Why Are My Cabbage Leaves Curling?

Mealy cabbage aphid

Mealy cabbage aphid (Brevicoryne brassicae) is one of the most damaging brassica pests in UK gardens. The grey-white, waxy aphids colonise the underside of outer leaves and then move into the heart of the cabbage, causing leaves to pucker, cup, and curl around the dense colonies. A heavy infestation in the heart of a cabbage renders the vegetable inedible and can cause the outer leaves to curl and yellow as the plant's resources are diverted. The waxy coating on the aphids makes them somewhat resistant to insecticidal sprays compared to other species.

What to do

• Inspect the undersides of outer leaves weekly from May for mealy cabbage aphid colonies. Early colonies are much easier to control than established ones that have penetrated the heart of the plant.
• Remove outer leaves carrying dense colonies by hand and dispose of them away from the vegetable garden. Crushing aphids by hand on lightly infested leaves reduces populations without the need for sprays.
• For larger infestations, spray with insecticidal soap, neem oil solution, or a pyrethrin-based spray, directing the spray at the underside of the leaves where the colonies are concentrated. Repeat every 4 to 5 days for 2 to 3 weeks.
• Cover brassica beds with insect mesh from transplanting to harvest to exclude the winged forms of cabbage aphid that colonise plants from late spring. The mesh must be fine enough to exclude the aphids as well as the larger cabbage white butterflies.

Caterpillars

Large white butterfly (Pieris brassicae) and small white butterfly (Pieris rapae) caterpillars are perhaps the most visible and well-known cabbage pests in the UK. The large white caterpillars are yellow and black striped and feed communally on the outer leaves, while the small white caterpillars are pale green and feed solitarily, often boring into the heart of the cabbage. Both species leave irregular holes in the leaves and deposit dark green frass (droppings) on the leaves below the feeding site. Leaves around the feeding areas may curl as they dry.

What to do

• Cover brassica crops with fine insect mesh or horticultural fleece from transplanting, secured at the edges to prevent butterflies from gaining access. This is the single most effective preventive measure and also excludes cabbage aphids and cabbage root fly simultaneously.
• Inspect leaves weekly for egg batches from late April: large white butterfly eggs are yellow and laid in clusters on the underside of outer leaves; small white butterfly eggs are pale yellow and laid singly. Remove and destroy egg batches before they hatch.
• Handpick caterpillars when populations are small. For larger infestations, spray with Bacillus thuringiensis (Bt), a naturally occurring bacterial biological control that specifically targets caterpillars and is harmless to birds, mammals, and beneficial insects. Repeat applications weekly while caterpillars are present.

Cabbage root fly

Cabbage root fly (Delia radicum) causes wilting and poor growth of transplanted brassicas from late spring. The adult fly lays eggs at the soil surface near the base of brassica transplants, and the white maggot larvae feed on the roots. The first symptom above ground is wilting that does not recover with watering, followed by yellowing and collapse of the plant. The maggots and their root damage are visible on uprooting the plant. Small, recently transplanted brassicas are most vulnerable.

What to do

• Place a collar of carpet underlay, cardboard, or purpose-made brassica collars around each transplant stem at soil level immediately after planting. The collar prevents the adult fly from laying eggs close to the stem. Collars should be at least 12 centimetres in diameter.
• Fine insect mesh placed over the bed immediately after transplanting provides more comprehensive protection against root fly, cabbage white butterflies, and mealy aphid simultaneously.
• Remove and destroy affected plants rather than attempting to rescue them. The larvae will have already caused extensive root damage by the time above-ground symptoms appear.

Clubroot

Clubroot (Plasmodiophora brassicae) is a soil-borne disease causing swollen, distorted roots and above-ground symptoms of wilting, yellowing, and poor growth that resemble drought or root fly damage. Pulling up an affected plant reveals grotesquely enlarged, club-shaped roots rather than the normal fibrous root system. Clubroot spores persist in the soil for 20 or more years, making infected beds effectively lost to brassica growing for the long term without significant soil treatment.

