Turnips are a fast root vegetable grown for both the swollen root and the greens, so a single sowing gives two harvests. They mature in 30 to 60 days and are a crop for early spring and fall, when the weather is cool; in summer heat they turn woody and bitter. The small white Japanese types, picked at golf-ball size, are sweet enough to eat raw.
Direct sow a half inch deep as soon as the soil can be worked, three or four weeks before last frost, and again in late summer for the fall crop, which is the sweeter one. They do not transplant, because of the taproot. Thin early to 4 to 6 inches so the roots have room, and keep the soil evenly moist to prevent splitting and tough texture. Pull the roots at 2 to 3 inches across; larger ones go to the greens.
Fall turnips can stay in the ground under mulch after frost, which improves the flavour. Flea beetles pepper the leaves in spring and root maggots tunnel the roots; a row cover from sowing stops both. Deer and rabbits graze the tops, so turnips belong inside the fence. The greens are a traditional cooked vegetable in their own right.
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.
White Rust
Raised white to cream-colored pustules on leaf undersides, with yellow blotches on upper surfaces. Infected tissue may swell or distort.
Remove and destroy infected leaves. Improve air circulation and avoid overhead watering. Rotate brassicas and avoid planting in previously infected soil.
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.
Alternaria blight
Brown circular spots with yellow halos on leaves; spreads rapidly in wet weather.
Remove affected leaves immediately. Water at the base only. A copper-based fungicide can help if caught early.
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.
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.