Kohlrabi, the fast crop: sow in batches, harvest tender

There are vegetables that are a multi-year project, and then there is kohlrabi. Between the day you drop the seed into the soil and the moment you twist the first crisp bulb out of the bed, there are often just eight weeks. For the impatient part in every gardener, that is a gift. And because a single sowing always ripens in one go, an old trick pays off here in particular: don't sow everything at once, but in batches, in small portions spread across half the year.

Why kohlrabi is the fast crop among the cabbages

Brassica oleracea var. gongylodes, as kohlrabi is called botanically, belongs to the large family of the brassicas, in the same breath as white cabbage, broccoli and Brussels sprouts. But unlike its relatives, which demand a whole gardening year of patience, kohlrabi picks up the pace. The swollen stem we eat is neither a root nor a fruit, but an above-ground stem that swells into a bulb. This bulb needs only around 650 growing degree days, and those add up quickly in spring and early summer.

Kohlrabi is a moderate feeder. It wants nutrient-rich, humus-rich soil, but no over-fertilising. Give it too much nitrogen and you will harvest large bulbs, but they store a lot of nitrate and the plant risks growing into the leaves instead of the bulb. A shallow dressing of mature compost in spring is plenty in most gardens.

Sow in batches instead of all at once

The most important sentence in this article: sow little, but often. A full row of kohlrabi all at once means you have to harvest twelve or fifteen bulbs within a few days, before they turn woody. No family manages that. Instead, scatter a small portion every two to three weeks, six to eight plants per batch. That way you have fresh, young bulbs in the right size from May right into October.

The first sowing starts early: from CW 12, that is mid to late March, you raise the first plants on the windowsill or in the cold frame. From CW 16 you can sow directly outdoors or set out pre-grown young plants. The last sensible batch goes in by around CW 30, late July, so the bulbs mature before the first hard frosts. Autumn varieties are more robust here than the tender spring types.

Planting: two to three centimetres deep, no more

The most common beginner's mistake happens when setting out the young plants. Kohlrabi wants to sit shallow. The root collar belongs just below the soil surface, two to three centimetres deep, no more. Set it too deep and the later bulb sits half in the soil, rots more easily and never forms a clean, round base. Plant spacing is around 30 centimetres in all directions. Large autumn varieties need up to 40 centimetres.

Good neighbours, good harvest

Kohlrabi is an easy-going bed companion. Next to lettuce, spinach, beetroot and bush beans it feels at home. The low-growing lettuce uses the space between the young kohlrabi plants before they need it. Avoid other brassicas as direct neighbours, though, and keep to the crop rotation.

Because kohlrabi belongs to the brassicas, the iron rule of crop rotation applies: at least four years' break on the same patch, and never directly after other brassicas like radishes or mustard. Otherwise the pathogen behind clubroot builds up in the soil, a fungal disease that makes the roots swell into misshapen lumps and leaves the plant stunted.

Watering: the key to a tender bulb

If kohlrabi has one single cardinal wish, it is even moisture. The bulb grows most beautifully when the soil stays constantly slightly moist. What harms it is the swing: a dry spell in which growth stalls, followed by heavy rain. Then the bulb suddenly shoots back into growth, the outer layer tears, and you harvest split, woody bulbs.

Woodiness is the other issue. A kohlrabi left too long in the bed stores fibres and turns tough. Harvest the early varieties as soon as they are tennis-ball to fist-sized, usually at seven to eight centimetres across. Only the modern autumn and storage varieties like 'Superschmelz' stay tender even at head size. With those you may take your time.

Keeping pests away without spraying

Kohlrabi has two classic adversaries, and both are most elegantly warded off with a physical barrier.

Flea beetle and cabbage white

The flea beetle loves dry, warm springs and riddles the young leaves like a sieve. Moist soil and a fine-meshed net slow it down. The cabbage white lays its eggs on the leaf undersides, and the green caterpillars later eat their way through the foliage. A crop protection net, laid over right after planting and well weighted at the edges, keeps both away reliably.

From seed to bulb: the sequence in six steps

1. Raise or sow directly

   From CW 12 raise on the windowsill, from CW 16 directly outdoors. One seed per pot, about one centimetre deep.

2. Plant shallow

   Set young plants with the root collar only two to three centimetres deep, spaced about 30 centimetres apart.

3. Lay the net

   Immediately stretch a crop protection net over the bed and weight the edges, against flea beetle and cabbage white.

4. Water evenly

   Keep the soil constantly slightly moist, mulch. Avoid drought stress, or the bulb will split later.

5. Harvest in good time

   Harvest early varieties at seven to eight centimetres, while they are tender. Don't wait for oversize.

6. Sow the next batch

   Sow a small new portion every two to three weeks, up to around CW 30, for a supply into autumn.

Harvesting, storing and what to do with the leaves

To harvest, twist or cut the bulb off just above the soil. Fresh from the bed it keeps in the fridge for a few days, wrapped in a damp cloth even for over a week. For winter you can blanch and freeze cleaned, diced bulbs. Late storage varieties keep for several weeks in a cool, moist box in the cellar.

And don't throw the leaves away. The young heart leaves are edible like chard or spinach, finely sliced in soup or briefly tossed in butter. That way you use the whole plant.

Frequently asked questions about kohlrabi