Direct seed or transplant in early spring, as soon as you can work the soil. To get an early start, prepare beds the previous fall by working in manure or compost and raking smooth to leave a fine seedbed. Seeds need light to germinate; sow at a very shallow depth by covering with a thin layer of growing medium.
Sow seed 1/8 inch deep, 1 inch apart in rows 12 to 18 inches apart. When plants have two or three true leaves, thin to 12-inch spacings for crisphead varieties, 6 to 10 inches for other types. You can also lightly broadcast seed (particularly of looseleaf varieties) in a patch instead of a row.
Sow in 1-inch cells 3 to 4 weeks before transplanting outside. Harden seedlings by reducing water and temperature for 3 days before transplanting. Hardened plants should survive 20 F. Space crisphead transplants 12 inches apart in rows 18 inches apart. Space other varieties 6 to 10 inches apart in rows 12 to 18 inches apart.