Why Companion Planting Earns Its Place

Companion planting has a reputation for being a bit folkloric, and it is true that not every old pairing stands up to scrutiny. But a handful of combinations work for solid, sensible reasons: strong-smelling plants mask the scent of their neighbours, flowering plants feed the predators that eat your pests, and a sacrificial crop can lure caterpillars away from the plants you actually want to eat. On a British allotment or a small back garden, that translates into less damage and fewer evening patrols with a torch.

The key is to treat companion planting as one tool among several, not a magic wand. Good soil, sensible rotation, netting where it matters and steady watering will always do more for your crops than any clever pairing. Used alongside those basics, though, the right neighbours can genuinely tip the balance in your favour.

Carrots and Onions: The Classic Partnership

This is the pairing that convinces most sceptics. Carrot root fly finds your carrots by smell, homing in on the foliage as it brushes against neighbouring plants. Interplant with onions, leeks, spring onions or garlic and that scent trail becomes much harder to follow. The onion fly, meanwhile, is reportedly put off by the smell of carrots, so both crops gain something.

• Sow alternate rows of carrots and onions about 25–30cm apart, or sow a thin line of spring onions directly between carrot drills.
• Spring onions and salad onions are ideal because they mature quickly and leave room for a later carrot sowing.
• Leeks work just as well and can follow on after early potatoes, giving you a second half of the season pairing.
• Keep the rows well thinned — carrot root fly is drawn to the bruised foliage you create when thinning, so do it in the evening and water afterwards.

Choose a reliable carrot such as 'Nantes' or 'Flyaway' and a hardy onion set, and you will have a bed that looks productive and confuses pests at the same time.

Nasturtiums: A Sacrificial Crop for Brassicas

If you grow cabbages, kale, sprouts or broccoli, you will know the frustration of cabbage white butterflies. Nasturtiums (Tropaeolum majus) are irresistible to them. Plant a generous row or two around the edge of your brassica bed and the butterflies will lay their eggs there instead, leaving your greens relatively untouched. The caterpillars strip the nasturtiums, which is exactly the point — treat them as a sacrifice, not a specimen.

Nasturtiums do a second job too. Their flowers are magnets for hoverflies, whose larvae munch through aphids, and for bees, which will pollinate your beans, courgettes and fruit later in the season. Blackfly often colonise the nasturtium stems before they find your broad beans.

• Sow seed direct in late May, about 60–90cm from the brassica rows, in full sun and poor-ish soil — rich ground gives you leaves at the expense of flowers.
• Trailing varieties such as 'Empress of India' or 'Alaska' work well at bed edges and in pots beside raised beds.
• Both leaves and flowers are edible, with a peppery kick that lifts a salad.
• Don't pull them out when they look tatty in late summer; the seed heads can be pickled as a caper substitute.

More Pairings Worth Trying

Once you start thinking in combinations, a few other reliable partnerships are worth working into the rotation.

• French marigolds with tomatoes — in greenhouse borders especially, the roots release compounds that suppress root-knot nematodes, and the flowers bring in pollinators.
• Basil among tomatoes — the flavour claims are arguable, but basil provides useful ground cover and its flowers feed bees.
• Borage with strawberries and courgettes — a bee magnet, and the young leaves taste faintly of cucumber.
• Lettuce between slower crops — quick to mature, it shades the soil and suppresses weeds while parsnips or brassicas get going.
• Peas and beans before leafy crops — legumes leave nitrogen behind, so follow them with cabbages or spinach.

One caution: keep fennel well away from most vegetables. It releases compounds that inhibit its neighbours, so give it a corner of its own. Alliums, similarly, are said to hamper beans and peas, so keep your onion and carrot bed separate from the legume patch.

Getting the Layout and Timing Right

Companion planting works best when you plan the season rather than the moment. Sketch your beds in winter and decide which pairings go where, then build in successional sowings so there is always something flowering. A few nasturtiums left to sprawl, a row of marigolds at the end of the greenhouse, chives in a pot by the door — these small habitats matter more than any single clever combination.

Spacing is important. Crowding plants to fit more companions reduces airflow and invites mildew, which is a real risk in a damp British summer. Leave proper paths, keep tall crops to the north side so they don't shade the bed, and water in the morning where you can.

Keeping Your Expectations Grounded

You will still lose some carrots to root fly and some kale to pigeons. Companion planting reduces pressure rather than eliminating it, and its effects are strongest in a garden that is already well managed. Rotate your crops, add organic matter each autumn, net your brassicas against pigeons and butterflies, and use these pairings as an extra layer of defence.

Start with one combination this season — carrots and onions is the easiest to judge, because you can compare a treated row with a plain one. Once you see the difference, you will find yourself planning the whole plot as a set of partnerships rather than a list of separate crops.