If you thought broccoli was a once-a-year, one-shot affair, think again. Just about any gardener in the United States can harvest crops two, or even three times a year. How? In most areas, by sowing seed in July. For that third harvest of the year, gardeners in zones US Hardiness Zone 7b through 9a can sow certain varieties again in late summer for an overwintering crop. You'll find all the whys and wherefores later in the article, but you'll have to know first what zone you live in.
Standard broccoli planting strategy calls for sowing seed in spring. Specifically, sow seeds indoors 6 to 8 weeks before the last spring frost and transplant into the garden 3 to 4 weeks later for an early summer harvest. While this schedule is fine in most regions, only gardeners in northern, short-season zones 3 to 4a are limited to it. These gardeners, in cities such as Bismarck, Minneapolis, and Montpelier, Vermont, should choose any of the standard southern European varieties from the list below.
Gardeners in zones 9b and 10 are similarly limited, but to the opposite season. Cool spring is too short, and summer is too hot in zones 9b and 10 for broccoli. In cities such as Houston, New Orleans, Phoenix, and Orlando, sow seed in fall for an early spring harvest. Choose any variety from the list below.
If you live in zones 4b to 6a: Although the cold temperatures and shorter days of fall come quickly in the North, you can harvest an early summer crop from a late spring planting; or harvest in September and October from an early summer (June or July) planting. For the latter, choose a variety that matures quickly and that has good frost tolerance, such as 'Green Valiant', in case there's an early frost.
Start seeds indoors or direct sow into the garden 10 to 12 weeks before the first expected frost in your area. If you buy transplants, plant them 6 to 8 weeks before the first frost. For best germination when direct seeding, sow seeds about 3/4 to 1 inch deep, keep the soil moist and shaded. Whether you start with seeds or transplants, planting in midsummer means heat is likely to stress young broccoli plants. Provide afternoon shade until harvest by erecting shade cloth, or grow broccoli plants in north-south rows on the east side of tall summer crops, such as pole beans and sweet corn.
If you live in zones 6b to 7a: Summer comes quicker in these regions, so gardeners here are better off forgoing the spring crop and planting only for a fall crop following the planting schedule outlined in the previous section.
To stagger the fall harvest, plant broccoli blends (such as George's Favorite Blend, which mixes three or four varieties together) or make your own blend by planting some early, midseason, and late-maturing varieties simultaneously. This way you can stretch your harvest season from October to perhaps January.
If you live in zones 7b to 9a: Gardeners in these zones can grow any of the standard broccolis for a fall harvest (again following the directions above) as well as the northern European overwintering varieties of broccoli. Transplanted in late summer these varieties grow slowly through the winter and head up in March to May.
NG test gardeners planted these overwintering broccolis last fall and several reported good results to temperatures as low as 10° F. And Tim Peters at Peters Seed and Research Company tells me that one variety, 'Spring Green Mix', is hardy to 0° F.
Most of these genetically distinct strains of broccoli look like familiar market broccoli, which originated in southern Europe. Others look more like cauliflower and some have purplish shoots.
These overwintering types need a cold period to head properly. That's why, in contrast to the standard varieties, these succeed only as overwintered crops in zones 7b to 9a.
Start overwintering broccolis early in summer and transplant them in August, so plants have time to grow by January. Plants that are either too small or too large at the onset of low temperatures will suffer. Also, don't overfertilize. Use one-half the recommended rate -- about 3/4 cup of 5-10-10 per plant -- at planting time and apply the other half the following spring.
All broccolis are heavy feeders. Before planting standard varieties, mix into your soil 2 cubic feet of aged manure or 3 pounds of a 5-10-10 fertilizer per 100 square feet. Sidedress with a pound of blood meal per 100-square-foot area when the transplants are about 6 inches tall. Broccoli is sensitive to some nutrient deficiencies -- especially boron (a common deficiency of coastal plain soils from New Jersey to Louisiana). Without adequate boron, plants develop small, mouse-ear-sized top leaves and hollow stems. If your soil tests low in boron, apply 1/2 tablespoon of borax mixed with compost over 100 square feet of garden.
Space plants 18 inches apart for large-sized heads. For better overall production, but smaller heads, space plants 12 inches apart. Mulch with a 4- to 6-inch-thick layer of straw to keep the soil cool and moist.
Summer insects can quickly kill a young broccoli plant. To control the most devastating ones (imported cabbageworm, cabbage looper, harlequin bugs, and cabbage maggots), cover broccoli transplants with a lightweight row cover, such as Agrofabric Pro-10. Leave it on throughout the season. The cover will let in light, air, and water, but unlike traditional row covers, it won't increase the temperature by more than 5oF. More important, the cover will prevent harlequin bugs from feeding and the adult forms of these other cabbage pests from laying eggs on the plants.
If you prefer not to use row covers, spraying with Bt will control any larvae, and pyrethrin will control harlequin bugs. Spray when you first see signs of insect damage. Place tar paper mats around young broccoli transplants to stop the cabbage maggot fly from laying eggs.
Here are the broccoli varieties I recommend for a fall or early spring harvest. It's based on discussions with gardeners and researchers throughout the United States, as well as my own experience here in Vermont. Information included is whether the variety is hybrid or open-pollinated (you can save your own seeds from the latter), the days to maturity from transplant (so you can mix early, midseason, and late varieties to extend harvest), and the variety's genetic ancestry, which indicates cold tolerance.