🥦 When to Plant Kohlrabi

Kohlrabi is the underrated brassica of the spring garden. It grows fast, tastes like a mild broccoli stem crossed with an apple, and matures in 45-60 days from transplant. Plant it while the weather is cool, harvest before the bulb passes 3 inches across, and you get one of the quickest, most rewarding crops a zones 2-11 gardener can grow.

Variety Selection

Kohlrabi comes in two skin colors: pale green and purple. Both have white flesh inside with the same mild, slightly sweet flavor. The difference is mostly cosmetic, though purple-skinned varieties often hold their quality in the garden a few extra days before turning fibrous.

• Grand Duke: F1 hybrid, about 45 days. Pale green skin, mild flavor, very uniform bulbs. The most widely grown variety for spring gardens.
• Kolibri: F1, about 45 days. Deep purple skin that stays attractive in the garden longer than most types. Fast and consistent.
• Early White Vienna: Heirloom, 55-60 days. The classic pale-green open-pollinated type; well suited to seed saving.
• Early Purple Vienna: Heirloom, 55-60 days. Purple skin, same mild flavor as the white type, and widely available from seed suppliers.
• Kossak: F1, about 80 days. Bred to stay tender at a much larger size than standard varieties. Worth growing if you want more flexibility on harvest timing.

For most gardens, a fast F1 hybrid in spring and a second sowing of the same (or an heirloom type) in late summer gives a two-season harvest without any one variety dominating the bed.

Spacing, Sun, and Soil

Kohlrabi needs full sun, at least 6 hours a day. In partial shade, bulb development slows and plants stretch rather than producing a well-formed globe.

Thin or transplant to 5-6 inches apart, with rows 12-18 inches apart. Crowded plants produce small, misshapen bulbs. If you direct sow, thin to the proper spacing as soon as seedlings reach 2-3 inches tall.

Soil should be fertile, well-draining, and slightly acidic, with a pH of 6.0-6.8. Work in 2-3 inches of compost before planting. Kohlrabi is a lighter feeder than broccoli or cabbage, but it benefits from consistent nutrient availability throughout its short season. Avoid heavy nitrogen applications, which push leafy growth at the expense of the bulb.

Top Growing Tips

Kohlrabi’s main enemy is warmth. Bulbs forming during sustained heat above 75°F turn fibrous quickly. Aim to have the crop growing during the coolest part of your season, which usually means two windows: early spring and late summer.

• Start seeds indoors 4 weeks before your last frost date, or direct sow outside starting 4 weeks before last frost. Zone 5a gardeners, for example, can direct sow as early as March 28.
• The swollen globe is a stem, not a root (it forms above the soil surface, which is normal and expected).
• Harvest when the bulb is 2-3 inches in diameter. Beyond 3 inches, toughness sets in quickly.
• Kohlrabi tolerates light frost, making early planting safe in nearly every zone.
• Thin to 5-6 inches early. Crowding is the most common cause of undersized bulbs.

In zones 9-11, treat kohlrabi as a winter crop. Zone 9a gardeners direct sow January 31 for a spring harvest before heat arrives. In zone 11a, sow in early December for a late-winter crop.

Watering

Consistent moisture is the biggest factor in getting a tender, juicy bulb. Aim for about 1 inch of water per week, from rain or irrigation. Drip irrigation works particularly well because it keeps soil moisture steady without wetting foliage, which reduces disease pressure.

Let soil moisture drop and the bulb develops a fibrous, sometimes cracked interior. Oversaturation in heavy clay soil can cause root rot. Mulching around plants keeps moisture stable between waterings and suppresses the weeds that compete with kohlrabi’s shallow roots.

Companion Planting

Good companions: beets, onions, celery, cucumbers, and aromatic herbs like dill and thyme. Nasturtiums planted nearby attract aphids away from kohlrabi while drawing in beneficial insects. Dill and celery are particularly good neighbors because they encourage populations of parasitic wasps that prey on cabbage worm larvae.

Avoid planting near: tomatoes, pole beans, and strawberries. These plants have documented growth-inhibition relationships with brassicas. Keeping kohlrabi away from peppers is also a reasonable precaution in a tight bed.

Crop rotation matters more than companion choices. Planting kohlrabi (or any brassica) in the same bed two years running builds up clubroot and other soil-borne pathogens quickly. Rotate brassicas on at least a three-year cycle.

Common Problems

Aphids: Cabbage aphids cluster on the undersides of leaves and on tender new growth. A strong spray of water dislodges most infestations. For heavier pressure, insecticidal soap applied to leaf undersides is effective and breaks down quickly.

Flea beetles: Tiny black beetles that chew small round holes in seedling leaves. Row cover installed at planting prevents them almost entirely. Established plants tolerate moderate flea beetle feeding without significant yield loss.

Cabbage worms: The pale green caterpillars of the imported cabbageworm butterfly feed on leaves and can skeletonize young plants. Row cover is the easiest prevention; Bt (Bacillus thuringiensis) is an effective organic spray once you spot them.

Bolting: When temperatures climb above 75°F, kohlrabi redirects energy toward flowering instead of bulb formation. The fix is timing: harvest before heat arrives, and plant fall crops early enough to bulk up before first frost.

Woody or hollow bulbs: Almost always a harvest-timing problem. Check plants starting at day 40 from transplant. If a bulb has passed 2.5 inches, pull it regardless of whether you are ready to use it.

Harvest Timeline

Kohlrabi matures in 45-60 days from transplant, and the harvest window is narrower than most vegetables. Unlike lettuce, which you cut repeatedly over weeks, each kohlrabi plant gives one bulb.

Start checking size at day 40. A 2-inch bulb is harvestable; 2.5-3 inches is ideal. Use a knife to cut the plant at soil level. The leaves are edible and taste like kale: strip the thick midrib and cook them as you would any hearty green.

After harvest, the bed is free for a follow-on crop. Succession-sow every 2-3 weeks during the cool-season window to spread the harvest instead of getting every bulb at once.

For a fall crop, count back 60 days from your first fall frost and direct sow or transplant at that point. Fall-grown kohlrabi often tastes sweeter and milder than spring-grown because light frosts in September and October convert starches to sugars in the bulb.