Kohlrabi

Origin & history

Kohlrabi is a young vegetable: descriptions of a cabbage with a swollen stem only appear in European herbals in the 16th century. It became so much at home in German-speaking gardens that English, Russian and Japanese simply borrowed the German word. The “bulb” is botanically not a root but a thickened stem above ground, which is why it sits visibly on the soil.

Varieties

White — really pale green — varieties grow faster and are the choice for the first sowings; blue ones are a little slower, stand longer without splitting and suit the autumn better. For the very earliest sowing it is worth looking for varieties explicitly described as bolt-resistant, because a cold spell after planting otherwise triggers flowering. Giant varieties such as “Superschmelz” grow to head size and still stay tender — with those, size is for once not a problem.

Care

The whole job is even watering: kohlrabi grows in spurts, and when heavy rain follows a dry week the bulb splits open. It is a moderate feeder — compost at planting is enough; too much nitrogen gives lush leaves and a small bulb. With a direct sowing, thin to 25 cm in good time, otherwise the plants push each other upward and stay flat.

What can go wrong

Split bulbs are almost always a water problem rather than a fault of the variety; a mulch layer keeps soil moisture steadier than watering ever can. Early sowings bolt when the young plants meet several cold weeks after planting out — better to plant a week or two later and lay fleece over them at first. The cabbage root fly lays its eggs at the root collar; insect netting from planting day or a cardboard collar round the stem deals with that, and cabbage should only return to the same bed every four years anyway.

Harvest, storage & kitchen

Harvest at about 7 to 10 cm across, roughly tennis-ball size. Twist the leaves off at the bed — they draw water out of the bulb in the fridge; the young heart leaves are good eating and belong in the soup rather than on the compost. Without its leaves kohlrabi keeps one to two weeks in the salad drawer, autumn varieties much longer in damp sand in a cool cellar.

Frequently asked questions

When can I sow kohlrabi directly outdoors?

Direct sowing works from the start of April until about 10 July, roughly 1 cm deep. For the earliest batches, raise plants in pots from 20 February and set them out from April onwards — in March the cold open ground gives slow, patchy germination.

Does kohlrabi grow back after harvesting?

No. Once the bulb is cut the plant will not form a second one, so the stump goes on the compost heap. The only way to keep supply going is fresh batches — sow again every three to four weeks and you will be picking into November.

How long does kohlrabi take from sowing to harvest?

Around eight to ten weeks for spring batches, nearer twelve in autumn. The season as a whole runs from 20 May to early November. Plants raised in pots spend about five weeks there and usually beat a direct sowing made on the same day.

Is kohlrabi frost hardy?

Plant and bulb shrug off light night frosts, but kohlrabi is only half hardy and will not sit out a winter in the bed. That is why harvest ends in early November; cover late batches with fleece when frost is forecast.

Can kohlrabi be grown in a pot or window box?

Yes — about five litres of compost and 20 cm of depth per plant is enough, since the bulb sits on top of the soil. Give it a spot in full sun and never let the compost dry out completely. Indoors on a windowsill it does not work.