What to do

• Raise soil pH above 7.2 by applying garden lime (calcium carbonate) to brassica beds in autumn. Clubroot thrives in acidic conditions and is significantly less damaging at pH above 7.0. Test soil pH before liming and apply lime at the rate recommended based on the test results.
• Never bring soil, compost, or plant material into the garden from outside sources without inspection: clubroot is most commonly introduced to new plots on contaminated soil or plant roots.
• Grow brassicas in large modules and delay transplanting until plants are well established with a robust root system: larger plants can tolerate some clubroot infection better than very young transplants.
• Rotate brassica crops to a different bed each year on a minimum 4-year rotation to reduce the soil spore load over time.

Drought

Drought causes the outer leaves of cabbage to curl outward and wilt, particularly in the heat of the day, recovering partially in cooler evening conditions. In severe or prolonged drought, the outer leaves yellow and die back and the plant fails to form a tight heart. Cabbages have a high water content and need consistent moisture throughout their growing season, particularly when forming the central head.

What to do

• Water deeply every 10 to 14 days during dry periods, saturating the soil to 30 centimetres depth. A heavy, infrequent watering is more beneficial than frequent light watering for encouraging the deep root development that makes plants more drought-resilient.
• Apply a mulch of compost around the plants (avoiding contact with the stem) to conserve soil moisture and reduce the frequency of irrigation needed in dry summer weather.

Frequently asked questions

Why are my cabbage leaves curling?

Cabbage leaves curl most often from cabbage whitefly, mealy cabbage aphid, or caterpillar damage. Mealy cabbage aphid (Brevicoryne brassicae) colonises the underside of outer leaves and the growing centre, causing the leaves to pucker, cup, and curl around the dense colonies of grey-white, waxy aphids. Large white and small white butterfly caterpillars (Pieris brassicae and Pieris rapae) eat the leaf tissue and create irregular holes, and leaves surrounding the feeding sites may curl around the caterpillars as the plant responds to the damage. Drought also causes the outer leaves of cabbage to curl and wilt in hot, dry UK summers.

What is clubroot and how do I prevent it?

Clubroot is a soil-borne disease caused by Plasmodiophora brassicae that distorts and enlarges the roots of all brassica family plants (cabbages, cauliflowers, Brussels sprouts, kale, broccoli, turnips, radishes, and related ornamentals such as wallflowers). Affected plants wilt, yellow, and fail to thrive even when watered, and pulling up the plant reveals grotesquely swollen, distorted roots. The organism that causes clubroot produces resting spores that can persist in the soil for 20 or more years, making infected beds effectively unusable for brassica crops for the very long term. Prevention is far more effective than treatment: raise soil pH above 7.0 by liming, rotate brassicas to a different bed each year, and never bring in soil or plant material from infected gardens.

How do I get rid of caterpillars on my cabbage?

Caterpillars on cabbage can be managed most effectively by covering the crop with fine insect mesh or horticultural fleece to prevent the adult butterflies from laying eggs on the leaves. Inspecting the leaves regularly from late spring and removing eggs (small, yellow, capsule-shaped clusters on the underside of outer leaves) before they hatch prevents the caterpillar problem from developing. Handpicking caterpillars is effective at small scale. The biological caterpillar control Bacillus thuringiensis (Bt) is a naturally occurring bacterial spray that kills caterpillars within 2 to 3 days of application without harming beneficial insects, birds, or mammals.

Why are my cabbage seedlings wilting and collapsing?

Cabbage seedlings wilt and collapse most often from damping off (caused by Pythium and Fusarium species in overly wet seedling conditions), cabbage root fly damage, or clubroot. Damping off causes the stem base to thin and collapse at soil level, typically affecting a batch of seedlings in a propagator or tray. Cabbage root fly (Delia radicum) larvae feed on the roots of young transplants, causing the plant to wilt despite adequate watering, with no visible pest above ground. Pulling up the plant reveals maggots on the root system and brownish rotted roots. Clubroot causes more gradual decline, with swollen, distorted roots and general yellowing rather than sudden collapse.