How to Grow Kohlrabi in South Africa: Purple & White Vienna

Want to harvest crisp, tender kohlrabi from your own garden? Kohlrabi is a compact cool-season brassica grown for the round stem that swells above the soil—the harvested part is a swollen stem, not a root or bulb. Use this South Africa-specific guide for Kohlrabi Purple Vienna Seed and Kohlrabi White Vienna Seeds. Both have white flesh; the outer stem is purple on Purple Vienna and pale light green on White Vienna.

Quick Grow Facts

• Crop and botanical name: Kohlrabi, Brassica oleracea Gongylodes Group; a biennial brassica normally grown as an annual vegetable
• Varieties in this guide: Purple Vienna and White Vienna only; not giant-storage kohlrabi or another cultivar
• Seed form and pretreatment: Raw seed; no routine soaking, chilling or scarification is needed for the standard covered-sowing method
• Method: Direct sow or start in cells; thin direct-sown plants at 3–4 true leaves, or transplant sturdy young plugs before they become root-bound
• Depth and light: Cover about 1 cm; light is not required before emergence
• Germination: Keep moderately warm and evenly moist; emergence usually takes less than two weeks in suitable conditions
• Position: Full sun in cool weather; light afternoon shade can reduce stress where the end of the crop overlaps with heat
• Final spacing: 25–30 cm between plants; keep rows about 40–45 cm apart for airflow and access
• Growth form and harvested part: Low upright leaf rosette around one swollen stem above the soil; it is not a root or bulb. No routine support, pinching or earthing-up is needed
• Harvest size: Cut while the swollen stem is about 5–8 cm across; larger Vienna stems become increasingly woody
• Timing: Size and tenderness are the harvest tests; weather and establishment change the timing

When to Sow Kohlrabi in South Africa

The month windows below are sowing windows from the SANSOR regional guide, narrowed by frost, drainage and heat risk. Succession-sow a short row every two to three weeks only while the remaining cool season still allows enough time for steady stem development.

Winter-rainfall Western Cape and South Coast: January–May

Sow from January through May, favouring late summer into autumn where midsummer heat is severe. Keep early sowings evenly moist through dry wind. As winter rain builds, use a raised or well-drained bed and leave the 40–45 cm rows open enough for airflow.

Frost-prone interior: January–March

On the Highveld and in the Northern Cape or Great Karoo, sow from January through March so stems size up in cooling weather. Do not assume that a brassica is immune to hard frost: young plants can be damaged, and prolonged cold can prime established plants to bolt later. In exposed districts, finish the final sowing early enough to establish before the severest frost.

Humid coast and KZN Midlands: February–May

The KZN Midlands window is February–May; the warmer KZN coast is best from March–May. Use the coolest practical part of the window, provide good airflow, water the soil early in the day and avoid leaving foliage wet overnight. Secure insect mesh before moths or flea beetles enter.

Hot-dry and subtropical interior

In the hot-dry Northern Cape and Great Karoo, use January–March. In the subtropical Lowveld, wait until the main heat has eased and sow March–May. Reliable drip irrigation, mulch after establishment and protection from drying wind help the shallow roots; plan to harvest before spring heat returns.

Direct Sow or Start in Cells

1. Prepare the soil. Choose a sunny bed with fertile, well-drained soil. Break the surface into a fine tilth and pre-water so the shallow seed zone starts evenly damp.
2. Sow about 1 cm deep. Cover the seed with fine soil or seedling mix and press gently for contact. Light is not needed until shoots emerge.
3. Keep the seed zone moderately warm. Keep the seed zone evenly moist, not saturated. Emergence usually takes less than two weeks in suitable conditions; cooler soil can take longer.
4. Thin direct sowings early. At 3–4 true leaves, keep the strongest plant at each station and snip extras so final plants stand 25–30 cm apart.
5. Move tray seedlings while young. Transplants are normally ready with 4–6 mature leaves and a well-developed but not circling root system, often after about 5–6 weeks. Harden them gradually, allowing one to two weeks as conditions require.
6. Plant at the original depth. Keep the root plug intact, set the young plant no deeper than it grew in the cell and water it in. Do not bury or earth up the stem that must swell above the soil.

Allow 25–30 cm between plants and about 40–45 cm between rows as a generous home layout for access and airflow. Leave room to inspect and harvest each plant without crowding.

