Kohlrabi

An unusual brassica forming a swollen stem above ground with a mild, crisp flavor similar to broccoli stems. Grow in cool weather with consistent moisture for the most tender bulbs, harvesting when they reach two to three inches in diameter. Both green and purple varieties exist, and the leaves are also edible and nutritious when cooked like collard greens. Kohlrabi is excellent eaten raw in slaws and salads, roasted, or added to stir-fries for a sweet, mild crunch.

Kohlrabi can be direct-sown or started indoors, and both methods work well given the plant's adaptable nature. For direct sowing, plant seeds 1 cm (1/2 inch) deep and 5 cm apart as soon as soil can be worked in spring — seeds germinate in 5-10 days at 10-30°C (50-85°F), with the fastest germination around 20°C (68°F). For an earlier harvest, start seeds indoors 4-6 weeks before the last frost in cell trays filled with a standard seed-starting mix. Transplant outdoors when seedlings have 3-4 true leaves, spacing them 15-20 cm apart.

Kohlrabi tolerates light frost down to about -5°C (23°F), so early spring transplanting is safe and even beneficial — cool weather produces the sweetest, most tender bulbs. For a fall crop, direct-sow 6-8 weeks before the first expected frost. Fall plantings are often more successful than spring ones in warm climates because the crop matures during increasingly cool weather rather than racing against rising summer heat. Succession plant every 2-3 weeks for continuous harvest throughout the season.

Kohlrabi does not develop a deep taproot, so transplanting is straightforward and seedlings recover quickly from the move. Harden off indoor-started seedlings for 5-7 days before transplanting by gradually exposing them to outdoor conditions. Seeds remain viable for about 4 years when stored in a cool, dry place. For the earliest possible spring harvest, start seeds indoors in late winter under grow lights and transplant under row covers as soon as the ground thaws — kohlrabi's frost tolerance makes this an achievable strategy even in cold climates.

Kohlrabi is one of the fastest-growing brassicas and one of the most unusual-looking vegetables in the garden, forming a swollen, bulb-like stem just above the soil surface. It thrives in cool weather between 7-24°C (45-75°F) and matures in as little as 45-60 days, making it an excellent crop for spring and fall planting. Sow seeds directly 1 cm (1/2 inch) deep and 5 cm apart in rows spaced 30 cm apart, or start transplants indoors 4-6 weeks before the last frost for an earlier harvest.

Thin seedlings to 15-20 cm apart once they develop their first true leaves. Keep the soil consistently moist, as drought stress causes the bulb to become tough and woody. Mulch around plants to maintain even soil moisture and moderate soil temperature. Kohlrabi has relatively shallow roots, so avoid deep cultivation near the plants. A balanced fertilizer applied at planting provides adequate nutrition for this fast-growing crop.

For spring planting, sow as soon as soil can be worked — kohlrabi tolerates light frost. For fall crops, sow 6-8 weeks before the first expected frost. Fall-grown kohlrabi is often sweeter than spring crops due to cool maturation temperatures. Harvest when bulbs reach 5-8 cm (2-3 inches) in diameter for the most tender, crisp texture. Giant varieties like Superschmelz can grow to the size of a softball while remaining tender, but standard varieties become woody if left too long.

Beets are excellent kohlrabi companions, as they occupy different soil depths and do not compete for nutrients. Onions planted nearby help deter cabbage worms and flea beetles with their pungent scent. Cucumbers benefit from the partial shade kohlrabi provides at their base during hot weather. Avoid planting near tomatoes, which are allelopathically incompatible with brassicas and may stunt kohlrabi growth. Keep away from strawberries, which compete for similar nutrients and space. Lettuce and spinach make good interplanting partners, harvested before kohlrabi bulbs need full space.

Kohlrabi grows best in fertile, well-drained soil with a pH of 6.0-7.5. Work 5-8 cm of well-aged compost into the top 20 cm of soil before planting to provide both nutrients and moisture retention for the fast growth rate. Apply a balanced fertilizer (10-10-10) at planting at approximately 60 grams per square meter. Sandy soils benefit from extra compost to improve water-holding capacity, while heavy clay soils should be amended with compost and coarse sand or grown in raised beds to ensure adequate drainage.

Kohlrabi is a moderate feeder and generally does not need side-dressing given its fast 45-60 day maturity. Avoid excess nitrogen, which can cause the bulb to crack or develop hollow, pithy centers. Calcium is critically important for cell wall structure and bulb integrity — lime the soil if pH is below 6.0, and consider a calcium foliar spray during rapid bulb expansion if cracking has been a problem in past seasons. Boron deficiency, while uncommon, can cause brown, corky patches inside the bulb — a light application of borax (1 gram per square meter) at planting prevents this.

Kohlrabi benefits from consistent moisture more than heavy feeding, so prioritize mulching and irrigation over additional fertilization. Apply 5-8 cm of straw, shredded leaves, or grass clippings around plants once they are established to maintain even soil moisture and suppress weeds. If growth appears sluggish midseason, a light side-dressing of compost tea or dilute fish emulsion provides a gentle nutrient boost without the risk of excessive nitrogen. Avoid fresh manure, which can introduce weed seeds, burn shallow roots, and promote lush leaf growth at the expense of bulb development.

Ideal Temperature

Hardiness Zone Compatibility

Harvest kohlrabi when the swollen stem reaches 5-8 cm (2-3 inches) in diameter — about the size of a tennis ball. Standard green and purple varieties become woody and fibrous if allowed to grow much larger, so check plants regularly once bulbs begin forming. Cut the stem at soil level with a sharp knife rather than pulling, which can disturb neighboring plants. The ideal harvest window is short for standard varieties — perhaps only 5-7 days between perfectly tender and unpleasantly woody — so daily monitoring is worthwhile once bulbs start sizing up.

