Brassicas are among the most satisfying crops to grow in a UK kitchen garden, but they attract a dedicated cast of pests. When leaves begin to curl, distort, or pucker, the plant is signalling that something is feeding on it. Catching the problem early makes all the difference between a minor setback and a ruined crop.
What causes brassica leaves to curl?
In the UK, the two overwhelmingly common causes of curled brassica leaves are cabbage aphids (Brevicoryne brassicae) and cabbage white caterpillars (Pieris brassicae). Both pests peak in the warmer months from May through September, which is exactly when your brassicas are doing most of their growing. Other factors, including nutrient deficiencies, fungal diseases, and environmental stress, can produce similar symptoms but are far less frequent. Starting your diagnosis with these two pests will solve the problem the vast majority of the time.
1. Aphids (cabbage aphid, Brevicoryne brassicae)
Cabbage aphids are small, soft-bodied insects roughly 2 mm long, covered in a distinctive waxy, grey-green powder that makes them look almost frosted. They colonise the undersides of leaves and cluster along stems, feeding on the sap by piercing the plant tissue. As they feed, they inject saliva that triggers the leaf to curl inward around the colony, creating a protective tent the aphids actively exploit. Large infestations can turn leaves pale yellow, stunt new growth, and coat everything in sticky honeydew that encourages black sooty mould. Cabbage aphids overwinter as eggs on old brassica stems, so clearing spent plants promptly at the end of the season reduces the following year's pressure.
How to fix it
Start by squashing accessible clusters by hand and cutting away the most tightly curled leaves, which tend to harbour the densest populations. Follow with a firm jet of water from a hose, directing the spray at leaf undersides to dislodge aphids mechanically. Repeat every two or three days. For persistent infestations, spray with insecticidal soap (SB Plant Invigorator is widely available in UK garden centres) or a dilute solution of washing-up liquid and water, coating undersides thoroughly. Planting poached-egg plant (Limnanthes douglasii) or phacelia nearby attracts hoverflies and parasitic wasps, which are highly effective long-term controls. Avoid broad-spectrum pesticides, which wipe out the predators that would otherwise do the work for you.
2. Cabbage white caterpillars (Pieris brassicae)
The large cabbage white butterfly lays clusters of pale yellow, ribbed eggs on the undersides of brassica leaves from May onwards, with a second generation appearing in late summer. When the eggs hatch, the young caterpillars initially feed in groups, skeletonising the leaf surface and leaving a papery, ragged appearance. As they grow, they disperse and begin boring into the heart of the plant. Mature caterpillars are up to 45 mm long, yellowish-green with black spots and a pale stripe along each side. Heavy infestations can strip a plant to the midribs within days. The leaf curling or folding associated with caterpillars tends to be less tight than the aphid-driven curl; you may find a caterpillar sheltering inside a loosely folded leaf rather than a dense grey colony.
How to fix it
Check the undersides of leaves weekly from mid-May and remove egg clusters immediately by rubbing them off with your fingers. Hand-picking caterpillars and dropping them into a bucket of soapy water is highly effective for small plots. For larger beds, fine mesh netting with openings of 0.8 mm or less, draped over hoops and pegged firmly at the edges, prevents the butterflies from reaching the plants at all. If caterpillars are already present and the infestation is severe, Bacillus thuringiensis var. kurstaki (Bt), sold under the brand name Dipel in the UK, is a biological spray that kills caterpillars without harming beneficial insects, bees, or birds. Apply in the evening when caterpillars are actively feeding.
Other causes to consider
If aphids and caterpillars do not match what you are seeing, a few other causes are worth checking. Cabbage whitefly (Aleyrodes proletella) produces pale, powdery adults on the leaf undersides and can cause some distortion, though it is a secondary concern compared to aphids. Downy mildew causes yellowing and a grey-white fungal coating on leaf undersides, sometimes accompanied by puckering. Clubroot, caused by the soil pathogen Plasmodiophora brassicae, produces wilting and yellow leaves along with dramatically swollen, distorted roots. In dry spells, bolting stress on spring-sown plants can cause outer leaves to cup slightly. A magnesium deficiency (common in UK soils) shows as yellowing between the leaf veins but rarely causes curling.
Prevention tips for healthy brassicas
- Cover transplants with fine mesh netting immediately after planting and keep it on throughout summer, checking regularly that there are no gaps at the edges.
- Practise crop rotation, moving brassicas to a fresh bed each year to break pest and disease cycles, especially clubroot which can persist in soil for 20 years.
- Remove spent brassica plants promptly at the end of the season to eliminate overwintering sites for aphid eggs and pupating caterpillars.
- Grow companion plants such as nasturtiums (which act as aphid sacrificial hosts), marigolds, and Limnanthes douglasii nearby to draw in hoverflies, ladybirds, and parasitic wasps.
Frequently asked questions
Why are my cabbage leaves curling inward?
Cabbage leaves curling inward is almost always caused by cabbage aphids (Brevicoryne brassicae) feeding on the underside of young leaves. As they feed, they inject a toxin that distorts the leaf tissue, causing it to curl tightly around the colony. Flip a curled leaf over and look for dense, greyish-green clusters to confirm. Cabbage white caterpillars can also cause leaves to fold if they shelter inside, but aphid infestation is by far the more common cause of the characteristic inward curl.
How do I get rid of cabbage aphids naturally?
The most effective natural approach is a strong jet of water directed at infested leaves, which dislodges the aphids without chemicals. Repeat every two or three days. Encouraging natural predators such as ladybirds, lacewings, and parasitic wasps by planting nectar-rich flowers nearby helps keep populations in check long-term. For stubborn infestations, spray with an insecticidal soap (such as SB Plant Invigorator) or a dilute solution of washing-up liquid and water, making sure to coat the undersides of leaves. Avoid broad-spectrum insecticides, which kill the beneficial insects that would otherwise control the aphids for you.
Are curled brassica leaves safe to eat?
Yes, in most cases the leaves themselves are still safe to eat once you have dealt with the pest. Wash them thoroughly under running water to remove any aphids, eggs, or caterpillar droppings. Leaves that have yellowed significantly or show signs of secondary fungal infection are best removed and composted (not in a cold heap if clubroot is suspected). Slightly distorted but otherwise healthy-looking leaves from kale or spring greens are perfectly edible once cleaned.
When should I net my brassicas in the UK?
Cover brassicas with fine mesh netting (ideally 0.8 mm or smaller) as soon as you transplant them in late spring, before cabbage white butterflies begin laying eggs in earnest from May onwards. In a typical UK season, the first generation of Pieris brassicae caterpillars appears in May to June, with a second generation in August. Keeping the net on throughout summer gives the best protection. Make sure the edges are fully pegged down at ground level so butterflies cannot get underneath.