Why Companion Planting Works

Companion planting is one of those old allotment tricks that turns out to have some genuine science behind it. The idea is simple: certain plants grown close together help each other out. Sometimes that means one plant's strong scent masks the smell of its neighbour, so pests struggle to find it. Sometimes it means a flowering herb pulls in hoverflies and parasitic wasps that munch their way through aphids. And sometimes it's purely practical — one crop shades another, or a quick-growing salad fills the gap while a slower crop is still finding its feet.

None of it is magic, and none of it will rescue a crop from poor soil or a slug plague. But used sensibly, companion planting can cut your pest problems noticeably and squeeze a great deal more out of a modest veg patch.

Carrots and Onions: The Classic Pairing

If you only try one combination this year, make it this one. Carrot fly finds your crop by smell, homing in on the foliage as it brushes against the soil. Onions, garlic, leeks and chives give off a pungent aroma that drowns out that signal — and in return, carrots help confuse onion fly. Grow them together and both crops tend to come through cleaner.

The practical way to do it is in alternating rows, roughly 15–20 cm apart, rather than in tight blocks. Sow your carrot seed thinly, then plant onion sets or leek seedlings either side. If you grow in raised beds, a band of onions down the middle with carrots on both flanks works just as well. Don't forget that alliums and carrots both dislike fresh manure, so prepare the bed in autumn with well-rotted compost instead.

A word of caution: keep the rows clearly labelled, because carrot foliage looks much like a dozen other seedlings and you'll want to know what's where when thinning.

Brassicas with Aromatic Herbs

Cabbages, kale, sprouts and broccoli are magnets for cabbage white butterflies, flea beetles and aphids. Surrounding them with strongly scented herbs — sage, rosemary, thyme, mint and dill — interferes with the butterflies' search pattern and hides the plants beneath. Purple and curly kale seem especially good at throwing pests off when interplanted this way.

Dill is particularly useful because it flowers profusely and attracts hoverflies and lacewings, whose larvae are voracious aphid predators. Let a few plants bolt rather than pinching them out. Rosemary and sage are best kept to the edges of the bed, where their woody roots won't compete too hard with shallow brassica roots.

• Sage and rosemary: plant at bed edges, about 45 cm from the crop.
• Thyme: a low carpet between rows, excellent on lighter soils.
• Mint: always in a sunken pot, or it will take over the whole bed.
• Dill: sow successionally every three weeks for a steady supply of flowers.

Nasturtiums deserve a mention too. They act as a sacrificial trap crop for blackfly and cabbage white caterpillars, luring pests away from your greens. Plant a few a metre or so from the brassica bed, not among them, or the caterpillars will simply move next door.

Making Better Use of Space

Companion planting isn't only about pests — it's also about getting two harvests from one square metre. Some of the most rewarding combinations are simply about timing and height.

Radishes and lettuce sown between slow-growing parsnips or Brussels sprouts will be harvested long before the main crop needs the room. Lettuce appreciates the light shade cast by taller plants in high summer, and its roots stay cooler and moister as a result. Leeks between rows of beetroot make good use of a bed that would otherwise sit half empty for months. Climbing beans up a wigwam with a ring of salad leaves beneath them is a classic use of vertical space.

The golden rule is to pair a shallow-rooted crop with a deep-rooted one, and a quick crop with a slow one. Then feed the whole bed well, because two crops draw more from the soil than one.

Herbs and Flowers Worth the Space

A few flowering plants earn their keep in any vegetable patch, and they'll bring in pollinators as a bonus.

• French marigolds: their roots release compounds that suppress soil nematodes, and they flower for months. Ideal around tomatoes and beans.
• Borage: a bee magnet, and traditionally grown near strawberries to improve flavour and vigour.
• Sweet alyssum: low, pretty and a favourite of hoverflies. Sow along bed edges.
• Chamomile: said to improve the flavour of nearby onions and cucumbers, and it attracts predatory insects.
• Basil: a natural partner for tomatoes, both in the bed and on the plate.

Pairings to Avoid

Not every neighbour is a good one. Alliums — onions, garlic, leeks and shallots — stunt peas and beans, so keep them well apart. Dill and fennel release compounds that can suppress carrot and tomato growth, so don't tuck them in among your roots. And keep vigorous herbs such as mint and oregano out of the main vegetable beds unless they're contained, or they'll outcompete everything around them.

Start with one or two combinations that suit your plot, keep notes on what worked, and build from there. A scrap of paper in the shed diary is worth more than any chart, because your soil, your shelter and your local pests are unique to your garden.