Broccoli is a cool-season brassica that wants cold nights and steady moisture; heat makes it bolt and button before it forms a proper head. The central head is the main crop, but the side shoots that follow give smaller heads for weeks, which is where most of the harvest from a home garden comes from.
Start seed indoors six to eight weeks before last frost and transplant two to four weeks before it, 18 to 24 inches apart in rich soil with compost worked in; it tolerates frost and improves in flavour after a cold spell. For a fall crop, which is usually the better one, count back 90 days from first frost and start transplants then. Keep it evenly watered and side-dress with compost when the heads start to form. Harvest the central head while the buds are tight and before any yellow shows.
Cabbage white caterpillars, cabbage loopers and flea beetles are the pests; a row cover from transplanting until the heads form keeps all three off without spraying. Rotate brassicas to a new bed each year to stay ahead of clubroot. Let a late plant flower and the yellow blooms are covered in bees. Deer and rabbits graze broccoli, so it belongs inside the fence.
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.
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.
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.
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.
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.