Kohlrabi is a cool-season brassica grown for the swollen, bulb-like stem that forms just above the soil line, crisp and mild, like a sweeter broccoli stem. It matures in 45 to 60 days, which makes it useful for both spring and fall plantings, and the purple and green types taste the same; the purple is prettier in the bed.
Grow it fast and steady. Sow directly a quarter inch deep a few weeks before last frost, or start indoors and transplant, 5 to 6 inches apart in rich, moist soil in full sun, and sow again in late summer for fall. It needs consistent moisture and cool temperatures to size up before it turns woody or bolts; heat and drought stress crack the bulbs and ruin the texture, so where summers run hot the fall crop is the one to count on.
Harvest the bulbs at 2 to 3 inches across by cutting the stem below the bulb; larger ones go tough and fibrous, except the giant storage types such as 'Kossak', which stay tender to softball size. The leaves are edible, cooked like kale. Flea beetles and cabbage worms are the pests, and a row cover handles both. Rabbits and deer graze it.
Clubroot
Swollen, distorted roots; wilting in midday heat; stunted growth and yellowing foliage. Plants recover poorly even with watering.
Remove and destroy infected plants with roots. Raise soil pH toward 7.2 with lime. Improve drainage and rotate out of brassicas for several years.
Black Rot
Yellow, V-shaped lesions spreading inward from leaf margins, with blackened veins. Severe cases cause wilting and leaf drop.
Remove and destroy infected plants. Use disease-free seed, rotate brassicas on a 3-year cycle, avoid overhead watering, and keep tools and beds clean.
Downy mildew
Yellowing on top of leaves with grey-purple fuzz underneath. Spreads rapidly in humid conditions.
Remove affected leaves. Improve air circulation. Avoid overhead watering. Copper spray as preventive.
Cabbage Worm
Pieris rapae →Ragged holes in leaves and green velvety caterpillars on undersides; dark green frass collects in leaf crevices and head.
Handpick caterpillars and eggs. Apply Bacillus thuringiensis (Bt) weekly during egg-laying. Use floating row cover to exclude white cabbage moths.
Flea Beetle
Alticini →Small round shot-holes scattered across leaves; tiny dark beetles that jump when disturbed. Heavy feeding stunts young plants.
Use floating row covers on seedlings. Remove crop debris and weeds that harbor beetles. Apply kaolin clay or spinosad if damage is severe.
Aphids
Aphididae →Clusters of small soft insects on new growth and flower buds.
Knock off with a strong jet of water. Ladybirds and lacewings are natural predators. Insecticidal soap as last resort.