Why Are My Broccoli Leaves Curling?

Cabbage white caterpillars

Caterpillars of the large white butterfly (Pieris brassicae) and small white butterfly (Pieris rapae) are among the most damaging pests of broccoli in UK gardens. The large white lays clusters of pale yellow eggs on the underside of leaves; the caterpillars are yellow and black, feeding in groups and capable of stripping entire plants to bare stems within days. The small white lays single pale yellow eggs and produces green, well-camouflaged caterpillars that feed alone and are harder to spot. Both species are active from late spring through to autumn, with multiple generations per year in the UK.

What to do

• Cover the crop with butterfly-proof mesh (sold as cabbage white mesh or brassica mesh) from transplanting through to harvest. This is the single most effective preventive measure, as it physically excludes the adult butterflies from laying eggs.
• Inspect the underside of leaves weekly during summer for egg batches (large white: clusters of upright pale yellow eggs; small white: solitary pale yellow eggs) and remove them by hand.
• Pick caterpillars off by hand: the large, clustered caterpillars of the large white are easily spotted; the small, green caterpillars of the small white require more careful searching along the midrib and underside of leaves.
• Apply Bacillus thuringiensis var. kurstaki (Dipel, XenTari) to young caterpillars. This biological insecticide is effective on young caterpillars but less so on older, larger ones.

Cabbage aphid

Cabbage aphid (Brevicoryne brassicae) is one of the most persistent pests of brassica crops and forms distinctive dense, compact, grey-blue, waxy colonies on the underside of leaves and at the growing tips. The colonies cause the leaves to curl inward around them, and heavily infested growing tips may be entirely distorted by aphid activity. The waxy coating of cabbage aphid makes it somewhat resistant to contact insecticides compared with uncoated aphid species.

What to do

• Squash small colonies by hand as soon as they are noticed. Early intervention before colonies become established is far more effective than treating large, entrenched populations.
• Apply insecticidal soap spray, ensuring thorough coverage of the underside of affected leaves where the colonies concentrate. Repeat every 5 to 7 days. The waxy coating requires good spray coverage to achieve effective control.
• Remove and destroy any growing tips with large, established aphid colonies in the centre: these are very difficult to control with sprays and the growth of that tip is already badly compromised.

Downy mildew

Downy mildew (Peronospora parasitica) on broccoli causes pale, yellowish, angular patches on the upper surface of the leaves, often with a distinct grey-purple, downy fungal growth on the underside of the patches in humid conditions. Affected leaves may curl and pucker around the infection sites. The disease spreads rapidly in cool, wet, humid conditions and is most common on closely spaced or poorly ventilated plantings.

What to do

• Remove and destroy affected leaves promptly to reduce spore spread within the crop. Do not compost them.
• Ensure adequate spacing between plants and good airflow through the crop. Broccoli plants should be spaced at least 45 to 60 centimetres apart.
• Rotate brassicas to a different bed each year to reduce the soil inoculum of downy mildew and other brassica pathogens.

Cabbage whitefly

Cabbage whitefly (Aleyrodes proletella) is a persistent pest of brassicas in UK gardens, forming flat, scale-like immature stages on the underside of the lower leaves of broccoli, calabrese, and Brussels sprouts. Adult whiteflies are tiny, white, and fly up in a cloud when the plant is disturbed. Heavy infestations cause yellowing and curling of the lower leaves, and the honeydew they produce encourages sooty mould growth. Cabbage whitefly overwinters on brassica crops in the UK and is not controlled by cold weather as effectively as greenhouse whitefly.

What to do

• Remove and destroy the most heavily infested lower leaves, which harbour the greatest density of immature stages. This reduces the population significantly without the need for any spray.
• Apply insecticidal soap spray or pyrethrin-based spray to the underside of affected leaves to target the adult whiteflies and accessible immature stages. Multiple applications every 5 to 7 days are needed as the scale stages are resistant to contact insecticides.

Drought

Drought causes broccoli leaves to wilt and roll inward, and drought stress during head development produces smaller, looser heads that mature more quickly and are harder to harvest at the right stage. Broccoli has higher water requirements than some other brassicas and benefits from consistent moisture throughout the growing season, particularly from transplanting onward and during the period of head development.

What to do

• Water broccoli consistently during dry spells, particularly from June to September when calabrese is actively heading up. A deep watering every 7 to 10 days in dry conditions is more beneficial than daily shallow watering.
• Mulch around the base of plants with compost or straw to conserve soil moisture and reduce the frequency of watering needed in summer.

Frequently asked questions

Why are my broccoli leaves curling?

Broccoli leaves curl most often from cabbage white caterpillar feeding, cabbage aphid infestation, or downy mildew. Large white butterfly caterpillars (Pieris brassicae) and small white butterfly caterpillars (Pieris rapae) feed on broccoli leaves from summer onward, and heavy feeding causes the leaves to curl inward as tissue is removed. Cabbage aphid (Brevicoryne brassicae) forms dense, waxy colonies on the underside of broccoli leaves, causing the leaf to curl around the feeding colonies. Downy mildew (Peronospora parasitica) causes pale, angular patches on the upper leaf surface with a grey-purple fungal coating below, leading to curling and yellowing.

How do I protect broccoli from cabbage white butterflies?

The most effective protection against cabbage white butterflies is to cover broccoli crops with fine butterfly-proof mesh or horticultural fleece from transplanting through to harvest, preventing the adult butterflies from laying eggs on the plants. If the crop is not covered, inspect the underside of leaves weekly from June onward for pale yellow egg batches (large white) or single pale yellow eggs (small white). Remove and destroy eggs and young caterpillars by hand. The biological control Bacillus thuringiensis var. kurstaki (sold as Dipel or XenTari) is approved for UK home garden use and kills young caterpillars without harming other organisms. Older, larger caterpillars are more resistant and should be removed by hand.

What is the difference between calabrese and sprouting broccoli?

Calabrese (Brassica oleracea var. italica) is the large-headed, green broccoli sold in supermarkets and widely grown in UK allotment gardens. It is sown in spring and produces a central large head in summer or early autumn, followed by smaller side shoots if the central head is cut promptly. Purple sprouting broccoli (also Brassica oleracea var. italica) is sown in spring, overwintered in the ground, and produces many small, intensely flavoured purple or white spears in late winter and early spring, filling the hungry gap when little else is available. The two crops share the same pests and diseases and both benefit from the same protective measures.

When should I harvest broccoli in the UK?

Calabrese should be harvested when the central head is fully developed but before the individual flower buds begin to open and turn yellow. Once the buds start to separate and show yellow, the broccoli becomes bitter and loses quality quickly. Cut the central head with a sharp knife leaving 5 to 10 centimetres of stem, then water and feed the plant to encourage side-shoot production. Purple sprouting broccoli is harvested from February to April by snapping off the individual spears when they are 10 to 15 centimetres long and the buds are tight; regular harvesting encourages the production of more spears and extends the harvest over 6 to 8 weeks.