The principle of companion planting works both above and below ground. Above, bright flowers can attract beneficial insects and pollinators or repel insect pests from vegetable crops, while fragrant foliage can repel insects. Below the soil surface, the range of different root systems that exist in polycultural environments can also be beneficial, with some plant roots spreading wide to improve soil structure and others reaching deep to bring nutrients to the surface. Some crops, especially legumes such as peas and beans, ‘fix’ nitrogen from the air and draw it into the soil to make it available to other plants. And finally, the risk of soil-borne disease is reduced if you have more than one type of plant in a bed.
There are endless recommendations for companion planting combinations, most of which have not been scientifically tested, but here are the ones that work best.
Marigolds and leafy greens
Growing marigolds among vegetables like kale, calabrese, and lettuce will attract beneficial insects like hoverflies, ladybirds, and lacewings, all of which are natural predators of aphids. Hoverflies are attracted to the color orange, so you can also try California poppies, tagets, or poached egg plants.
Nasturtiums and Brassicas
Like marigolds, nasturtiums are very easy to grow from seed and will spread widely to cover any open ground. These plants are full of mustard oil that will attract a variety of insects, including cabbage-eating caterpillars, so they can be used as sacrificial plants for your more valuable crops of broccoli, cabbage or kale.
Hyssop and Brassicas
A member of the mint family, hyssop attracts cabbage white butterflies away from brassicas. Its dark blue flowers are also a magnet for bees.
Basil and tomatoes
These two plants work in the garden as well as on the plate. Growing pots of basil in the greenhouse will attract whiteflies away from tomatoes, and its fragrant leaves will repel other pests. They need similar growing conditions, with plenty of heat and water.
Tagetes and potatoes
Tagetes roots release a chemical that is resistant to soil pests like eelworms, so it works well with potatoes. It can have similar effects with weeds such as bindweed and ground alder.
Lavender and leeks
Lavender or oregano can be used as companion plants for leeks and carrots because the fragrant leaves can deter aphids and other pests such as allium leaf miners.
Borage and broad beans
Broad beans can often be plagued by blackflies, but borage’s beautiful blue flowers will attract pollinating insects, including hoverflies, whose larvae will feed on blackflies.
Dwarf French Beans and Kale
The kale will benefit from the nitrogen that the beans pull into the soil, and in turn the kale will give strong support to the beans with their more spreading habit.
Thyme and rose
Thyme’s strong scent can repel blackflies, making it a good companion for roses. A tea made by steeping thyme leaves in water can be used to spray cabbage against whiteflies.
