Bok choy is a cool-season leafy vegetable in the cabbage family, grown for crisp white or pale green stalks and tender dark leaves, and it is the quickest of the brassicas, ready in 30 to 50 days. The baby types ('Toy Choy', Shanghai green-stem) are harvested whole at 4 to 6 inches; the full-size heads reach a foot or more. It goes into stir-fries, soups and braises and is best cooked only minutes.
It grows best in cool weather and bolts when days lengthen and temperatures climb into the upper 70s. Direct sow a quarter inch deep a few weeks before last frost, or start indoors and transplant, 6 to 12 inches apart depending on the type, in rich, moist soil in sun or part shade, and sow again in late summer for the fall crop, which is the better one; the plants take light frost and sweeten after it. Keep the soil evenly moist, since drought and heat both trigger bolting.
Harvest the whole head by cutting at the base, or take the outer stalks and let the centre grow on. Flea beetles pepper the leaves in spring and cabbage worms arrive in summer; a row cover from sowing handles both. Slugs like the stalks. Rabbits and deer graze it, so it belongs inside the fence. If a plant does bolt, the yellow flowers are edible and the bees take the rest.
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.
White Rust
White, blister-like pustules on the undersides of leaves with yellow spots on the upper surface; severe infections distort leaves and stems.
Remove and destroy infected leaves, improve air circulation, avoid overhead watering, and rotate brassicas to a new bed each year.
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.