(B. oleracea) Start heirloom broccoli seeds for transplants 4-6 weeks prior to desired transplanting date. Seedlings need bright artificial light or full sun; a cold frame is ideal. Set the plants in the garden about 4 weeks before last frost date in the spring, or about 4-6 weeks before first frost when fall-planting. Plants need rich soil and abundant moisture to produce a good crop. After the main head is harvested, plants often produce smaller florets over a long season. Always harvest broccoli just before the first flowers open. Spring plantings often bolt to seed in hot summer weather.