What Are the Best Companion Plants for Brassicas?

Brassicas — cabbage, kale, broccoli, cauliflower, Brussels sprouts, and their relatives — are among the most pest-prone crops in a kitchen garden. Cabbage white caterpillars, cabbage aphids, and club root disease can devastate a whole bed. Companion planting cannot replace physical protection like fine mesh netting, but it can reduce pest pressure, attract the insects that control caterpillars, and improve soil health around these heavy-feeding crops.

Dill and Fennel to Attract Parasitic Wasps

Parasitic wasps are one of the most effective natural controls for caterpillars. Tiny braconid wasps lay their eggs inside caterpillars, killing them from the inside. These wasps need nectar to fuel the adults, and they are strongly attracted to umbelliferous flowers — the flat-topped flower heads of dill, fennel, and their relatives. Planting dill in blocks at the ends of your brassica beds significantly increases parasitic wasp populations in the immediate area. Do not plant fennel too close to other vegetables (it is allelopathic), but dill is safe to interplant directly in the brassica bed.

Nasturtiums as a Trap Crop

Nasturtiums are magnets for cabbage aphids and black aphids. Rather than repelling pests, nasturtiums work as a sacrificial trap crop — the aphids colonise the nasturtium plants and concentrate there rather than spreading throughout your brassicas. Once the nasturtiums are heavily infested you can pull them out and compost them or hose them off. Plant nasturtiums around the perimeter of the brassica bed rather than in the centre, so they catch incoming aphids before they reach your crops.

Alliums for General Deterrence

Planting garlic, onions, or chives among brassicas provides a general aromatic deterrence that appears to reduce the egg-laying behaviour of several brassica pests. The strong sulphur compounds in allium foliage interfere with the scent signals that cabbage white butterflies use to identify host plants. For this to have any effect the alliums need to be present during the main butterfly flight period — mid-spring through to early autumn in most climates. Garlic is particularly useful as it can be planted in autumn and will be established when the first butterflies appear.

Clover as a Living Mulch

White clover sown between brassica transplants serves several purposes. It fixes nitrogen from the air into the soil via root nodules, which directly benefits the nitrogen-hungry brassicas growing alongside it. It keeps the soil moist and suppresses weeds. And its flowers attract hoverflies, whose larvae eat aphids. Mow or cut it back if it begins to compete with young transplants, but once brassicas are established, clover makes an excellent living mulch beneath them.

What to Avoid Near Brassicas

Keep strawberries, tomatoes, and climbing beans away from brassicas. These crops all have different nutrient needs and timing, and they tend to create environments that benefit different pests. Onions make good neighbours when interplanted, but a solid block of onions next to a solid block of brassicas is less effective than mixing them together. Never plant brassicas in the same bed as other brassicas year after year — crop rotation is more important than any companion for controlling club root.