With short seasons and lower light levels than the tropics, intercropping has not been a major element of UK agriculture, but has potential for small scale vegetable production through opportunistic mixing of crops
with different cropping times.
In the UK Companion Planting usually refers to a strand of gardening lore which holds that certain combinations of flowers and vegetables deter insect or fungus pests. Tables of specific ‘companion plant’ combinations are provided in many gardening books, and it is suggested that gardeners follow these instructions to reduce pest damage.
It is often suggested anecdotally that combinations of crops grown together reduce pest damage, but as far as we are aware there is no scientific evidence that supports the supposition that particular pairs of species of plants are good ‘companions’. The combinations in ‘companion planting’ tables should be regarded as myths until proven true experimentally.
Strong smelling plants are claimed to confuse and deter insects seeking their normal food plants by scent. Commonly suggested species include marigolds Calendula officinalis,
garlic chives Allium tuberosum
, and lavender Lavandula angustifolia
are suggested actively to deter aphids, whitefly and carrot root fly. Wikipedia has a page with an extensive referenced list of companion plants
, but relatively few citations are of experimental work.
There is probably good reason to mix crop species together with other types of plants, but it doesn't have to be the traditional ‘companion plant’ combinations. Research suggests that many species of plants, including weeds, can perform the same role. Research by many scientists, including the papers cited below, suggests that it is simply an effect of mixing different plants together that hinders insect pests from locating their host plants. In a monoculture, each plant contact will be a crop plant, but with other species mixed in, insects are likely to encounter several non-crop plants before they find what they are looking for.
Evidence suggests that insects use tactile stimuli to identify the host plant, and a certain number of approaches to the correct host plant are needed to stimulate egg laying. The idea that scents or odours are involved is not supported by the evidence. Insect senses of smell can be extremely powerful, but usually with a specific chemical receptor that would not (as in our noses) be confused by a strong but unrelated scent. There is also no experimental evidence to support the idea that mixed planting deters fungus pests such as Black Spot on roses.
Advice to Gardeners
Intermixing a "desired" or crop species which is the host plant of an insect pest with any other plant species makes it less likely that the pest will land on the plant it is looking for, and slow it down. This in turn will reduce the likelihood of insect pests laying eggs on the host plant.
The prophylactic use of insecticides is not appropriate in a wildlife-friendly garden, so a ‘mixed planting’ approach to growing food crops may be an alternative strategy to help reduce insect pest damage. Instead of growing crops in long rows or large patches, you could try mixing your vegetables so that the beans for example are not all in contact with each other. There is no law of gardening that says you can't mix "flower" areas with "vegetable" areas, and this will help reduce pest spread. A tall tripod of bright red flowering runner beans is a handsome addition to any flower bed. Mixing plants also gives an opportunity to grow more flowers that provide pollen and nectar among vegetables, thereby providing food for pollinators. It is even possible that leaving some "weeds" growing around your vegetables may reduce pest damage - but that has to be balanced with their competition for light and water.
Finch, S and Collier, R (2003) Insects can see clearly now the weeds have gone Biologist 50:132-135