Companion planting is the practice of growing certain plants together because they benefit each other — either by deterring pests, attracting beneficial insects, improving soil, or simply using space more efficiently. In a raised bed where every centimetre counts, smart companion planting can noticeably increase yield and reduce pest pressure.
The Three Sisters
The most famous companion planting combination is the Three Sisters: corn, beans, and squash grown together. Corn provides a trellis for beans to climb. Beans fix atmospheric nitrogen into the soil, feeding the corn and squash. Squash leaves shade the soil, suppressing weeds and retaining moisture. The combination has been used by Indigenous North American farmers for thousands of years and remains one of the most productive combinations in a large raised bed.
Tomatoes and Basil
Basil grown near tomatoes is widely believed to improve tomato flavour and deter aphids. Whether the flavour claim has strong scientific backing is debated, but basil certainly attracts pollinators and repels thrips. Practically speaking, planting basil at the base of tomatoes makes good use of the shaded ground beneath the canopy where basil thrives in partial shade during hot summers.
Carrots and Onions
Carrots and onions are classic companions because each deters the other main pest. Carrot fly is repelled by the scent of onions; onion fly is repelled by the scent of carrots. Alternating rows of each doubles the repellent effect. Add chives or leeks to strengthen the allium side of the partnership.
Marigolds as a General Companion
French marigolds (Tagetes patula) are the most versatile companion plant in any raised bed. They produce a root exudate that suppresses soil nematodes, and their flowers attract hoverflies — whose larvae eat aphids. Plant marigolds at the corners and edges of beds growing tomatoes, peppers, and courgettes for broad-spectrum pest deterrence.
Plants to Keep Apart
Not all companions are beneficial. Fennel is allelopathic — it produces chemicals that inhibit the growth of most vegetables including tomatoes, peppers, and beans. Grow fennel in its own bed. Brassicas (cabbage, broccoli, kale) and strawberries compete and suppress each other. Potatoes and tomatoes share late blight disease and should be kept well separated.
FAQ
Does companion planting actually work?
The evidence is mixed. Some combinations (Three Sisters, marigolds for nematodes) have good scientific support. Others are largely anecdotal. What is clear is that diverse plantings generally support more beneficial insects and have fewer pest outbreaks than monocultures.
How close should companion plants be?
Close enough to interact but not so close that they compete for light and nutrients. As a rule of thumb, companion plants should be within 30-60cm of the plant they are meant to help.
Can I companion plant in a small raised bed?
Yes — in fact, small beds benefit most from companion planting because it maximises use of limited space. Edge plantings of marigolds or herbs do not significantly reduce the growing area for main crops.
What herbs deter pests?
Basil deters aphids and thrips; rosemary and sage deter bean beetles and cabbage moths; dill and fennel attract beneficial predatory insects; mint deters ants and aphids (but should be grown in a container as it spreads aggressively).
Should I plant companions at the same time as main crops?
It depends on growth rate. Fast-growing companions like basil and marigolds can be started at the same time as tomatoes and peppers. Slower companions like bulb onions should be started first so they are established before carrot seeds are sown nearby.
See also: Raised Bed Gardening for Beginners and Best Soil Mix for Raised Beds.