Kohlrabi, a unique member of the Brassica family that includes cabbage and broccoli, is grown for its bulbous, edible stem that forms just above the soil line. Known for its mild, slightly sweet flavor and crisp texture, this cool-season vegetable is straightforward to cultivate from seed. Success in growing kohlrabi relies primarily on precise timing, consistent moisture, and protection from common brassica pests.
Timing and Starting Your Seeds
Kohlrabi thrives in cool weather; high temperatures cause the bulb to become tough and woody, making timing crucial. Gardeners aim for two main planting windows: an early spring crop for summer harvest and a mid-summer planting for fall. For the earliest start, sow seeds indoors four to six weeks before the last expected frost date.
Start seeds in a cool room (60°F to 70°F) under bright light to prevent leggy growth. Sow seeds shallowly (one-quarter to one-half inch deep) in cell trays. For the spring crop, you can also direct sow seeds into the garden two to four weeks before the last frost, once the soil is workable.
The fall crop is often considered more reliable because the plants mature as the weather cools, which maximizes sweetness and tenderness. To calculate the planting date for fall, count back 90 days from the date of your first expected fall frost. Direct-sown seeds should be placed about one inch apart initially, with a final planting depth of one-quarter to one-half inch.
Transplanting and Essential Growing Conditions
Once indoor-started seedlings develop four to six true leaves and the danger of hard freeze passes, transplant them into the garden. Final spacing between plants should be six to nine inches to allow the bulbs adequate room to swell. This spacing applies whether transplanting or thinning direct-sown rows.
Kohlrabi requires rich, well-draining soil high in organic matter to support rapid growth. An ideal soil pH is slightly acidic to neutral (6.0 and 7.5), which helps prevent diseases like clubroot. As a moderate feeder, the plant benefits from a balanced fertilizer worked into the soil before planting.
Consistent moisture is paramount, as moisture stress is the primary cause of tough, woody, or cracked bulbs. Aim to provide about one to one-and-a-half inches of water per week, ensuring the soil remains evenly moist but not waterlogged. A steady supply of water is especially important during the period when the stem begins to swell above the soil line. Applying an organic mulch, such as straw or shredded leaves, once the plants are established helps to conserve soil moisture and regulate temperature around the developing bulbs.
Protecting Your Kohlrabi from Pests
Kohlrabi is susceptible to several common pests that damage leaves and stunt growth. The imported cabbageworm and cabbage looper leave holes in the leaves and are identifiable by their green droppings. Flea beetles are another nuisance, particularly to young seedlings, as they chew tiny “shot-holes” that can severely weaken the plant.
The most effective preventative measure is using floating row covers, which create a physical barrier against adult insects like the white cabbage butterfly. Place row covers over the plants immediately after transplanting or thinning. For existing caterpillar infestations, manual removal is effective, or apply Bacillus thuringiensis (Bt), a soil bacterium that targets and eliminates the larvae.
For soft-bodied pests like aphids, which cluster on the undersides of leaves and can stunt growth, a strong jet of water can dislodge them from the plant. Alternatively, a gentle application of neem oil or insecticidal soap can manage a mild infestation without relying on harsh chemical treatments. Maintaining a healthy, well-fed plant is also a strong defense, as stressed plants are often more vulnerable to pest damage.
When and How to Harvest
Correct timing is essential to guarantee crisp, sweet flavor. Harvest the enlarged stem (the edible part) when it reaches two to four inches in diameter, roughly the size of a tennis ball. Allowing bulbs to grow larger risks them becoming tough, fibrous, and woody.
Best quality kohlrabi results from fast, uninterrupted growth, so monitor the swelling stem closely. To harvest, use a sharp knife or garden shears to cut the stem just below the bulb, right at the soil line. This method leaves the root system in the ground.
The leaves are also edible and can be used as cooking greens, similar to collards or kale. Snip a few outer leaves for consumption at any point without harming the developing bulb. Avoid removing too many leaves, which could slow the plant’s growth.