Which companion plants actually help, and which pairs to keep apart

The pairings with the best reasons behind them are French marigolds near tomatoes, nasturtiums as a decoy for cabbage pests, and sweetcorn, climbing beans and squash grown together. The pairs most often kept apart are onions and their relatives near peas and beans, and potatoes near tomatoes.

Beyond those, companion planting is a mix of sound ideas and old garden lore. The chart below is a fair starting point. Just don’t expect it to fix problems on its own.

Companion planting chart

These are the good neighbours and pairs to keep apart from VegLog’s crop library, which draws on sources such as the RHS and university extension services.

Crop	Good neighbours	Keep apart from
Tomato	Basil, French marigold, carrot, garlic, parsley	Potato, sweetcorn
Potato	Broad bean, French bean, French marigold	Tomato, squash, cucumber, courgette
Carrot	Onion, leek, chives, pea, tomato	Dill, parsnip
Onion	Carrot, beetroot, lettuce, strawberry, parsnip	Broad bean, pea, runner bean, French bean
Leek	Carrot, celeriac	Broad bean, pea, runner bean, French bean
Garlic	Tomato, carrot, beetroot, strawberry, rhubarb	Broad bean, pea, runner bean, French bean
Broad bean	Potato, carrot, lettuce, spinach	Garlic, onion, shallot, leek
Pea	Carrot, radish, turnip, cucumber, spinach	Garlic, onion, shallot, leek
Runner bean	Sweetcorn, squash, nasturtium	Garlic, onion, shallot, leek
French bean	Sweetcorn, cucumber, potato, aubergine, celeriac	Garlic, onion, shallot, leek
Cabbage	Nasturtium, celeriac, dill, mint, beetroot	Strawberry
Lettuce	Carrot, radish, beetroot, strawberry, onion	None listed
Courgette	Nasturtium, sweetcorn, French marigold	Potato
Cucumber	Dill, nasturtium, radish, pea, French bean	Potato
Sweetcorn	Runner bean, French bean, squash, courgette, melon	Tomato
Beetroot	Onion, garlic, lettuce, cabbage, kale	None listed

Most of these pairings are traditional. Some have a clear mechanism, a few have been tested, and plenty simply describe crops that are happy in the same conditions. The sections below sort out which is which.

Pairings with a good reason behind them

French marigolds and tomatoes. French marigolds (Tagetes patula) are dotted among tomatoes, especially in greenhouses, because their scent is said to put off whitefly. Their roots also release compounds that can reduce some root nematodes, though that effect needs a dense planting grown for a whole season, not a few plants at the end of a row. They’re easy, cheap and bring in hoverflies, so there’s little to lose.

Nasturtiums as a sacrificial crop. Nasturtiums draw blackfly and cabbage white butterflies, which lay their eggs on them. Planted a short way from your brassicas or beans, they act as a decoy: you check them, pick off or squash what’s there, and fewer pests end up on the crop. Planted right next to the crop, they can work the other way and bring pests closer.

Sweetcorn, climbing beans and squash. This is the three sisters method from Indigenous North America. The corn gives the beans something to climb, the beans add some nitrogen to the soil, and the squash leaves cover the ground and smother weeds. In a cool, short summer it needs care: sow the corn first and let it get a head start, or vigorous runner beans can pull it over. Grow sweetcorn in a block rather than a single row, because it’s wind-pollinated.

Carrots with onions or leeks. The idea is that the strong smell of alliums hides carrots from carrot fly, which finds its target by scent. It’s widely repeated, but the effect seems to depend on having plenty of onions to each carrot, and it fades once the onions stop growing leaf. Fine mesh over the carrots is far more reliable. Grow them together if you like, but use mesh too.

Flowers for predators. Dill, French marigolds, calendula and other open flowers feed hoverflies, lacewings and parasitic wasps, whose young eat aphids and caterpillars. This is one of the better-supported ideas in companion planting, and it works best with flowers spread through the plot, not in one corner.

Pairs to keep apart

Potatoes and tomatoes. This one is solid. They’re the same family and both catch blight, which spreads from one to the other. They also share soil pests, so keep them apart in space and in your rotation.

Onions, garlic and leeks near peas and beans. Almost every companion chart lists this, and the crop library does too. The usual claim is that alliums hold back the growth of legumes. The evidence is thin, but since the two belong to different rotation groups, you’d usually grow them in separate beds anyway.

Carrots with dill or parsnips. All three are in the same family. Parsnips are hosts for carrot fly too, so growing them side by side gives the fly a bigger target.

Strawberries and brassicas. Strawberries and cabbages are a traditional bad pairing. The reasons given are vague, but strawberries are a permanent bed and brassicas move round the rotation, so they rarely end up together.

Tomatoes and sweetcorn. In North America the two share a caterpillar pest that feeds on both corn cobs and tomato fruit. In the UK that pest isn’t a common garden problem, and the main issue is that tall corn shades sun-loving tomatoes.

How much to rely on it

Treat companion planting as a small extra, not a pest control plan. Some honest rules of thumb:

• Spacing comes first. A companion that crowds or shades your crop does more harm than any benefit it brings. Basil under tomatoes is fine. Squash sprawling over lettuce is not.
• Barriers beat scent. Mesh, fleece and netting stop carrot fly, cabbage whites and pigeons far more reliably than any neighbouring plant.
• Diversity helps. A plot with a mix of crops and flowers usually has more predators and fewer big pest outbreaks than long rows of one thing.
• Flavour claims are mostly folklore. There’s little evidence that basil makes tomatoes taste better, though they’re good together on the plate.