Broccoli is treated much as cabbage. Grow broccoli as rapidly as possible. Give broccoli plenty of moisture and be sure to feed it through the season–a planting bed amended with aged compost is an important start. While broccoli is hardy at maturity, young plants should not be subjected to frost.
For broccoli growing tips see How to Grow Broccoli or Broccoli Growing Success Tips at the bottom of this post.
Here are common broccoli growing problems with cures and controls:
• Seedlings fail to emerge from soil; seedlings are eaten; roots are tunneled. Cabbage maggot is a small gray-white, legless worm to ⅓-inch long; adult is the cabbage root fly, looks like a housefly. Flies lay eggs in the soil near the seedling or plant. Maggots will tunnel into roots leaving brown scars; some plants may be honeycombed with slimy tunnels. Exclude flies with floating row covers. Remove and dispose of damaged plants. Apply lime or wood ashes around the base of plants; time planting to avoid insect growth cycle. Plant a bit later when the weather is drier.
• Seeds rot or seedlings collapse with dark water-soaked stems as soon as they appear. Damping off is a fungus that lives in the soil, particularly where humidity is high. Do not plant in cold, moist soil. Make sure soil is well drained.
• Seedlings are eaten or cut off near soil level. Cutworms are gray grubs ½- to ¾-inch long that can be found curled under the soil. They chew stems, roots, and leaves. Place a 3-inch paper collar around the stem of the plant. Keep the garden free of weeds; sprinkle wood ash around base of plants.
• Young sprouts fail to grow or die back; bluish-black spot on leaves and stems. Blackleg is a fungal disease which causes sprouts to girdle and rot at soil level–“blacklegs.” Blackleg is spread by cutworms and cabbage maggots. Remove and destroy infected plants; keep the garden free of plant debris. Add organic matter to planting bed; make sure soil is well-drained. Rotate crops.
• Small scattered heads form; mature heads do not. Young plants will form small heads prematurely if temperatures fall below 40°F shortly after planting. Protect young plants with hot kaps or floating row covers.
• Young plants flower. Cold will cause young plants to prematurely flower and produce seed without forming a head. Protect young plants from cold weather with floating row covers; set transplants into the garden no sooner than 1 to 3 weeks before the last average frost date in spring.
• Irregular yellowish to brownish spots on upper leaf surfaces; grayish powder or mold on undersides. Downy mildew is caused by a fungus. Improve air circulation. Plant resistant varieties. Rotate crops. Keep garden free of plant debris.
• Plant wilts; roots are swollen and misshapen, roots rot. Clubroot is a soilborne fungal disease. Fungus interferes with water and nutrient uptake of roots. Keep the garden clean of plant debris and weeds that can harbor the fungus. Remove and destroy infected plants including soil around roots. Clubroot is found in acid soils; add lime if soil pH below 7.2. Rotate crops for at least 2 years. Purchase transplants from disease-free supplier.
• Leaves become dull yellow, curl, and plant may die. Cabbage yellows is caused by the Fusarium soil fungus that infects plants usually where the soil is warm. The disease is spread by leafhoppers. Remove infected plants. Control leafhopper. Keep the garden free of weeds which can harbor disease. Keep soil evenly moist, but not wet. Rotate crops.
Plant resistant varieties: Early Snowball.
• Leaves yellow; plant stunted; small glistening white specks on roots. Root cyst nematode is a microscopic worm-like animal that lives in the film of water that coats soil particles. Rotate cabbage family crops. Solarize the soil with clear plastic in mid-summer.
• Plant stunted; worms tunnel into roots. Plump grayish grub with brown head is the larva of the June beetle, a reddish brown or black hard-shelled beetle to 1 inch long. Wireworms are the soil-dwelling larvae of click beetles; they look like wiry-jointed worms. Check soil before planting; hand pick and destroy pests; flood the soil if wireworms are present. Remove infested plants and surrounding soil. Keep the garden free of refuse that could shelter beetle eggs.
• Leaves are yellowish and slightly curled with small shiny specks. Aphids are tiny, oval, whitish-green, pink, or black pear-shaped insects that colonize on leaves. They leave behind sticky excrement called honeydew which can turn into a black sooty mold. Remove with a blast of water. Use insecticidal soap solution. Mulch with aluminum foil to disorient aphids.
• Leaves have whitish or yellowish spots; leaves are deformed; plant wilts. Harlequin bugs or stink bugs. Harlequin bugs are black with bright red yellow or orange markings. They suck fluids from plant tissue causing white and yellow blotches. Handpick and destroy bugs and egg masses. Keep garden free of crop residue and weeds where bugs breed. Spray plants wit Sevin, pyrethrum and rotenone. Stink bugs are gray or green shield-shaped bugs about ¼-inch long; they feed on fruits. Remove garden debris and weeds where bugs can overwinter. Hand-pick egg masses and bugs and destroy.
