Because it’s a cool weather crop, you’ll see your best results growing broccoli in the spring or fall. Florets that mature in autumn when nights turn chilly taste the sweetest.
Broccoli prefers full sun, but partial shade can prevent plants from bolting (going to seed) in areas with warm spells. Provide a rich, well-drained soil, with plenty of compost.
Cool days and nights are essential once the flower heads start to form. There’s a wide range of days to maturity, so pick a cultivar that will mature before the weather in your area turns hot or use a heat tolerant variety. Gardeners in most temperate areas can harvest both spring and fall crops.
Set the young plants 1 to 2 inches deeper in the garden than they grew in the pots or flats. Space them 1 to 2 feet apart in rows 2 to 3 feet apart. Closer spacing will produce smaller heads. Firm the soil and water well.
Protecting young broccoli plants from temperature extremes is also critical for a successful crop. A prolonged period of nights around 30°F and days in the 50° to 60°F range can produce tiny, immature heads called buttons. To prevent this, protect plants with cloches or row covers during cool weather. Unexpected warm spells can cause the heads to open too soon.
For fall crops, you can start seedlings indoors or sow seeds directly in the ground in July or August. In mild-winter climates, plant in the late fall for a spring harvest.
The trick to producing good broccoli is to keep it growing steadily and well fertilized. Two to three weeks after transplanting, topdress with compost tea or side-dress with blood meal or fish emulsion, and water deeply. Repeat monthly until a week before harvesting the flower head. This regimen also encourages large and tender side shoots, which you can harvest until hot weather or a heavy ground freeze ends the broccoli season.
Cultivate around young plants to get rid of weeds and keep the soil loose. When daytime temperatures exceed 75°F, put down a thick layer of organic mulch to cool the soil and conserve moisture. Broccoli needs 1 to 1½ inches of water a week. A lack of water will result in tough stems, so soak plants extra well during dry spells. A fall crop of broccoli needs steady (but slightly less) water.
Of all cabbage-family plants, broccoli is often the least affected by pests, and fall crops tend to have fewer problems than spring ones. Possible pests include aphids, cabbage loopers, imported cabbageworms, cabbage maggots, cutworms, and flea beetles.
Diseases are seldom a problem. Black leg produces dark spots on leaves and stems. Symptoms of black rot include yellowing leaves and dark, foul-smelling veins. Prevent these diseases with good cultivation and crop rotation. In case of club root, which shows up as weak, yellowed plants with deformed roots, destroy the infected plants. Plant your next crop in another part of the garden, and before planting, apply lime to boost soil pH to about 7.0.
Leaf spot shows up as enlarging, water-soaked spots that turn brown or purplish gray. Fusarium wilt, also known as yellows, causes lower leaves to turn yellow and drop off and makes broccoli heads stunted and bitter. Destroy plants afflicted with leaf spot or Fusarium wilt to prevent these diseases from spreading.
Harvest before the florets start to open and turn yellow. Cut just below the point where the stems begin to separate. Once you’ve harvested the main head, tender side shoots will form in the leaf axils all along the lower stalk. Keep cutting, and broccoli will keep producing until the weather turns too hot or too cold. You can, freeze, or pickle broccoli, or keep it refrigerated for up to 2 weeks. Green cabbage loopers and imported cabbageworms often go unnoticed on harvested heads and can end up in your cooked broccoli. To prevent this, drive them out by soaking the heads in warm water with a little vinegar added for 15 minutes before cooking.