Companion Planting Finder

How companion planting works

Companion planting is the idea that some plants grow better together than apart — and worse together than apart. It is one of the oldest ideas in gardening, and it works through a handful of straightforward mechanisms. Some plants repel pests: the strong scent of basil, marigolds and onions confuses insects hunting by smell, which is why tomatoes ringed with basil suffer less damage. Some plants attract beneficial insects: flowers such as marigolds, calendula and borage bring in pollinators and the predators that eat aphids. Some plants feed their neighbours: beans and peas fix nitrogen from the air and leave some of it in the soil for the heavy feeders planted after them. And some plants simply use different space — a deep-rooted carrot under a shallow-rooted lettuce, or a tall corn plant shading a low squash — so the bed produces more from the same ground.

The pairings with the best support

The most reliable companions are the ones with a clear mechanism. Tomatoes and basil are the classic pair: basil's scent is said to repel tomato hornworms and whiteflies, and the two are harvested together in the kitchen as well as the garden. Marigolds are the workhorse of the vegetable garden — their roots discourage root-knot nematodes and their flowers attract hoverflies that eat aphids — which is why they appear as a companion for so many crops in our database. The “three sisters” — corn, beans and squash grown together — is a system that has fed people for centuries: corn gives beans a pole to climb, beans feed the soil with nitrogen, and squash shades the ground and suppresses weeds. Carrots and onions are another proven pair: onions mask the scent of carrots from carrot fly, and carrots do the same for onions against onion fly. These are the combinations worth building a bed around.

The pairings to avoid

The keep-away list matters as much as the companion list, and it is usually about shared problems rather than active harm. Plants in the same family share pests and diseases, so tomatoes, potatoes, peppers and eggplant — all nightshades — should not follow each other in the same bed, because blight and other soil-borne diseases build up. Beans and peas grow poorly near onions and garlic, because alliums can stunt legumes. Fennel is the great exception: it is allelopathic, meaning it releases compounds that inhibit most vegetables, so it is best grown on its own. Mint is not poisonous to its neighbours, but it spreads so aggressively by runners that it will take over a bed — grow it in a pot, sunk into the soil if you want it in the garden at all. The finder lists the conflicts that are widely reported in standard references; where it says there are none, that means none is widely reported, not that every pairing is proven safe.

How much should you believe?

It is worth being honest about the evidence. Companion planting is largely traditional knowledge, passed down by gardeners and recorded in books, and only some of it has been tested in formal trials. The pest-confusion and shared-disease effects are real and well understood. Some of the more romantic claims — that one plant “improves the flavour” of another, or that certain pairs are mysteriously incompatible — have little evidence behind them and may reflect other factors, such as competition for water or nutrients being mistaken for incompatibility. The practical approach is to treat companion planting as a useful planning framework, not a set of laws: use the strong pairings, respect the clear conflicts, mix families and forms in every bed, and judge by what happens in your own garden. A diverse bed with flowers, herbs and vegetables mixed together is more resilient than a monoculture whatever the specific pairings — and that, more than any single combination, is the real lesson of companion planting.

Related tools: lay out the bed with the Square-Foot Garden Planner, check spacing with the Plant Spacing Calculator, and plan what goes in each month with the Garden Planner. For per-crop growing advice, browse the growing guides hub.