Why Are My Brussels Sprouts Leaves Curling?

Cabbage aphid

Cabbage aphid (Brevicoryne brassicae) is one of the most persistent pests of Brussels sprouts in the UK, and it can be particularly damaging because its colonies become established deep inside the developing sprout buttons, making them impossible to spray once the sprouts have started to form. The compact, grey-waxy colonies colonise the underside of leaves, the growing tip, and the axils of the leaves where sprouts are developing, causing leaf curl, distorted sprouts, and significant loss of quality. Cabbage aphid populations peak in late summer and autumn, exactly when sprout development is most critical.

What to do

• Inspect plants weekly from June onward for the first cabbage aphid colonies. Early intervention before colonies multiply across the plant is far more effective than treating large, entrenched populations.
• Squash small colonies by hand or remove them with a strong water jet. For larger colonies, apply insecticidal soap spray, ensuring thorough coverage of the underside of leaves.
• Cover plants with insect mesh from transplanting through the main aphid flight season. Mesh protection also prevents cabbage white butterfly egg-laying and is worthwhile for both reasons.
• Remove and destroy any plants with colonies established deep inside developing sprout buttons: these sprouts will be unusable regardless of treatment.

Cabbage whitefly

Cabbage whitefly (Aleyrodes proletella) is a persistent pest of Brussels sprouts in the UK and does not die back in winter as greenhouse whitefly does: it overwinters on brassica crops in the garden. Flat, scale-like immature stages feed on the underside of the lower leaves, and adult whiteflies fly up in a white cloud when the plant is disturbed. Heavily infested leaves yellow, curl, and are covered with sticky honeydew that encourages sooty mould growth. The scale stages are resistant to most contact insecticides.

What to do

• Remove and destroy the most heavily infested lower leaves, which carry the greatest density of scale stages. This is the single most practical control measure for established infestations.
• Apply insecticidal soap or pyrethrin-based spray repeatedly every 5 to 7 days to the underside of affected leaves, targeting adult whiteflies. Complete elimination is rarely achievable but consistent treatment keeps populations at a manageable level.
• Clear away Brussels sprouts plants at the end of the season rather than leaving stumps in the ground: overwintering whitefly populations on old stumps colonise the following year's brassica crops.

Caterpillars

Caterpillars of the large white butterfly (Pieris brassicae) and small white butterfly (Pieris rapae) feed on Brussels sprouts leaves from summer onward, causing holes and ragged edges that make the leaves curl inward as tissue is lost. In serious attacks, caterpillars also burrow into developing sprouts, making them inedible. Large white caterpillars are highly visible: yellow and black, feeding in groups on the outer leaves. Small white caterpillars are green and camouflaged, feeding alone and difficult to spot.

What to do

• Cover plants with butterfly-proof mesh from transplanting to prevent adult butterflies from laying eggs on the plants.
• Inspect the underside of outer leaves weekly from June through September for egg batches and remove them by hand. Large white egg batches are distinctive: upright, ribbed, pale yellow eggs in compact clusters.
• Hand-pick caterpillars. Apply Bacillus thuringiensis var. kurstaki spray to young caterpillars.

Downy mildew

Downy mildew (Peronospora parasitica) on Brussels sprouts produces the same angular, pale-yellow patches on the upper leaf surface and grey-purple downy growth below that it causes on other brassicas. Leaves curl and yellow around infection sites. Downy mildew on Brussels sprouts is most common in the autumn and early winter months when cool, humid conditions favour spore germination and spread.

What to do

• Remove and destroy affected lower leaves, which are most susceptible and carry the heaviest infection loads. Good airflow around the base of the plant reduces downy mildew pressure significantly.
• Space plants adequately: at least 60 centimetres between plants and 75 centimetres between rows for full-sized varieties.

Wind rock

Wind rock (loosening of the stem in the soil due to wind) does not cause leaf curl directly but causes yellowing of the lower leaves and progressive deterioration of plant health as root function is disrupted. Tall Brussels sprouts plants are particularly vulnerable to wind rock in exposed gardens. A plant that is being blown about rather than standing firm will direct energy into survival rather than sprout development, reducing both the quantity and quality of the harvest.

What to do

• Stake each plant with a stout cane at transplanting and tie the stem to the cane as the plant grows. Check and re-tie throughout the season.
• Firm the soil around the base of each plant after transplanting and after any wind event that has visibly rocked the plant. Earth up around the stem base to provide additional anchorage.

Frequently asked questions

Why are my Brussels sprouts leaves curling?

Brussels sprouts leaves curl most often from cabbage aphid infestation, cabbage whitefly, or caterpillar feeding. Cabbage aphid (Brevicoryne brassicae) forms dense, grey-waxy colonies on the underside of leaves and in the axils of developing sprouts, causing leaf curl and distortion. Cabbage whitefly (Aleyrodes proletella) feeds on the underside of the lower leaves, causing yellowing and curling as populations build. Caterpillars of the large and small white butterfly feed on the outer leaves, causing holes and curl, and can also burrow into developing sprouts.

Why are my Brussels sprouts plants blown over?

Brussels sprouts are tall, top-heavy plants that are highly susceptible to wind rock, which can cause the stems to loosen in the soil and the plants to lean or topple. Wind rock disrupts root development and reduces both plant vigour and the quality of the sprouts. Prevention involves: planting deeply (burying the stem up to its lowest leaves); staking each plant individually with a stout cane from the time of transplanting; firming the soil around the base of the plant thoroughly after transplanting and after any subsequent loosening; and choosing a sheltered position or erecting a windbreak around exposed plots. Earth up the base of the stem through the growing season to provide additional support.

When are Brussels sprouts ready to harvest in the UK?

Brussels sprouts are ready to harvest in the UK from October onward, depending on variety, and continue through to March the following year. Early varieties produce sprouts from October to December; maincrop varieties from November to January; late varieties from January to March. The sprouts are ready when they are firm and approximately the size of a large marble to a small walnut (2 to 3 centimetres in diameter). Pick from the bottom of the stem upward: the lowest sprouts on the stem mature first. Snap or cut individual sprouts off the stem rather than removing the whole stem.

Why are my Brussels sprouts loose and leafy rather than tight?

Loose, leafy Brussels sprouts (also called blown sprouts) are most commonly caused by overly rich soil with too much nitrogen, which promotes leaf growth at the expense of the compact bud development needed for tight sprouts. Other causes include wind rock (the plant's stem loosening in the soil), harvesting too late (sprouts that are left on the plant too long naturally open and become leafy), and growing unsuitable varieties. Brussels sprouts need firm soil, so incorporate plenty of organic matter before planting but avoid adding high-nitrogen fertilisers during the growing season. Lime the soil if the pH is below 6.5.