Marigolds with hoverfly

Companion planting is the catch-all term for exploiting the many ways in which plants interact with each other. Companion planting is often portrayed as a simple matter of which plants grow well together, but the small amount of scientific research that has been done suggests that this is not the case.

One way in which plants influence each other is by forming microclimates. We exploit this by growing hedges and windbreaks, but each plant in your garden is affecting the climate around it. Tall shrubs and trees cast shadows; ground cover plants prevent evaporation and raise the humidity close to the soil.

Plant roots have a profound effect on soil that can benefit other plants nearby. Roots form channels through which air and water can percolate, and each plant will draw a slightly different combination of nutrients from the soil.

Plants can attract or repel insects and other wildlife. They can even attract or repel the gardener – and a reasonable amount of attention from the gardener can be a big factor in plant survival!

An obvious negative influence is competition – your plants compete with weeds for sun, water and nutrients, which is why we try to keep weeds at bay.

The sheer number of influences on plant growth and the diversity of garden environments mean that companion planting – growing different species together to benefit one or both of them – is an art, rather than a science. What works in one garden may not work in another, or the following season.

However, there are some simple ways in which gardeners can maximize the positive effects their plants will have on one another. One of the easiest is to ensure that you have a wide variety of plants growing. In the kitchen garden, grow flowers and herbs among your vegetables to attract beneficial insects and repel pests and they will also help to support bees and encourage pollination. Big, open flowers rather than showy double varieties are better for attracting bees; umbellifers are good for attracting hoverflies, whose larvae feed on aphids.

There is some evidence that surrounding rows of carrots with strongly scented plants (onions are often used) prevents the carrot fly from sniffing their way to the carrots – but you need to have a high ratio of scented plants to carrots for there to be a noticeable effect.

The use of green manures can be seen as an example of companion planting, because you are making use of one plant to improve conditions for another. Green manures display many ‘good companion’ traits – they can improve the soil texture, add nutrients to the soil, prevent a build up of plant diseases and attract beneficial insects if allowed to flower.

Four examples of companion planting effects

# A good companion plant for attracting beneficial wildlife is Limnanthes douglasii, the poached egg plant, which is often recommended for planting around soft fruit.
# The Black walnut is a very bad companion, producing a chemical that inhibits the growth of other plants around it. The dense shade cast by the English walnut can have an equally negative effect.
# The Native American technique of planting corn, squash and beans together (known as the Three Sisters) uses corn to support climbing beans. The beans fix nitrogen in the soil for the benefit of all the crops, and the squash happily ramble across the ground in the shade of the sweetcorn.
# French marigolds can be used to keep whitefly from bothering your tomatoes, inside the greenhouse or outside. The effect only works when they’re in flower, so you need to keep up with dead heading.

This article first appeared in Country Gardener in May 2008. Your choice of green manure plants depends on the season – for more information read spring green manures and autumn green manures.