20 March 2014
Companion planting is a way of growing different plants next to each other for the benefit of one or both of the plants, and is an old gardening tradition. It's not just about pest control either - by combining the right plants they can help each other by giving protection from wind or sun, by attracting beneficial pests or acting as a decoy for harmful ones, and by providing nutrients in the soil.
Many of the recommended plant combinations are based on garden folklore rather than scientific evidence, although many gardeners swear by them. There is, however, growing scientific evidence to support certain widely used plant combinations.
Many pests locate their food by sense of smell, so by mixing up your plants you can confuse the predators while saving your crop. The cabbage white butterfly can be fooled into passing by a crop of brassicas simply by planting French marigolds in between the plants, while parsley planted among carrots repels carrot flies. Other plants that produce confusing scents include garlic, lavender, rosemary, chives, sage and thyme.
Some plants are especially attractive to pests and can be used near other susceptible crops to draw potential pests away. Nasturtiums, for example, are so popular with aphids that they will be attacked in preference to other plants. There is scientific evidence to show that Tagetes Sinuata can be used to keep potato crops free of eelworm, as the pest is attracted to it's roots instead.
Companion plants can also give their neighbours a tonic. Parsley gives increased vigour to tomatoes and asparagus, while horseradish planted near potatoes make them stronger and more disease resistant. However, make sure that you dig up horseradish every year to prevent it spreading too much.
Some combinations to try are:-
French marigolds among tomatoes. Marigolds give off a strong odour that repels both greenfly and blackfly.
Sage with carrots or plants in the cabbage family to ward off pests. Both have strong scents that repel each other's pests.
Nasturtiums with cabbages. Caterpillars are drawn to the nasturtiums rather than the cabbages.
Carrots and leeks. Carrots repel onion fly and leek moth while leeks repel carrot fly.
Other beneficial plants are chives, whose scent helps to keep aphids from tomatoes, and chervil, which keeps aphids off lettuce.
Yarrow increases the vigour of other plants and accumulates calcium, silica and phosphorous, which can help your compost heap when plants are added. It also attracts many beneficial creatures such as hoverflies and ladybirds.
Just remember that you need to plant companion plants at the same time as the crops you want to protect.
Enjoy a pest-free gardening year.