• Tiny shot-holes in leaves of seedlings. Flea beetles are tiny bronze or black beetles a sixteenth of an inch long. They eat small holes in the leaves of seedlings and small transplants. The larvae feed on roots of germinating plants. Spread diatomaceous earth around seedling. Handpick off plants, Cultivate often to disrupt life cycle. Keep garden clean. Spade garden soil deeply to destroy larvae in earl spring. Treat plants with Sevin, pyrethrum, or rotenone.
• Leaves partially eaten; leaves webbed together; eggs in rows on undersides of leaves. Cabbage webworms are green with a light stripe to ¾ inches long; the webworm is the larvae of a brownish yellow moth with gray markings. Larvae spin light webs. Clip off and destroy webbed leaves. Destroy caterpillars. Keep garden weed free.
• Leaves are eaten and plants are partially defoliated. Blister beetles are slender gray or metallic black beetles to ¾-inch long; they may have striped spots on their wings. Handpick beetles and destroy. Keep the garden weeds and debris. Cultivate in spring to kill larvae and interrupt the life cycle. Spray or dust with Sevin or use a pyrethrum or rotenone spray.
• Large holes in leaves; leaves skeletonized. Cabbage loopers or armyworms. (1) Cabbage looper is a light green caterpillar with yellow stripes running down the back; it loops as it walks. Keep garden clean of debris where adult brownish night-flying moth can lay eggs. Cover plants with spun polyester to exclude moths. Pick loppers off by hand. Use Bacillus thuringiensis. Dust with Sevin or rotenone. (2) Armyworms are dark green caterpillars the larvae of a mottled gray moth with a wingspan of 1½ inches. Armyworms mass and eat leaves, stems, and roots of many crops. Armyworms will live inside webs on leaves. Handpick caterpillars and destroy. Cultivate after harvest to expose the pupae. Use commercial traps with floral lures.
• Leaves are chewed and slimed. Snails and slugs eat leaves. Collect these pests at night. Set beer traps at soil level to attract and drown snails and slugs.
• Leaves chewed. Imported cabbage worm is a pale green caterpillar with yellow stripes to about 1¼ inches long; the adult is a white moth with two or three black spots on the forewing. Use Bacillus thuringiensis. Destroy all remains and weeds after harvest. Companion plant with mint. Encourage the predatory trichogramma wasp.
• Leaves and head become pale green; leaves wilt; slimy rot develops in stem, leaves, and head. Bacterial soft rot is caused by Erwinia bacteria. Water-soaked spots appear on leaves and roots; spots enlarge and turn dark and mushy. Black ooze develops in cracks in roots and stems. Rot can not be cured. Collect and burn infected plants Promote good drainage by adding aged compost and organic materials to planting beds. Avoid over-head watering. Rotate crops.
• Plant stops producing heads or buds; older buds flower. Harvest heads regularly, at least every 3 days. If buds are allowed to flower, the plant will stop producing new heads.
• Plant suddenly flowers. Warm temperatures over 85°F will trigger flowering, small yellow flowers bloom. Plant earlier so that plant matures before heat. Plant mid-summer for a fall crop; plant matures in cool weather. Plant early-maturing varieties: Green Comet Hybrid, Spartan Early, Premium Early.
Broccoli Growing Success Tips:
Planting. Broccoli is a cool-season crop. Plant broccoli in full sun where temperatures do not exceed 80°F. Where temperatures are warmer, grow broccoli in partial shade. Grow broccoli in well-drained soil rich in organic matter. Add aged compost to the planting bed before planting. Broccoli can be sown directly in the garden but is best started indoors where it can be protected from early temperature fluctuations and pests.
Planting time. Sow broccoli indoors as early as 6 to 8 weeks before the average last frost date in spring. Set plants in the garden as early as 2 weeks before the expected last frost. Plant early: warm weather in late spring will cause plants to bolt and flower. As well, weather too chilly in spring–just three or four days below 50°F–will cause broccoli to form button-like flower heads that will never develop. Do not plant out broccoli until the weather is settled. For a fall crop, sow broccoli in the garden 10 to 12 weeks before the average first frost in fall. Protect mid-summer planted broccoli from the heat by planting between taller crops such as tomatoes or corn. In late fall, use floating row covers to protect maturing broccoli from temperatures in the 20°sF.
Care. Keep broccoli evenly moist; do not allow the soil to dry out. Side dress broccoli with compost tea about 2 weeks after transplanting into the garden. Later, side dress plants with aged compost when the main flowerhead begins to form.
Harvest. Broccoli will be ready for harvest soon after flower heads are 1 inch in diameter. Once the main flower head is harvested, broccoli will produce side shoots for up to 3 months. (For a sustained harvest, space broccoli plants at least 24 inches apart at planting time.)
More tips at How to Grow Broccoli.