Brassicas are known for their cold tolerance. Traditionally in the South, collard greens grow all winter on tall stalks and are harvested a few leaves at a time, says Powell Smith, an educator with the Clemson University Extension in Lexington, South Carolina. But in most of North America, brassicas are spring and fall crops. Wherever they’re grown, cabbage and its relatives don’t do well in hot, dry weather.
Brassicas can be seeded where they are to grow in the garden, but more often they are started indoors or in a greenhouse and transplanted to the garden as seedlings. Spring crops can be tricky; it’s important to get the timing right so the crops are harvested before hot summer weather that tends to make them bolt. Up north and in the Great Plains, look for early varieties—those with the fewest days to maturity. Gardeners can save several weeks of growing time by purchasing transplants of broccoli, cabbage, and other brassicas at garden centers in spring.
As fall crops, brassicas often are started indoors in early summer and transplanted to the garden in July or early August, when there will be 90 to 100 days before the first frost. To withstand the summer heat, the tender transplants benefit from a little shade from nearby vegetables or floating row covers and careful watering as they become established. In fact, all brassicas need a steady supply of water. Soaker hoses or drip irrigation is a good idea, at least until autumn rains come. An organic mulch over the root zone will hold in moisture, reduce weed competition, and insulate soil against temperature extremes.
Brassicas need a soil pH between 6.5 and 7.0 to reduce the risk of club root, a fungal disease associated with acid soils. It’s always best to get a soil test before starting a vegetable garden, to determine not only your soil’s acidity but also its nutrient level. Like most vegetables, brassicas thrive in well-drained soil with lots of organic matter. They need ample nitrogen, so soil should be enriched with plenty of mature compost. A nitrogen-fixing cover crop such as clover helps increase soil fertility, or simply plant brassicas where you’ve recently harvested beans. It’s important to rotate crops anyway, since these plants are subject to a number of soilborne diseases. Four to six weeks after the plants are set out in the garden, side-dress with more compost, or apply seaweed emulsion or other organic fertilizer in moderation. Midseason applications of fertilizer are especially helpful in nutrient-lean sandy soil.
Among insect pests, cutworms—easily defeated by collars made from cut-down paper towel rolls—and flea beetles may plague new transplants. All pests of cabbage—cabbage loopers, cabbageworms, aphids, and maggots—will happily munch its vegetable cousins as well. Some farmers spray Bacillus thuringiensis (BT), a predatory bacterium, as a control for leaf-eating caterpillars. Cindy Tong, postharvest horticulturist with the University of Minnesota Extension, recommends using lightweight floating row covers all season long to deter flying insect pests from laying eggs.
Smith, who grows plenty of brassicas in his home garden, advises another pest-defeating strategy. He surrounds the vegetables with a diverse garden of nectar-bearing flowers to lure predatory insects and parasitic wasps to prey on the brassica pests.