Kohlrabi

Kohlrabi (Brassica oleracea var. gongylodes) matures in 45–60 days - faster than any brassica except radish. It occupies a productive window in spring and fall when most summer crops can’t yet go in the ground, and it uses that window efficiently. Fresh kohlrabi at retail runs $2–$4/lb; at farmers markets, purple varieties and large specialty kohlrabi fetch $3–$5/lb (USDA AMS Market News, 2023).

What it actually is

Kohlrabi is not a root vegetable. The swollen structure you eat is a modified stem - the stem of the plant enlarged just above the soil line. This distinction matters for growing because it means compaction and rocky soil affect kohlrabi much less than they affect carrots or beets. The edible portion grows above ground.

Both the swollen stem and the leaves are edible. The leaves are nutritionally dense - similar in profile to kale, which is the same species - and can be cooked exactly like kale. Most gardeners eat the leaves as a bonus harvest while waiting for the bulb to size up.

Cultivar choices are limited but important. White Vienna and Green Vienna are the standard green types, fast-growing, mild flavor. Purple Vienna grows at the same rate but develops anthocyanin pigmentation on the skin; the flesh is still white. Purple types have stronger farmers market appeal. Kossak and Gigante are large-bulb varieties that can reach 10 inches in diameter without becoming woody - useful if you want a later, larger harvest.

The ROI case

A $2.49 packet contains 200–300 seeds. Direct sow in a 10-foot row at 4 inches apart and you get 30 plants. At 0.5–1 lb per bulb (Penn State Extension, Kohlrabi, 2021), that’s 15–30 lb per 10-foot row. At $2.50/lb, that’s $37.50–$75 per 10-foot row from a $2.49 seed packet.

The real advantage is timing. You can run two kohlrabi crops in a season - one in spring (sow 6 weeks before last frost, harvest by early summer) and one in fall (sow 8 weeks before first frost, harvest after light frosts improve flavor). That doubles the value from the same bed footprint.

Growing requirements

Kohlrabi is a cool-season crop. Seeds germinate at soil temperatures as low as 40°F and growth is best between 60–75°F (University of Illinois Extension, Watch Your Garden Grow: Kohlrabi, 2020). At temperatures consistently above 80°F, the bulb becomes tough and pithy and the plant may bolt. Time plantings to finish before summer heat arrives.

Direct seeding is preferred. Sow seeds 0.25 inch deep, spacing 1–2 inches apart, then thin to 4–6 inches when plants are 3–4 inches tall. Wider spacing (6 inches) produces larger bulbs; tighter spacing (4 inches) is acceptable if you prefer smaller, more tender kohlrabi.

Soil pH 6.0–6.8. Consistent moisture is critical to prevent woody development. The bulb becomes pithy and tough if the plant experiences water stress during development. Mulch 2 inches around plants to buffer soil moisture; kohlrabi doesn’t compete well with weeds, and cultivation near the shallow stem base can damage plants.

Side-dress with a balanced fertilizer 3 weeks after sowing. Kohlrabi is a light to moderate feeder - excess nitrogen encourages leafy growth at the expense of bulb development.

What goes wrong

Cabbage aphid (Brevicoryne brassicae) forms dense gray-green colonies on leaf undersides and inside forming bulbs. A hard blast of water disrupts colonies; insecticidal soap for persistent infestations. Aphid pressure in fall crops is often heavier than spring crops.

Imported cabbageworm (Pieris rapae) is the larva of the white cabbage butterfly. Pale green caterpillars feed on leaves; heavy infestations can defoliate small plants. Row cover is the cleanest preventive measure. Bacillus thuringiensis var. kurstaki (Bt) applied to foliage kills young larvae on contact.

Clubroot (Plasmodiophora brassicae) is a soilborne pathogen causing gall-like swellings on roots and wilted, yellow plants. Infected plants are stunted and produce poorly. Clubroot persists in soil for 20+ years. Do not plant brassicas in infected soil; raise soil pH to 7.0–7.2, which reduces but does not eliminate the pathogen. Purchase transplants from certified disease-free sources.

Splitting happens when the bulb grows too large - typically over 3 inches in diameter for standard varieties. The bulb also becomes woody. Harvest on time.

Harvest and storage

Harvest kohlrabi when the swollen stem is 2–3 inches in diameter for standard varieties. At this size the flesh is crisp, sweet, and tender. Let it go beyond 3 inches and the texture becomes woody and fibrous. The exception is large-bulb varieties like ‘Kossak,’ which are specifically bred to stay tender at larger sizes.

Cut at the soil line. Leaves and stem are both edible - store the leaves separately and use them within 2–3 days. The bulb keeps refrigerated for 2–3 weeks. For longer storage, peel, cube, and blanch 3 minutes before freezing.

Raw kohlrabi sliced thin is one of the better raw vegetable preparations in a brassica. The flavor is mild - somewhere between broccoli stem and apple. It doesn’t need cooking.

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Related crops: Arugula, Kale

Related reading: Succession Planting Calendar - how to schedule spring and fall kohlrabi crops around your summer plantings