Start transplants in a greenhouse for the earliest plantings, in a cold frame/high tunnel for slightly later crops, and in outdoor seed-beds or directly in the field when the weather is warm enough for germination and growth (above 50°F). For most brassica crops, the minimum temperature for germination and growth is 40°F, the optimum range is 50-85°F (ideal 75°F), and the maximum greater than 90°F. It takes 6 to 8 weeks for plants to reach transplant size, and 1 oz. of seed will produce about 5000 transplants.
In the greenhouse, seed directly into plug trays or soil blocks. Plants are commonly grown in 128 cell trays. Smaller celled trays (256’s) cost less per plant but require more management. A day/night temperature difference of 10°F in the greenhouse tends to grow the best plants (for example: 70d/60n). Temperatures in cold frames/high tunnels or outdoor seed-beds should be similar but are harder to achieve.
For out-door or ground grown seed-beds, plant in well-drained, disease free, sandy soils. Plant enough seed to produce 20-25 plants per row-foot and space rows 10-12 inches apart.
Transplants take 6-8 weeks after seeding (depending on growing temperature) to grow 5 to 7 true leaves (transplant size). Seedlings from a greenhouse require some hardening-off while those from outdoor seedbeds can be moved directly without conditioning. Do not over-harden since buttoning (see other production problems on pg. 34) is more likely to occur. Thoroughly water the outdoor grown plants prior to pulling. Undercut seedbed plants for easy removal and to minimize root breakage. Outdoor or ground grown plants are generally planted bare root. Set transplants slightly deeper in the field (cover root ball), irrigate after planting, and add a starter solution to aid in establishment.
Sometimes, brassicas are direct-seeded in the field. Seedbed conditions should be worked to a fine tilth that is free of surface trash, firm, and level. Precision seeders ensure more uniform seed spacing but these require size graded or pelleted seed. Seed requirement is about 0.5 to 1.5 lbs./acre depending on seed size and row spacing. Under most soil conditions, seed should be plant ⅓ to ½ inches deep. Direct-seeded crops will need to be thinned to the desired plant spacing when they have 3-5 leaves. Direct-seeded plants often mature several weeks earlier than transplants.