Kohlrabi

Kohlrabi is a fast, unusual vegetable — not a root, not a leafy green, but a swollen stem that sits at soil level and bulges outward like a small alien spacecraft. The edible part is the enlarged base of the stem, crunchy and mild, tasting something like a cross between broccoli and a water chestnut. It matures in about seven weeks from seed and tolerates cold better than most spring vegetables. The main thing most gardeners get wrong is not missing it — it's that they let it get too big.

Direct sow seeds about four weeks before your last frost. Kohlrabi germinates quickly in cool soil and the seedlings are cold-tolerant. Thin to six inches apart once plants are a few inches tall. In a small bed, succession planting every two to three weeks can extend the harvest window through late spring. For a fall crop, sow about eight weeks before your first expected fall frost.

Kohlrabi wants consistent moisture and soil with reasonable fertility. It does not need heavy feeding — too much nitrogen and the plant puts energy into leaves rather than the swollen stem. Side-dress lightly with compost if the plants look pale, but otherwise the fertility at planting is usually enough for the short season. Keep the soil evenly moist; drought stress causes the stem to become stringy and fibrous rather than crisp.

Harvest when the stem reaches about 2 to 3 inches in diameter. At this size, the texture is mild and crisp. Left to grow larger — 4 inches or more — the stem turns fibrous and woody, especially in warm weather. There is no recovering a woody kohlrabi; the texture doesn't improve with cooking. Check the garden every few days once the stems start to swell. The window from ideal to overripe is often just a week.

Bolting is the other timing problem. If temperatures spike above 80 degrees while the plant is still developing, it may send up a flower stalk rather than completing the stem. This is why kohlrabi works best as an early spring or fall crop — it does not tolerate summer heat. The Gigante variety is one exception, bred to stay tender at larger sizes and to handle warmer conditions better than most, but even it has limits.