How to grow and care for kohlrabi

Grow kohlrabi in a sunny site in light, fertile soil, kept moist and weed-free.

When to plant kohlrabi

Sowing time depends on the variety and also whether seed is being sown direct outside or started off under cover before planting outdoors. Purple kohlrabi is slower growing but hardier than green varieties, so it's best to sow faster-growing green kohlrabi from March to June for early harvests, then sow the hardier, purple type in summer.

Kohlrabi is fairly quick to mature so make several sowings through spring and summer to ensure a regular harvest from summer to autumn.

How to plant kohlrabi

Sow seeds under cover in late winter and early spring. Kohlrabi seedlings dislike root disturbance, so sow seed into modular trays filled with moist, peat-free seed compost. Sow two seeds per module and, if both germinate, thin to leave one seedling. Grow on in a well-lit frost-free place and harden off to acclimatise to the outdoors before planting out in mid- to late-spring when all risk of frosts has passed. Take care to plant the seedlings with the top of the root ball level with the soil surface, as the swollen stems will develop from just above ground.

Outside, sow kohlrabi seed direct in moist soil in shallow drills 1.5cm deep, with rows spaced 30cm apart. Thin seedlings to space individual plants 15cm apart.

Here, Monty Don sows kohlrabi for an autumn crop, and explains how to grow and care for this unusual vegetable:

Keep kohlrabi watered during dry spells to avoid plants bolting (running to seed) before the stems are properly formed.

When to harvest kohlrabi

Kohlrabi is ready to harvest within two to three months. Harvest when the stems have reached between golf and tennis ball size – any larger and they will become tough and woody. Cut them at the root and remove the oldest leaves to help keep the plant fresh. Kohlrabi is best eaten fresh but can be stored for a short time in the fridge in a perforated bag. Harvest until December, or before if you notice the leaves starting to yellow.

Pests and diseases

Kohlrabi is a brassica, which means it's subject to the same pests and diseases that commonly affect other brassicas like cabbage and broccoli. Protection and prevention are the best courses of action – most of the problems below can be prevented by using fine mesh netting.

• Weak, dying plants could be a symptom of cabbage root fly, which lays eggs around the roots of the plant. These hatch into maggot-like larvae that eat the roots
• Caterpillars on leaves are those of the large and small white butterflies, which can be removed and transferred to a sacrificial crop of nasturtiums
• Tiny holes in the leaves are caused by flea beetle, which cause no real damage to the plant
• Clouds of white flies are caused by whitefly. These mainly concentrate in the leaves so the swollen kohlrabi stem shouldn't be affected
• Swollen and distorted roots and yellowing leaves are caused by clubroot. Avoid growing brassicas in the same soil for several years. Regularly adding lime to acidic soils can help to prevent this fungal disease taking hold