Why Companion Planting Earns Its Keep

There is no such thing as a pest-free kitchen garden, and anyone who tells you otherwise is probably selling something. What you can do, though, is make life considerably harder for the pests while making it considerably easier for the creatures that eat them. That is the whole logic of companion planting, and it is one of the few old-fashioned gardening ideas that stands up to a bit of scrutiny.

On a smallholding or a busy allotment, every square foot has to pull its weight. Companion planting lets you do three jobs at once: confuse the insects that home in on your crops by smell, offer food and shelter to hoverflies, lacewings and parasitic wasps, and fill the awkward gaps between slow-maturing brassicas with quick salads or herbs. Stack those functions together and you get a garden that mostly looks after itself.

Confuse the Pests With Scent and Clutter

Most of the troublesome insects in a UK kitchen garden find their host plants by following a scent trail, backed up by the shape and colour of the leaves. Carrot root fly, onion fly, cabbage root fly and flea beetle are all remarkably good at it — and remarkably bad at coping with a muddle.

The trick is to break up the signal. Instead of long, neat rows of carrots, sow them in blocks and run a row of spring onions, chives or leeks alongside. The onion scent masks the carrot foliage, and the carrot scent masks the onion, so neither fly quite knows where to land. The same principle works for brassicas: tuck strong-smelling herbs such as sage, rosemary, thyme or mint among your cabbages and kale and the cabbage white butterflies have to work much harder to find a suitable spot to lay.

It also helps to avoid bare soil. A crowded bed with flowers, herbs and foliage jumbled together is far less inviting to a pest than a clean expanse of one crop.

Flowers That Call in the Cavalry

The second half of the equation is attracting predators. Adult hoverflies, lacewings, ladybirds and the small parasitic wasps that lay their eggs inside aphids all feed on nectar and pollen as well as on pests. Give them a reason to stay and they will patrol your beds for you.

Flat, open flowers are the easiest for them to feed from, which is why the umbellifers are so valuable:

• Dill, fennel, coriander, chervil and parsley — let a few plants bolt and flower rather than pulling them up.
• Phacelia and buckwheat — cheap to sow, quick to flower, and magnets for hoverflies.
• Pot marigold (calendula), borage and poached egg plant — cheerful, hardy and endlessly useful.
• Sweet alyssum and yarrow — low-growing, so they fit neatly along bed edges.

Sow a small pinch of these every three weeks or so through the season and you will always have something in flower. A garden with a gap in its flowering is a garden where the predators drift off elsewhere.

Pairings That Genuinely Work

Some combinations have earned their reputation over generations of kitchen gardeners. These are the ones I would start with:

• Carrots with onions, leeks or spring onions — mutual scent camouflage against carrot root fly and onion fly.
• Brassicas with sage, rosemary, thyme or mint — masks the scent and confuses cabbage white butterflies.
• Tomatoes with basil, French marigolds and parsley — marigolds in particular seem to help with whitefly and soil-dwelling nematodes.
• Beans and peas with nasturtiums and borage — the nasturtiums act as a sacrifice, drawing blackfly away from the crop.
• Strawberries with borage — borage brings in pollinators and is said to improve the flavour of the fruit.
• Beetroot with onions or garlic — good use of space and a useful deterrent for several root pests.
• Sweetcorn with climbing beans and squash — the old three sisters arrangement, still a brilliant use of vertical space.

Living Barriers and Sacrificial Crops

Not every companion needs to be planted between the rows. Some are at their best in a defensive ring around a bed, or in a pot at the corner.

Nasturtiums are the classic trap crop. Sown near beans, courgettes or brassicas, they will attract blackfly and aphids in numbers, and as long as you leave the pests on the nasturtiums rather than panicking and spraying, your main crop stays clean. French marigolds are worth dotting through the greenhouse and among tomatoes, where they seem to unsettle whitefly. Chives and garlic planted under fruit trees and roses are a long-standing deterrent against aphids, and tansy has a reputation for discouraging ants and cabbage moth.

Plant them as a boundary, a border or a scatter of pots — the point is simply to put something unpalatable or distracting between the pest and the crop it is hunting for.

Making It Work in Your Own Beds

The temptation is to treat companion planting as a set of rigid rules, but it is really just good gardening. Keep the soil covered, let a few herbs flower, and never leave a bed empty when there is a quick crop or a pinch of seed you could tuck in.

Use the edges — the strip along a path or the foot of a fence is prime real estate for calendula, alyssum or a row of chives. Sow fast crops such as radish, rocket or lettuce between slower brassicas so the ground is earning while you wait. And keep a notebook, because what works in your soil and your particular corner of the country will be slightly different from what works in mine.

Watch, adjust, and let the garden do more of the work. Once the hoverflies and lacewings have moved in for good, you will wonder how you ever managed without them.