These are some suggestions for plants that you can grow in close proximity to one another and which have properties that can help each other’s development. Companion planting is by no means an guarantee that your crops won’t suffer from pests, but is a useful tool in permaculture, helping to achieve an edible garden in which the plants help themselves, and the gardener inputs as little labour as possible.
Marigolds are considered the ultimate companion plant, excreting allelochemicals which repel several pests. These chemicals also inhibit seed germination, which is very useful in weed suppression. Their bright colours will attract pollinators to the site.
Nasturtiums are particularly useful when planted with potatoes or tomatoes. They serve as a sacrificial crop, luring aphids away from your crops.
Chamomile, Dill, Mint, Rosemary and Sage are thought to repel the cabbage white butterfly, so plant in abundance around Brassicas (cauliflower, cabbage, broccoli etc). Plant Brassicas withpeas, potatoes and onions, but not with lettuce, which will compete for nutrients (both rely on large quantities of nitrogen for leafy growth).
Keep potatoes and tomatoes away from Brassicas and fennel, and from each other. Plant them with marigolds, nasturtiums,basil, asparagus, peas, tarragon and thyme.
Plant Legumes (peas and beans) with carrots,spinach, beetroot and brassicas (they will fix nitrogen in the soil for use by the brassicas).
Alliums (onions and garlic) have a strong smell which can mask the smell of other crops from pests – plant them near carrots to ward off carrotfly – and plant lettuce around them for (some) protection from slugs. Growing lettuce around onions also means that the lettuce will take up excess nitrogen, ensuring better bulb development. For the same reason, avoid planting beans close to alliums as the beans will fix nitrogen, which is exactly what you don’t want for your onions.
Alliums accumulate sulphur, producing a fungicidal effect which is useful to prevent fungal attack to your crops (strawberries, for example, are highly susceptible to moulds). You can also grow them at the base of fruit trees (plums and cherries also suffer from fungal attack).
Tomatoes should be kept away from fennel and all brassicas. Plant them instead with basil, parsley, alliums, marigolds, lettuce and nasturtiums. Asparagus roots excretes a chemical which kills Trichodorus, a nematode which attacks tomatoes, so these two make a very good pair.
Potatoes are protected from eelworm by marigolds, from aphid attack by nasturtium, and from Rhizocotonia infections by onions. Avoid planting them with sunflowers, as both have a high nutrient requirement and they will compete.
Legumes, as discussed, fix nitrogen which leafy vegetables and brassicas rely on, so they make good companions. Legumes can be planted with most vegetables, with the exception of alliums.
Rosemary, thyme, and sage, I’ve found, go some way in repelling slugs, so plant around seedlings, lettuce and spinach.
Planting in guilds makes use of the properties of your plants, creating beneficial relationships which is an integral part of a permaculture approach to gardening. They are also a good way way of maximising limited space. The most well known is the Three Sisters, used by American Indians. Corn serves as a structure for runner beans to climb up. The runner beans nitrogenate the soil for the courgettes, which in turn provide shade which suppresses the weeds that would otherwise be competing with all three crops. Another guild uses sunflowers as a support for trailing courgettes, with beetroot and lettuce planted at the base (the lettuce would need to be harvested before the courgette leaves became too large though!)