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Kohlrabi

This Allotment Pro editorial draft summarises the cited RHS guide. Check the source and adjust timings for local conditions.
RHS source guide
Kohlrabi crop illustration

Month by month

Sow

  1. SpringSow early batches indoors in modules, especially on cold or heavy soil.

  2. March–JulySow outdoors once temperatures remain above 10°C (50°F).

  3. August in mild areasSow hardy purple cultivars for autumn and early-winter cropping.

Plant out

  1. Mid- to late springTransplant hardened module plants while no more than 5cm (2in) tall and outdoor temperatures remain above 10°C.

Harvest

  1. May–NovemberLift repeated sowings between golf-ball and tennis-ball size.

  2. December in mild areasFinish protected late crops before severe winter weather.

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1 - Getting Started

Kohlrabi is a fast-growing brassica that forms an edible swollen stem base above the soil. This round structure is often called a bulb, although botanically it is stem tissue. Its skin may be green, white or purple, while the flesh remains white or pale green. Harvested young, it is tender, crisp and juicy with a sweet flavour resembling mild turnip or broccoli.

Grate or slice the peeled stem for salads and coleslaw, or steam, roast, sauté or add it to stews. Young leaves are edible as greens. Baby kohlrabi can be ready in six weeks and full-sized stems in about twelve, making it useful for gaps between slower crops.

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2 - Choosing What To Grow

Green and white cultivars generally mature quickly and suit early sowing. Purple cultivars are often hardier and slower, making them useful for late-summer sowing and autumn or winter harvest. F1 hybrids tend to be vigorous and uniform. Resistance to bolting and fungal disease is valuable because temperature or moisture stress can interrupt stem swelling.

Recommended varieties

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3 - Preparing The Ground

Choose a sunny position with fertile, moisture-retentive soil. Kohlrabi needs less nitrogen than many brassicas and grows in light sand or heavier clay. Some cultivars tolerate partial shade, particularly during summer heat.

Test acidic soil: where pH is below 6, consider applying lime to raise it and reduce club-root risk. Rotate kohlrabi with other brassicas. Weed thoroughly, then add about two bucketfuls of garden compost per square metre or square yard. Firm and rake the surface. In a no-dig bed, apply organic matter as a surface mulch and sow or plant through it.

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4 - Sowing

Make early sowings indoors because cold can trigger premature flowering before the stem swells. Outdoors, sow several small batches through spring and summer. Spring crops grow fastest; late purple crops extend harvesting into colder weather.

Sowing indoors

Fill modular trays with damp peat-free seed compost. Sow two seeds per cell 1.5cm (½in) deep and remove the weaker seedling if both emerge. Keep in bright light and water regularly. Germination normally takes about two weeks. Plants are ready outside three to four weeks later, while no more than 5cm (2in) tall.

Sowing outdoors

Sow from March to July, or into August in mild districts. Wait for temperatures above 10°C (50°F), protecting unexpected cold snaps with cloches or biodegradable fleece. Make drills 1.5cm deep, water their bases, sow thinly and set rows 30cm (1ft) apart.

When seedlings reach 2.5cm (1in) or form their first true leaves, thin to 15cm (6in). Protect the remaining plants from slugs and snails.

Sowing in containers

Use a container at least 30cm (1ft) wide and deep in sun or semi-shade. Fill with peat-free multipurpose compost, scatter seed thinly and cover with 1.5cm compost. Thin seedlings to 15cm, water consistently and harvest at golf-ball size for a quick mini crop.

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5 - Planting

Harden indoor-raised plants gradually for two weeks. Plant only while seedlings are no more than 5cm (2in) tall, because larger kohlrabi transplants less successfully. Ensure outdoor temperatures are consistently above 10°C; sudden chilling can cause bolting. Mid- to late spring is usually best, and warming the soil beforehand may help.

Remove modules without disturbing roots. Set plants 15cm (6in) apart in rows 30cm (1ft) apart, firm gently and water well. Do not bury the stem base so deeply that swelling is obstructed.

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6 - Plant Care

Temperature and moisture stress can make kohlrabi bolt or turn woody. Water seedlings and young plants regularly until rooted, then keep the soil consistently moist. Cool damp weather produces the best stems. On dry summer ground, prioritise early and late sowings that avoid peak heat.

Apply a thick organic mulch once plants grow strongly. This holds moisture and suppresses weeds. Hand-weed close to each stem because a hoe can damage the swelling. Use insect-proof mesh against cabbage butterflies and other brassica pests, and protect young seedlings from slugs and snails.

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7 - Harvesting

Harvest swollen stems between golf-ball and tennis-ball size, when their flesh is sweetest, mildest and most succulent. Overgrown kohlrabi loses flavour and becomes fibrous or woody, especially in hot dry weather. Repeated sowings provide crops from May to November and potentially December in mild locations.

Use cloches or biodegradable fleece to finish late crops as winter arrives. Cut or pull the whole stem, remove roots and peel the outer skin before use. The young leaves can be cooked like kale.

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8 - Problem Solving

Heat, drought and cold shocks are the main physiological risks. Maintain moisture, avoid very early outdoor sowing, harden transplants gradually and harvest young. Kohlrabi grows quickly, so insect damage is often less severe than on slower brassicas, but supported fine mesh remains worthwhile.

Common problems

Bolting in vegetables

Premature flower stalks prevent a useful swollen stem. Avoid temperature shocks, drought and over-aged transplants.

Problem source

Club root

This soil-borne problem produces swollen distorted roots and stunted tops. Rotate brassicas, avoid contaminated transplants and manage strongly acidic soil.

Problem source

Cabbage caterpillars

Larvae chew large holes in foliage. Inspect for eggs and caterpillars or keep the crop beneath insect-proof mesh.

Problem source

Cabbage root fly

Maggots feed on roots and can check or collapse young plants. Use close-fitting collars or mesh and firm plants securely.

Problem source

Cabbage whitefly

Small white insects gather under leaves. Light populations are often tolerated, but large colonies reduce plant quality.

Problem source

Flea beetles

Small beetles pepper young foliage with holes. Keep plants growing strongly and cover seedlings with fine mesh.

Problem source

Mealy cabbage aphid

Waxy grey-green aphids form colonies and distort young growth. Encourage predators and remove badly affected leaves.

Problem source

Pigeons

Birds tear brassica foliage and may strip plants. Secure mesh over a supporting frame so birds cannot reach the leaves.

Problem source

Slugs and snails

These molluscs attack seedlings in damp conditions. Reduce hiding places and protect the crop until well established.

Problem source