9 Companion Plants the RHS Pairs with Brassica Crops

Cabbage whites set the early pattern

Large and small cabbage white butterflies, Pieris brassicae and Pieris rapae, shape much of the companion planting around brassicas. From June to September their larvae can strip leaves from cabbages, kale, broccoli and Brussels sprouts. Female butterflies locate host plants partly by sight and partly through volatile glucosinolate cues released by brassica foliage.

Nasturtium, Tropaeolum majus, uses that behaviour against the pest. Cabbage whites readily lay eggs on it, which makes it useful as a trap crop beside brassicas. Egg clusters and caterpillars can then be removed from the nasturtium leaves before the larvae shift onto the cabbages. The placement matters less than inspection access: a trap crop that is allowed to fill with caterpillars becomes a nursery beside the crop it was meant to protect.

Thyme and sage aim at the same butterfly problem through scent masking. A close block or dense collar of either herb raises the level of aromatic volatiles around the crop canopy, so the brassica signal is harder for butterflies to pick out. The effect remains partial. With heavy egg laying, cabbage whites can still find cabbages, so sparse herbs at a distance add little.

Trials of intercropping with aromatic herbs have given inconsistent reductions in caterpillar damage. Results shift with herb density, garden size and butterfly population. A thyme border on its own, with no netting, is still likely to leave caterpillars in cabbages by August.

Roots, rhizomes and the soil around young plants

French marigold, Tagetes patula, belongs in the ground before it belongs in the brassica plan. Its roots release alpha-terthienyl and related thiophenes that suppress some plant-parasitic nematodes in the nearby soil. The effect is tied to the marigold root zone, so it helps most where marigolds have already occupied soil that brassica roots will later explore.

That pushes French marigold into rotation planning. In a common four-bed rotation, brassicas follow legumes, which leave residual nitrogen in the soil from their root nodules. Tagetes patula can be slotted into the preceding season or used in gaps before the brassicas arrive, giving the thiophene effect time to build in that ground.

Mint, Mentha species, is aimed at cabbage root fly, Delia radicum. Its volatile menthol load is associated with deterring the adult fly whose larvae tunnel into the root collar of young transplants. Mint spreads strongly by rhizome, so a sunken container or a confined strip keeps it from taking over open bed space. In a 3-metre bed, a 20 cm plastic pot sunk to its rim at the most exposed corner keeps the rhizomes contained while placing menthol volatiles near the transplant collars.

Clubroot needs separate handling. It is caused by the soil organism Plasmodiophora brassicae, and these companion plants do not change its course; the controls are a crop rotation of four years or more, raising soil pH toward 7.0 with lime, and improving drainage.

Dill, borage and calendula earn space by feeding predators

Dill, Anethum graveolens, carries flat umbel flowers with nectar that is easy for parasitic wasps and hoverflies to reach. The aphid control comes later through hoverfly larvae, which feed on cabbage aphid colonies. Dill grown only for leaves and clipped before flowering never supplies that predator food.

Borage, Borago officinalis, produces nectar heavily through summer. It draws in hoverflies and bumblebees, and the hoverfly presence helps build the local supply of larvae that feed on aphids. Calendula, Calendula officinalis, adds another route because its sticky stems and flowers trap aphids directly while also attracting hoverflies.

These flowers work best scattered through and around the brassica area, close enough for wasps and hoverflies to forage near aphid colonies. Concentrating every dill, borage and calendula plant in one corner leaves nectar in the plot, but places less of it beside the leaves where aphids settle.

Take a bed three metres long and one metre wide, planted with summer cabbages at 45 cm spacing. That gives roughly six cabbage plants down the centre line. The companion layer has to share the same footprint without taking the light, nitrogen and root room needed for head formation.

One workable arrangement puts a nasturtium at each end of the bed. Those end positions are easy to check during watering, so egg clusters can be picked off before caterpillars spread. Between the cabbages, three calendula and two dill plants can alternate down the row, using gaps the cabbage roots have not yet filled.

Dill kept for predator support is allowed to flower and may reach 60 to 90 cm. In a temperate-latitude garden it fits best on the north side of the bed, where its height casts less shade across the crop. With two nasturtiums, three calendula, two dill and one potted mint, the companion count comes to about eight plants, enough to alter pest behaviour while cabbages remain the main crop.

Cabbages are heavy feeders. Dense companion planting can draw nitrogen below the level needed during head formation. A spring topdressing of garden compost before transplanting helps cover the early demand, and comfrey liquid feed during the heading weeks tops up potassium and nitrogen without disturbing the companion layer. Comfrey leaves steeped in water for four to six weeks produce a feed diluted at roughly one part liquor to ten parts water.

Dill, borage and calendula are annuals, yet they often behave like semi-permanent residents because they self-seed freely. Once established, they may appear without deliberate sowing. That helps predator support when early flowers feed insects before aphid colonies build, though volunteers can crowd young brassica transplants if left thick.

When mesh takes over

Fine insect mesh is the strongest physical defence when butterfly numbers are high. Mesh over hoops, sealed at the soil edges, stops cabbage whites laying eggs and also blocks cabbage root fly and pigeons. Companion plants lower pressure and support predators that deal with pests that slip through the barrier.

Nasturtium gives visible evidence because egg clusters and caterpillars can be counted on the leaves. Dill, borage and calendula can also be checked, since hoverfly larvae can be seen feeding on aphid colonies. Scent masking by thyme and sage is harder to judge from a single bed.

Perennial edges and annual volunteers

Mint, thyme and sage fit awkwardly into an annual vegetable rotation because they are perennial. A permanent herb edge or a row of containers along one side of the vegetable plot lets the brassica bed rotate past them across years. The menthol and aromatic volatiles stay near the growing area without the herbs being lifted and replanted each season.

French marigold is different because its useful work is tied to the soil it has occupied. A bed that will receive brassicas later needs the marigold roots in place beforehand, especially where the aim is nematode suppression rather than a flower strip at the edge.

In dense mixed planting, some reduced pest pressure may also come from breaking up the block of brassica leaves that cabbage whites are scanning. That sits alongside the chemistry of nasturtium, thyme and sage and the nectar value of dill, borage and calendula. The practical reading still comes from the leaves: eggs on nasturtium, aphids on calendula stems, hoverfly larvae in aphid colonies, and chewed brassica foliage where caterpillars have already crossed over.

By late summer, self-sown dill and calendula can leave the brassica row with both predator flowers and crowded transplants. The unresolved patch is the shaded strip under flowering dill, where hoverfly food and cabbage leaf space compete in the same few centimetres of soil.