Emily Hartley
How companion planting actually confuses pests
Most garden pests hunt by smell. Carrot fly finds a row of carrots by the scent of the foliage, cabbage root fly homes in on the mustard oils released by brassicas, and aphids simply drop onto the first soft shoot they find. Companion planting works largely by breaking that scent trail. Mix strong-smelling plants among your crops and the pest has to work much harder to locate its host — and often gives up and moves on.
There is a second job, and it is just as important. Many flowers and herbs feed the predators that do the real pest control: hoverflies, ladybirds, lacewings, parasitic wasps and ground beetles. Adult hoverflies need nectar and pollen before they can lay eggs, and their larvae then eat aphids by the hundred. A bed of nothing but cabbages gives them nowhere to live.
None of this is magic. Treat companion planting as one layer of defence, alongside rotation, netting, healthy soil and a bit of hand-picking on a warm evening.
The classic pairings that really earn their keep
- Carrots and onions (or leeks). The traditional pairing, and still the best. The alliums mask the scent of carrot foliage, while the carrots mask the onion fly. Sow in alternate rows about 20cm apart, or dot spring onions through a carrot row.
- Brassicas and strong-smelling herbs. Dill, sage, rosemary and thyme around cabbages and kale make life harder for cabbage white butterflies and cabbage root fly. Keep the herbs close to the base of the plants, where root fly lays its eggs.
- Radish with brassicas and lettuce. Flea beetle is the worst pest of young brassicas in Britain. A quick row of radish nearby draws them off — and if they riddle the radish leaves instead, you have lost nothing but a few pence of seed.
- Tomatoes and French marigolds. Marigolds around tomatoes look cheerful, and their roots release compounds that suppress root-knot nematodes in the soil. Whitefly often seems less troublesome too.
- Climbing beans and sweetcorn. Beans fix nitrogen, and sweetcorn is a tall, hungry crop. Add squash at ground level and you have the Three Sisters — though in a wet British summer, give the squash the sunniest corner and plenty of room.
Flowers that pull their weight
If you plant only one thing for pest control this year, make it a flower. French marigolds are the hardest working: compact, cheap, and useful both in the greenhouse and among tomatoes. Do not confuse them with pot marigolds (calendula), which are a different plant doing a different job — they are superb for attracting hoverflies and bees, and happily soak up aphids that would otherwise find your beans.
Nasturtiums are the great sacrificial crop. Blackfly adore them, and cabbage white butterflies will often lay on them in preference to your kale. Plant them near brassicas and under fruit trees, and accept that they will look tatty by August. The flowers are edible, which is a bonus.
Sweet alyssum is underrated. Sown as a low edging along a path, it flowers for months and draws in parasitic wasps and hoverflies in numbers. Borage is the one for strawberries and tomatoes; folk tradition claims it improves their flavour and vigour, and the bees certainly agree. Dill and coriander, left to flower, are magnets for beneficial insects — but keep dill away from fennel, as the two cross-pollinate freely.
Getting the layout right
Do not scatter companion plants at random. Think about height, scent and timing. Tall plants such as sweetcorn, fennel and sunflowers belong on the northern side of a bed so they do not shade shorter crops.
Respect the plants that dislike company. Fennel is allelopathic — it suppresses its neighbours — so give it a corner of its own. Alliums and legumes are uneasy bedfellows: onions and garlic can hold back beans and peas, so keep them apart. Beans, on the other hand, get on well with carrots, cucumbers and sweetcorn.
Use your edges. A 30cm strip of alyssum, calendula and dill along a path costs almost nothing and shelters a huge number of predators. Intersow quick crops — radish, lettuce, spinach — between slower ones, so the ground is never bare and pests never find a solid block of their favourite food.
Where companion planting will not save you
Be honest about the limits. Slugs ignore every clever pairing ever devised; deal with them by encouraging frogs and hedgehogs, watering in nematodes in spring, and keeping mulch away from tender seedlings. Cabbage white butterflies will find your sprouts regardless, so net them from May to September over a sturdy frame.
Carrot fly is another one where a physical barrier beats a companion plant. The fly flies low to the ground, so a fleece or mesh barrier at least 60cm high around the bed is more reliable than onions alone. Aphids, though, are where companion planting genuinely shines — flowers, herbs and a little patience will bring in enough predators to keep them in check.
A simple plan for a British plot
- Keep a permanent strip of flowers — calendula, nasturtium, alyssum, dill — along one edge of the plot.
- Alternate rows of carrots and onions, roughly 20cm apart.
- Ring brassicas with radish and dill, and net them anyway.
- Dot French marigolds through the greenhouse and around the tomatoes.
- Confine fennel to the herb bed, well away from dill, beans and peas.
- Write down what worked and what did not. Every plot differs — clay or sand, sheltered or windswept — and the best combinations are the ones you discover in your own soil.
Start with two or three pairings this season. Watch closely, adjust next year, and you will build a garden that works with its insects rather than against them.
