What is companion planting?
Companion planting is based on the theory that certain plants, when planted in close proximity to certain other plants, can assist in nutrient uptake, pollination, pest management, increased yields and, in some cases, improved flavour. Companion planting is often used simply to reduce the need for pesticides – a most important factor in organic gardening.
Fact or fiction?
The long history of companion planting suggests it really does work – but in most cases, no-one understands why. Research has proven some cases to be scientifically valid. For instance, findings show that marigolds release the chemical thiopene, which is an excellent worm repellent. It is also likely that other plant combinations rely on scent or root excretions to carry out the task they are credited with.
Intercropping
one of the most significant crop failures in history was the Great Famine in Ireland, caused by widespread potato blight in the 19th century. It was responsible for the deaths of one million people, due to starvation and disease. Home garden crops are of course grown on a much smaller scale, but intercropping (mixing different crops) is recommended to confuse pests and reduce their numbers.
oasking
Plants with a strong or pungent odour are used to confuse or repel pests. For instance, mixing strong-smelling parsley with carrots apparently deters carrot rust flies as they cannot smell the carrots! other good masking plants include basil, rue, marigolds and mints.
Biochemical pest suppression
In simple terms, this is the excretion of a chemical compound that suppresses or repels pests – as in thiopene from marigolds. This can have adverse effects, also known as plant allelopathy. For instance, black walnuts exude the compound juglan, which suppresses the growth of a wide range of other plants.
Trap cropping
This technique is used to draw pests away from crop plants. For instance, when nasturtiums are planted close to beans, they attract black aphids away from the beans. Similarly, when planted in a ring around the base of an apple tree, nasturtiums draw woolly aphids away from the apples.
Beneficial habitat
An area of plants that attracts and supports populations of beneficial insects, such as ladybirds, hoverflies, lacewings, parasitic flies and wasps, is called a beneficial habitat. Beneficial insects feed on insect pests, keeping their populations in check. Cottage garden flowers, umbelliferous plants (carrots and parsley), alyssum and phacelia are all good beneficial-insect attractants.
Soil improvement
More correctly termed “symbiotic nitrogen fixation”, this strand of companion planting uses plants to replenish nutrient levels in the soil. Legumes (peas and beans) fix atmospheric nitrogen via a symbiotic relationship with certain soil bacteria. Plants grown with, or in succession to, the legumes benefit from the added nitrogen.
‘The three sisters’
This is an ancient method of companion planting used by the Iroquois indians. Dry field corn was planted as a support structure for climbing beans. The beans replenished the soil with nitrogen and squash provided living mulch, suppressing weeds and conserving soil moisture. All three crops were vital to the Iroquois’ survival and, planted in this manner, produced high yields in a small space with little environmental impact.
Trap Cropping
This technique is used to draw pests away from crop plants. For instance, when nasturtiums are planted close to beans, they attract black aphids away from the beans. Similarly, when planted in a ring around the base of an apple tree, nasturtiums draw woolly aphids away from the apples.
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