Why Are My Kohlrabi Leaves Curling?

Flea beetle

Flea beetles (Phyllotreta species) are tiny, jumping beetles that chew numerous small, round holes in kohlrabi leaves. The leaf tissue between the holes curls and dies in heavy attacks, giving the leaves a characteristic shothole and curl appearance. Seedlings and young plants are most vulnerable: a heavy flea beetle attack on a newly germinated kohlrabi seedling can kill the plant before it establishes. Established plants with their swelling globe underway are less vulnerable, as the leaf damage does not prevent the globe from continuing to swell.

What to do

• Cover kohlrabi sowings with fine insect mesh from sowing through to harvest. This is the most effective flea beetle control and eliminates the risk of seedling losses entirely.
• Keep plants well-watered: flea beetles are most active on dry, stressed plants and are less damaging to well-watered kohlrabi growing in moist soil.
• Sow kohlrabi successionally in small batches every 3 weeks rather than in large single sowings: this spreads the risk and avoids a large, uniformly vulnerable seedling flush that flea beetles can devastate.

Cabbage aphid

Cabbage aphid (Brevicoryne brassicae) is a grey-green, waxy aphid that forms dense, grey colonies on the underside of kohlrabi leaves and on the developing globe. The colonies cause the leaf to curl inward around the feeding sites. In heavy infestations, the growing tip is distorted and the globe development may be impaired. Ants often attend cabbage aphid colonies and protect them from predators.

What to do

• Inspect the underside of kohlrabi leaves weekly from May for cabbage aphid colonies. The grey, powdery, waxy coating on the aphids is distinctive and different from the green aphids on other vegetables.
• Squash small colonies by hand or remove with a strong jet of water. Apply insecticidal soap spray to larger infestations, covering the underside of leaves and the developing globe. Repeat every 5 to 7 days.
• Insect mesh from sowing excludes aphids as well as flea beetles: one measure prevents both problems.

Caterpillars

Large and small cabbage white caterpillars (Pieris brassicae and Pieris rapae) and cabbage moth caterpillars (Mamestra brassicae) all attack kohlrabi leaves, rasping holes in the leaf blade and causing the surrounding tissue to curl and dry around the damage. Large white caterpillars feed in groups and can strip a young kohlrabi plant rapidly; small white and cabbage moth caterpillars feed singly and are more difficult to spot.

What to do

• Inspect kohlrabi plants weekly from June for eggs (yellow clusters on the underside of leaves) and caterpillars. Remove and destroy eggs and caterpillars by hand.
• Fine insect mesh prevents adult butterflies and moths from laying eggs on the plants: the most comprehensive protection against caterpillar damage as well as flea beetles and aphids.
• Bacillus thuringiensis (Bt) spray is an approved biological control that kills caterpillars when they eat treated leaf tissue, without harming other insects.

Downy mildew

Downy mildew on kohlrabi (Hyaloperonospora parasitica) produces pale yellow patches on the upper surface of the leaves and a grey-white downy growth on the underside. Affected leaves curl and yellow. It is most common in cool, wet conditions and on plants that are densely spaced or poorly ventilated.

What to do

• Space kohlrabi at 20 to 25 centimetres apart to ensure good airflow. Remove and destroy affected leaves promptly.
• Rotate brassica family crops each year. Avoid overhead watering and working among wet plants.

Drought

Drought causes kohlrabi globes to crack, develop a woody texture, and become strongly flavoured. Irregular watering (alternating drought and flood) is particularly likely to cause the globe to split. The leaves droop and curl inward in drought conditions. Kohlrabi is less demanding than many brassicas but still requires consistent moisture to produce the sweet, crisp globes that make it worth growing.

What to do

• Water kohlrabi regularly during dry spells, giving a thorough soaking every 5 to 7 days. Avoid a dramatic shift from dry to very wet conditions, as this is the primary cause of globe splitting.
• Harvest kohlrabi promptly at the correct size: globes left too long in dry conditions become woody and strongly flavoured regardless of watering.

Frequently asked questions

Why are my kohlrabi leaves curling?

Kohlrabi leaves curl most often from flea beetle damage or cabbage aphid infestation. Flea beetles (Phyllotreta species) chew numerous small holes in kohlrabi leaves, and the tissue between the holes curls and dies in heavy infestations. Cabbage aphid (Brevicoryne brassicae) colonises the underside of the leaves and causes them to curl inward around dense colonies. Kohlrabi is a brassica family member and is subject to the same range of pests as cabbage, broccoli, and kale, though its rapid growth rate means it can outpace pest damage when healthy.

What does kohlrabi taste like?

Kohlrabi has a mild, crisp, slightly sweet flavour similar to a cross between broccoli stalks and a mild turnip, without the sharpness of raw turnip. Raw kohlrabi is crunchy and refreshing; it is delicious peeled and sliced into sticks for dipping, grated into slaws, or julienned for salads. Cooked kohlrabi has a more mellow, slightly sweet flavour and a texture similar to turnip or swede: it can be roasted, steamed, added to stir-fries, or mashed. Purple kohlrabi is identical in flavour to green kohlrabi; the colour is in the skin only, not the flesh.

When is kohlrabi ready to harvest?

Kohlrabi is ready to harvest when the swollen stem (the globe above the soil) reaches approximately 5 to 8 centimetres in diameter. Most varieties can be harvested from as small as golf-ball size (around 4 centimetres). Do not leave kohlrabi in the ground too long: roots left to grow beyond 8 to 10 centimetres become woody, fibrous, and less pleasant to eat. Summer varieties are typically ready 6 to 8 weeks from sowing. The leaves are edible and can be used like kale, harvested as needed. In the UK, kohlrabi sown in April is typically harvested from June to July; late sowings in July and August produce autumn harvests.

How do I grow kohlrabi in the UK?

Kohlrabi is one of the easiest brassicas to grow in the UK. Sow direct or in modules from March to July for a succession of harvests. Direct sowings can be made every 3 weeks for continuous production. Thin or plant out to 20 to 25 centimetre spacings. Kohlrabi grows best in an open, sunny position in reasonably fertile, well-drained soil. Unlike many brassicas, it tolerates poorer soils and is less demanding than cabbage or Brussels sprouts. Water during dry spells to prevent the globes from cracking or becoming woody. Cover with fine insect mesh from sowing to keep off cabbage white butterflies, flea beetles, and aphids, which are the primary pests.