When to sow kohlrabi

Kohlrabi is the fastest of the cabbages: from sowing to harvest takes eight to twelve weeks, and as little as six for baby bulbs. That makes it ideal for sowing little and often. A new batch every three to four weeks from March to July keeps the kitchen supplied from May to November.

The earliest sowings, in February and March, belong indoors or in a greenhouse. Outdoors you can sow from April, once the soil warms up, until July; in the UK and other mild areas, August sowings still crop in autumn.

Early crops have one real risk: cold. If young plants spend a spell of days at about 41–54°F, especially four to nine weeks after sowing, they can switch to flowering and bolt instead of forming a bulb. Wait for mild weather before planting out, and keep the earliest plants under fleece or perforated film until about the end of April.

Sow 0.4–0.8 in deep, either one or two seeds per module or thinly in rows outdoors. Seedlings appear within about ten days. Indoors, thin to the strongest per spot; the young plants are ready to go out after three to five weeks with three or four leaves.

In an outdoor row, thin to the final spacing once the seedlings are big enough to handle.

Plant so the bulb can form above the soil: set the plant no deeper than it grew, never burying the base of the first leaves, or the bulb ends up sitting on the ground and rots more easily. Water in well.

Space plants 10–12 in apart each way. Small early varieties can go 8 in apart in rows 12 in apart; giant varieties such as 'Superschmelz' need 16 in each way. A 120 × 80 cm raised bed holds about 12 plants, or up to 18 small early ones.

Kohlrabi grows fast, and fast growth needs steady water. In dry spells, water thoroughly about once a week, around 30 litres per square metre, rather than a splash every day. That soaks the whole root zone.

In German terms kohlrabi is a Mittelzehrer, a medium feeder, and less hungry than most cabbages. In a bed that gets compost every year, a few litres of compost per square metre replace what the crop takes out.

Like all brassicas it prefers a pH at the upper end, around 7 to 7.5, which also keeps clubroot in check.

Pick kohlrabi young, when the bulbs are between golf-ball and tennis-ball size, roughly 2.8–4 in across for most varieties. They are most tender then. Left too long, most varieties turn woody, some more than others.

Giant varieties are the exception: they are bred to grow to 8 in across and more, and take longer to get there.

Pull or cut the bulb just above the root. The young inner leaves are edible too; chop them and add them to the pan with the bulb.

Kohlrabi comes in pale green ('white') and purple ('blue') forms; both are white inside and taste much the same. Choose varieties by season rather than colour: some are bred for the earliest sowings and are slow to bolt, while others suit summer and autumn.

Most varieties form bulbs 3–6 in across. Giant types such as 'Superschmelz' reach 8 in and more and take longer to grow.

Kohlrabi gets all the usual cabbage pests. Flea beetles pepper the leaves of young plants with small holes; cabbage root fly maggots eat the roots of new plantings; cabbage white caterpillars chew the leaves from about May to October.

Because kohlrabi is quick and small, insect mesh is easy to use: lay it straight after sowing or planting and tuck the edges in. Mesh of about 0.8 mm keeps out flea beetles, root fly, cabbage whites and mealy aphids alike; 1.3 mm is enough for the whites and root fly. Where root fly is a known problem, a collar 3–6 in across round each stem helps too.

Slugs love young kohlrabi, and in shade with long leaf wetness downy mildew can appear. Give plants full sun and air.

Clubroot makes roots swell into solid clubs; plants wilt in warm weather and stay small. Its spores live in soil for many years, up to 20. Keep the pH around 7 or a little higher, bin infected plants, and leave at least four years before cabbage-family crops go back in the same bed, seven or more after an outbreak.

Kohlrabi belongs to the cabbage family (Brassicaceae), with cabbage, kale, broccoli, cauliflower, radishes, turnips and rocket. All of them, and mustard as a green manure, count for rotation. Leeks, onions, courgettes and sweetcorn are good crops to grow before it.

Kohlrabi is small and quick, so it fits well in mixed beds. Rows of kohlrabi alternating with lettuce, leeks or onion sets are a common, practical layout. Whether these partners protect each other from pests has not been shown; the benefit is mostly good use of space.

Two rules have better grounds. Don't mix kohlrabi with other cabbages, which only concentrates the pests they share. And a low undersowing of clover among brassicas makes them harder for cabbage root fly to find; mixed plantings generally carry fewer pests than a single crop.