minimize aphid attack.

It is more difficult to attract some of the other predators because they do not necessarily feed on flowers. Some, such as ladybugs, lacewings and several species of wasps that feed on and lay their eggs inside aphids and other soft-bellied pests including caterpillars, can be encouraged by providing as varied a collection of plant life as possible.

It has also been found that some pests are attracted to their host plants by sight. By mixing ornamental plants and vegetables in an ornamental border, you can camouflage the host plants which deters the pests. There is also evidence that vegetable plots that are left weed-infested suffer less than clean ones. However, the yields are also lower because the weeds compete for the same light, nutrients and water.

By careful consideration of which plants are cultivated together, you can promote healthy growth and utilize a completely natural form of pest control.