Soil, Water and Feeding

• Soil: Aim for fertile, moisture-retentive but freely drained loam, with any pH correction based on a soil test. Improve very sandy soil with mature organic matter; use raised rows where heavy soil stays saturated.
• Water: Kohlrabi roots are shallow. Supply roughly 25–50 mm a week from rain and irrigation, adjusted for soil and weather, and water deeply enough to wet the root zone. Alternating drought and heavy watering makes stems tough, woody or split.
• Mulch: Once seedlings are established, apply a light clean mulch to conserve moisture. Keep it clear of the swollen stem and do not bury the plant.
• Feeding: Use a soil test where possible. Work mature compost and any required complete fertiliser into the bed before planting. If growth is pale or slow and the soil test supports it, side-dress lightly about three weeks after thinning or transplanting, then water it in. More fertiliser cannot correct heat, compaction or irregular moisture.
• Weeds and rotation: Weed early and cultivate only shallowly. Rotate away from cabbage, broccoli, cauliflower, kale and other brassicas for three to four years where practical.

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Prevent Bolting and Woody Stems

Kohlrabi prefers cool days, but “cool-season” does not mean unlimited cold. Hard frost can injure young plants. Prolonged cold can prime established plants to flower later. Sustained heat slows growth and reduces stem tenderness, so keep the crop within a reliably mild growing period.

• Sow inside the regional month window and use the earliest-maturing seed first where the remaining cool period is short.
• Harden transplants gradually and avoid holding large, stressed seedlings in trays.
• Keep moisture even, thin promptly and do not let weeds or overcrowding interrupt growth.
• If the centre begins elongating into a flower stalk, harvest immediately; bolting cannot be reversed.

Pests, Diseases and Safe Crop Care

Inspect seedlings and the undersides of leaves at least twice a week. Flea beetles make small shot holes and can destroy a young stand. Cutworms, slugs and snails remove seedlings; aphids curl leaves; cabbageworms, loopers and diamondback moth larvae chew larger holes. Brassica diseases can include black rot, downy mildew, damping-off and clubroot where susceptible crops are repeatedly grown in poorly drained or contaminated soil.

• Install fine insect mesh immediately after sowing or transplanting and seal the edges before pests enter.
• Hand-pick caterpillars and snails, remove badly affected leaves, control brassica weeds and clear old crop debris.
• Water at soil level early in the day and keep rows open for airflow, especially on the humid coast.
• Rotate brassicas and avoid moving contaminated soil between beds. Do not plant into a bed with a known clubroot problem.
• If treatment is needed, identify the pest first and use only a product registered in South Africa for kohlrabi or the edible brassica crop being treated and for the identified target pest, following the label and harvest withholding period.

Harvest the Swollen Stem at 5–8 cm

Check developing stems regularly rather than waiting for a fixed day count. Measure the rounded swollen stem, not the leaf spread. Cut the plant at soil level while the stem is 5–8 cm across, firm and uncracked. Do not wait for a giant “bulb”: these small-stem types are best harvested young, before they become fibrous and woody.

Trim the roots and leaves, refrigerate the stem in a perforated bag and use the leaves promptly. Peel any firm outer skin. The crisp white flesh can be grated raw, sliced for slaw, roasted, steamed or stir-fried; tender young leaves can be cooked like other brassica greens. Wash produce under clean running water before preparation.

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Why Germination or Stem Development Fails

• No emergence: the seed zone dried, stayed saturated, was too cold, overheated, crusted or was buried too deeply. Restore about 1 cm depth, fine tilth and steady moisture.
• Patchy stand: sowing depth or watering varied, seed washed into clumps, or pests removed emerging seedlings. Inspect before resowing.
• Seedlings collapse: the mix was waterlogged, airflow was poor or damping-off developed. Remove failures, improve drainage and use clean trays or soil.
• Transplants stall: plants were root-bound, not hardened, set too deeply or moved into hard frost or heat. Use younger plugs and keep the stem base above soil.
• Many leaves, little swelling: plants are crowded, shaded, overfed with nitrogen, growing in compacted soil or repeatedly moisture-stressed.
• Split, tough or woody stems: watering fluctuated, heat arrived, plants were left too large or growth stopped and restarted.
• Flower stalk instead of a stem: prolonged cold may have primed plants to bolt later, or the crop was stressed and left too late. Harvest what is usable and shift the next sowing window.
• Shot holes or missing seedlings: inspect for flea beetles, cutworms, slugs and snails; protect the next sowing before damage starts.
• Yellow V-shaped leaf lesions or persistent wilting: isolate the plant and investigate black rot, root disease, clubroot or drainage failure rather than adding fertiliser blindly.

Shop Kohlrabi Seeds in This Guide

• Kohlrabi Purple Vienna Seed
• Kohlrabi White Vienna Seeds

Browse more cool-season choices in our vegetable seed collection and winter vegetable seeds.