Transplant Production

Start transplants in a greenhouse for the earliest planting, in a cold frame/high tunnel for slightly later crops, and in outdoor seedbeds 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 °F to 85°F (ideal 75 °F), and the maximum is greater than 90 °F. It takes 6 to 8 weeks for plants to reach transplant size, and 1 ounce of seed will produce about 5,000 transplants.

In the greenhouse, seed directly into plug trays or soil blocks. Plants are commonly grown in 128-cell trays. Smaller-celled trays (256s) 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 seedbeds should be similar but are harder to achieve.

For outdoor or ground-grown seed beds, plant in welldrained, 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 five to seven 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 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 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 pounds per acre, depending on seed size and row spacing. Under most soil conditions, plant seeds ⅓ to ½ inches deep. Thin direct-seeded crops to the desired plant spacing when they have three to five leaves. Direct-seeded plants often mature several weeks earlier than transplants.