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.
All broccolis are heavy feeders.
Space plants 18 inches apart for large-sized heads. For better overall production, but smaller heads, space plants 12 inches apart. Insects and Other Pests Summer insects can quickly kill a young broccoli plant. 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.