COMPANION planting has been around a long time, in the stripes and patches of old cottage gardens and the mixed fields of peasant farmers down the centuries. It's an ancient practice with new appeal for a generation of organic, environmentally aware gardeners and farmers determined to reduce spraying and still harvest a worthwhile crop.
Here's the basic idea: positive pairings of a variety of plants can deflect pests, improve soil and plant health and pollination. Some pairings are well known – carrots and leeks, chervil and lettuce, nasturtiums and pumpkins, garlic and roses.
But do they really work? It's hard to prove.
"Apart from one or two exceptions, most examples are unproven, and unprovable," Bob Sherman, chief horticultural officer at Garden Organic, says. And traditional horticulture scientists remain cautious, if not downright sceptical.
"Companion planting has its advocates, and it is possible that interplanting crops may reduce pest incidence, for instance that of cabbage aphid and root fly where brassica plant rows are alternated with some other crop," the Royal Horticultural Society concedes.
But there's much more to it than simply planting leeks and carrots or rosemary and cabbages together. Companion planting is one important element in an organic, holistic view of gardening that emphasises the interdependence of life forms, from different plant components to microbes, insects and birds – in fact, all the life that is to be found in a healthy garden.
MIXED BLESSINGSThe Three Sisters, a traditional system native to Guatemala and Mexico that combines runner beans with maize and squash, is an excellent example of companion planting that functions in several different mutually beneficial ways: the maize provides a natural support for the beans, which are nitrogen fixers, improving soil fertility for the following year's crop.
They also help to stabilise the maize plants and the shallow-rooting squashes provide living mulch that suppresses weeds and reduces evaporation from the soil. Because the plants are growing at different levels, the overall yield of the planting area can be increased.
Forest planting is another ancient system, common in the tropics, which takes the interaction at different levels even further. Fruit or palm trees, vines, herbs, shrubs and vegetables are grown together as they would in a natural forest, creating a real ecosystem that maximises land use to produce a variety of crops in a relatively small area.
Square-foot gardening is a clever modern system ideal for small spaces, which packs companion plants together in small blocks. This is made viable by using companion plants, which can be planted in close proximity. This can help to discourage pests while attracting predatory insects and encouraging beneficial micro-organisms.
WAYS AND MEANS
There are many other ways in which plants benefit each other and the whole garden
Protection: one type of plant may serve as a windbreak or provide shade for another.
Chemical warfare: some key companion plants, which may help prevent pest insects or pathogens from damaging the crop, through chemical means. Marigolds (Tagetes spp) which are able to combat soil nematodes are a prime example. The effect of marigolds is greatest when you grow them as a solid planting for an entire season. The scent of other plants (such as rue, wormwood and tansy) may also help to repel pests.
Attraction: in a vegetable garden, nectar– and pollen-rich flowers attract pollinators and a variety of beneficial insects that will help to control pests. Some beneficial predatory insects consume pests in their larval form and are nectar or pollen feeders in their adult form.
Distraction: some key companion plants attract pests away from your prime crop. Nasturtiums are magnets for aphids as well as caterpillars. And egg-laying insects will hopefully prefer them to your lettuces.
Confusion: specific pests such as cabbage and carrot flies pick out their host plants first via scent. They will not land on bare soil but on the first available green leaf. They will then fly from leaf to leaf and if they receive enough positive signals to confirm that this is their host plant, they will proceed to lay eggs.
In fields or beds of a single crop this a piece of cake, but if your cabbage or carrots are interplanted with repellent or distracting plants they may just give up and go away. Recent studies on host-plant finding have shown that flying pests are far less successful if their host plants are surrounded by different plants – or even fake decoys of green plastic or cardboard.
BY THE BOOK
There's another excellent reason for companion planting: it makes for an extremely interesting garden – and a pretty one.
The Magaliesberg garden of South African herb authority Margaret Roberts is a prime example and convincing proof of its efficacy. Roberts, a lifelong devotee of organic gardening practice, has used hundreds of different plants in companion planting throughout her farm and garden, to impressively vigorous effect, "and I have very few pests and very little plant damage," she says.
Her book, Companion Planting (published by Briza), is a bible on the subject and also a fascinating insight into the many uses of plants. Roberts details over 100 plants and their recommended companions, as well as their uses in both home and garden.
Separate sections deal with useful weeds, natural sprays, fertilisers and mulches and other methods to improve the soil. She also singles out a few key aromatics as generally beneficial to most plants: basil, elder, garlic, myrtle and rue – and comfrey as a valuable soil tonic.
Companion Planting may be ordered by e-mailing firstname.lastname@example.org or visit www.margaretroberts.co.za