The leaves are also edible and nutritious when young and tender — cook them like collard greens, add small ones to stir-fries, or blend into smoothies for a mild brassica boost. Harvest leaves selectively while the bulb is still developing, taking only one or two outer leaves at a time to avoid reducing the plant's photosynthetic capacity. Giant varieties like Superschmelz can be harvested at much larger sizes (up to 25 cm across) while retaining remarkably good texture, making them ideal for gardeners who prefer fewer, larger harvests.

For the crispest texture and mildest flavor, harvest in the morning after dew has dried but before the heat of the day. Fall-grown kohlrabi exposed to light frost actually develops a sweeter flavor as the plant converts starches to sugars in response to cold — a phenomenon shared with many brassicas. Succession plantings every 2-3 weeks extend the harvest window from a single concentrated picking to weeks of continuous supply. Pull or cut the entire plant at harvest rather than trying to leave the root in place, as kohlrabi does not resprout from the base.

Storage & Preservation

Remove leaves immediately after harvest — they draw moisture from the bulb and accelerate wilting — and store them separately in a damp paper towel inside a plastic bag for up to 5 days. Whole, unwashed bulbs keep in a perforated plastic bag in the refrigerator crisper drawer for 2-3 weeks. For extended root cellar storage, layer unwashed bulbs in damp sand or peat moss at 0-2°C (32-35°F) and 90-95% humidity, where they can keep for 1-2 months while maintaining their crisp texture.

Kohlrabi freezes well for long-term preservation. Peel and dice bulbs into 1-2 cm cubes, blanch in boiling water for 2 minutes, immediately cool in ice water to halt cooking, drain thoroughly, and spread on a parchment-lined baking sheet to flash-freeze before transferring to labeled freezer bags. Frozen kohlrabi keeps for up to 10 months and works best in cooked dishes like soups, stews, and gratins — the crisp raw texture is lost after freezing, so plan accordingly.

Kohlrabi can also be lacto-fermented like sauerkraut for a tangy, probiotic-rich preserve that keeps for months in the refrigerator. Peel and shred or thinly slice the bulbs, toss with 2% salt by weight, pack tightly into a jar, and allow to ferment at room temperature for 5-7 days. Pickled kohlrabi in a vinegar brine with mustard seeds, dill, and garlic makes an excellent condiment. Dehydrating thin slices at 57°C (135°F) for 6-8 hours produces crispy kohlrabi chips that store in airtight containers for months — a healthy, addictive snack seasoned with sea salt or smoked paprika.

Troubleshooting Common Problems

Woody, fibrous bulbs are the most common issue — caused by harvesting too late, heat stress, or drought. Always harvest standard varieties at 5-8 cm diameter and grow in cool seasons. The single best prevention is daily monitoring once bulbs start sizing up, because the window between perfectly tender and unpleasantly tough can be as short as 5-7 days in warm weather. Growing giant varieties like Superschmelz largely eliminates this problem, as they remain tender at much larger sizes.

Cracked or split bulbs result from irregular watering — sudden heavy rain or irrigation after a dry spell causes rapid expansion that splits the skin. Maintain consistent moisture with drip irrigation and mulch. Bolting (going to seed without forming a proper bulb) occurs when young transplants are exposed to prolonged cold below 10°C (50°F) followed by warmth, a process called vernalization. Avoid setting out transplants too early in spring, or use row covers to buffer temperature swings. Direct-sown kohlrabi is less prone to bolting than transplants that experienced cold stress.

Hollow or pithy centers develop from excessive nitrogen fertilization or wide temperature fluctuations between day and night. Slow growth and poor bulb formation usually indicate overcrowding — thin seedlings to 15-20 cm apart early and do not delay. Purple discoloration of leaves in cool weather is typically a temporary phosphorus uptake issue that resolves as soils warm and is not cause for concern. Root maggots can occasionally tunnel into the bulb base — floating row covers at planting provide the best prevention.

Growing Tips

1. Timing is everything with kohlrabi. Plant in early spring or late summer for a fall harvest — avoiding the heat of midsummer, which causes tough, woody bulbs and triggers bolting. Fall crops generally produce sweeter, more tender bulbs than spring ones.
2. Thin seedlings early and ruthlessly to 15-20 cm (6-8 inches) apart. Overcrowded kohlrabi produces small, misshapen bulbs that never reach their full potential. Use thinned seedlings as microgreens — they have a mild, pleasant brassica flavor.
3. Consistent moisture is the single most important factor for tender, crack-free bulbs. Use drip irrigation or soaker hoses and mulch heavily with straw or shredded leaves to maintain even soil moisture throughout the growing period.
4. Harvest standard varieties promptly at 5-8 cm (2-3 inches) diameter — do not wait for them to grow larger. The window between perfectly tender and unpleasantly woody is very short. If you want large bulbs, grow Superschmelz instead of leaving standard varieties in the ground too long.
5. Floating row covers are your best friend for kohlrabi. They protect against flea beetles and cabbage worms from day one, eliminating the two most common pest problems without any spraying. Leave covers on from planting through harvest.
6. Succession plant every 2-3 weeks to avoid a glut of kohlrabi all maturing at once. Stagger sowings from early spring through late summer for a continuous supply of perfectly sized bulbs from May through November.
7. Do not discard the leaves — they are nutritious and delicious. Young kohlrabi leaves can be sautéed with garlic and olive oil, added to soups, or used anywhere you would use collard greens or kale. They are higher in calcium and iron than the bulb itself.
8. Kohlrabi is one of the best vegetables for container growing. Its compact size, shallow roots, and fast maturity make it ideal for pots at least 20 cm (8 inches) deep. Grow 3-4 plants per 40 cm container for a satisfying harvest on a balcony or patio.