Growing Kohlrabi

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Purple kohlrabi bulb sticks up from the soil in the garden

The above-ground swollen stem of kohlrabi is like a cross between cabbage and turnip.

Kohlrabi is an odd-looking member of the cabbage family grown for its bulb-like stem that tastes like a mild, sweet turnip. You can also eat the leaves. High in fiber and vitamin C, kohlrabi is a fast-growing, cool-season crop for both spring and fall. Plants are ready to harvest just a few weeks after planting. The key to good kohlrabi is steady growth from the start. Avoid root-bound transplants.

Set out spring transplants about 4 weeks before the last frost so they mature in cool weather. Transplants just out of a greenhouse need initial protection from freezes. Set out fall transplants about 6 weeks before the first frost. In fall, plants “hardened” by gradual exposure to cool weather are tolerant of frost. Kohlrabi that matures in cool weather is deliciously sweet.

Kohlrabi needs at least 6 hours of full sun each day; more is better. Give it fertile, well-drained, moist soil with plenty of rich organic matter. A soil pH between 6.5 and 6.8 discourages clubroot disease. To check pH, test the soil with a purchased kit, or get a soil test through your regional Cooperative Extension office. Fertilize and lime according to test recommendations.

Mulch around kohlrabi plants in the garden to promote better bulbs and keep water in the soil.

Kohlrabi needs plenty of water to form solid bulbs. Mulch around plants to help retain moisture.

Without a soil test, add nitrogen-rich amendments such as blood meal, cottonseed meal, or composted manure to the soil. Or, work a timed-release vegetable food such as 14-14-14 into the soil according to label directions. Also use liquid starter fertilizer such as fish emulsion or Bonnie Herb & Vegetable Plant Food at planting for a boost. Fertilize with a liquid fertilizer again after plants begin to develop new leaves and when they start forming heads.

Space according to directions on the Bonnie label, or about 12 inches apart; 6 to 9 inches apart is okay in intensive gardens. Exact spacing is not critical as long as you give the plants enough room to develop their swollen stems.

Kohlrabi needs an even supply of moisture to produce good bulbs. Mulch with compost, finely ground leaves, or finely ground bark to keep the soil cool and moist and to keep down weeds. Apply 1 to 1.5 inches of water per week if it doesn’t rain. You can measure the amount of water with a rain gauge.

Although the same pests that like cabbage can also attack kohlrabi, it is generally less troubled by aphids, root maggots, cabbageworms, and other caterpillars. Clubroot and black rot diseases in the soil can be a problem.

Harvest kohlrabi stems when they are still young and tender, usually about 2 1/2 to 4 inches in diameter. Harvest by cutting them from the base of the plant. You can trim the leaves from the stem and save them to cook separately. Kohlrabi keeps for 2 to 3 weeks in the fridge. You can peel and slice kohlrabi tubers to eat them raw with dips or in salad, or you can cook them like turnips. The leaves are cooked until just tender, like cabbage or turnip greens.

Is there a secret to growing great kohlrabi?

Kohlrabi grows very fast, which means plants need plenty of water and fertilizer. Keep soil moist with an organic mulch, such as straw, compost, or grass clippings. Apply mulch when plants are 6 inches high, mulching right up to the bulb. Fertilize plants every two to three weeks using a complete liquid fertilizer, such as fish emulsion.

Does kohlrabi need a lot of water?

Although established kohlrabi plants are drought-tolerant, dry soil tends to produce bitter stems. Keep soil consistently moist to harvest the best tasting kohlrabi.

How can I keep weeds under control without damaging the plants?

Kohlrabi are shallow rooted, so it’s best not to hoe deeply near the plants. If you have a heavy crop of large weeds, water the bed first to loosen soil, then hand-pull the weeds. A layer of mulch will keep weeds in check and helps soil stay moist, which yields the best-tasting kohlrabi. If you mulch with compost and a few weeds sprout, remove them when they’re small enough that dragging the hoe over soil dislodges them.

The kohlrabi I have picked so far are tough and bitter. What is wrong?

The ideal size to harvest kohlrabi is when stems are 2 inches across. Larger stems become tough and pithy, difficult to chew, and bitter tasting. Also, if plants are stressed for water they can turn tough and bitter while small. After picking, cut off roots about an inch below the thick bulb and remove leaves. Stems will store in the refrigerator for up to two months.

Can I eat the leaves of kohlrabi?

Kohlrabi leaves are very similar to turnip greens. Pick them small to toss into salads. (Just don’t harvest too many or the stems won’t grow as large.) When harvesting the plant, snip leaves from stems and cook as you would turnip greens.

They’re calling for frost tomorrow night. Do I need to pick the kohlrabi?

Frost actually enhances the flavor of kohlrabi. A hard freeze – the kind that kills all vegetation – will harm kohlrabi. If a hard freeze is imminent, go ahead and harvest the kohlrabi. But if it’s just a light frost, don’t worry. You can also use frost protection blankets (similar to floating row covers) to cover plants during heavier frosts. Frost blankets can add weeks to your growing – and harvesting – season.

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