Companion planting

Companion planting is the practice of growing particular plants together because one is thought to benefit the other, by repelling pests, attracting useful insects, improving the soil or giving shelter. It is an old idea with a mixed record: a few combinations are supported by research, and many more are repeated from book to book with little evidence behind them.

How it might work

• Confusing pests: many insects find their host plant by sight and smell. Research at Warwick in England showed that cabbage root fly and other specialists lay fewer eggs on a crop surrounded by other green plants than on one standing in bare soil, because they land repeatedly on the wrong leaves and give up.
• Attracting predators and pollinators: open flowers supply nectar and pollen to hoverflies, lacewings and parasitic wasps, whose larvae eat aphids, and to the bees that pollinate beans, courgettes and fruit.
• Trap crops: a plant more attractive to the pest draws it away from the crop.
• Physical help: tall plants give shade, shelter or support to shorter or climbing ones.
• Soil: peas, beans and clover host bacteria that fix nitrogen from the air, some of which becomes available to later crops when the roots are left in the ground.

Combinations with some evidence

French marigolds with greenhouse tomatoes. A study at Newcastle University published in 2019 found that French marigolds planted among tomatoes from the start reduced whitefly, and identified limonene, a chemical released by the marigolds, as the repellent. Planting marigolds after whitefly had arrived worked much less well.

Undersowing brassicas. Cabbages and their relatives grown through a low carpet of clover or other plants suffer less from cabbage root fly and some other pests, in line with the Warwick findings. The cover competes with the crop for water and nutrients, so yields can fall unless it is kept trimmed.

Flowers among vegetables. Strips or clumps of pot marigold, poached egg plant, phacelia, cornflower and sweet alyssum increase the number of hoverflies and other aphid predators nearby. This is the most dependable form of companion planting, and it is used on a field scale on some farms.

Sweetcorn, beans and squash. The "three sisters" system of Native American farming grows climbing beans up maize stems with squash covering the ground beneath. In a British summer the maize is often too slow to support vigorous runner beans, and the combination works best with sweetcorn started early under cover and less rampant French beans.

Combinations with weaker support

Onions and carrots. The scent of onions is said to hide carrots from carrot fly. Trials suggest some effect, but only with several rows of onions to each row of carrots and only while the onions are in active leaf growth. Fine mesh is far more reliable.

Nasturtiums as a trap crop. Nasturtiums attract blackfly and the caterpillars of cabbage white butterflies. Whether that spares nearby beans and brassicas or simply raises pest numbers in the area is not settled.

Basil with tomatoes, garlic with roses. Both are widely recommended; neither has consistent support from trials. The plants grow well together because they like similar conditions.

Plants that hinder

Some pairings are advised against. Fennel is reputed to suppress its neighbours. Walnut trees release juglone, a substance that harms tomatoes, potatoes and apples growing within reach of their roots. Most other warnings about bad companions come down to competition: any vigorous plant set too close to a slow one takes its light and water.

In practice

Companion planting does not replace barriers, crop rotation or resistant varieties, but mixing flowers and herbs into a vegetable plot is easy and has clear benefits for pollinators. The timing matters: companions need to be in flower or in full leaf when the pest arrives, which usually means sowing them at the same time as the crop or earlier.