Lettuce grows best in full sun, though excessive heat can cause plants to bolt to seed, or leaves to wilt. For an early start, seeds can be started in flats 4 weeks prior to the last frost and transplanted outdoors in mid to late spring. If growing in summer, select a partially shaded location, or one that receives primarily eastward exposure to mitigate the potentially damaging effects of excessive heat upon lettuce

Lettuce is tolerant of a wide range of soils, but prefers well-drained, cool, loose soil with plentiful moisture and pH 6.2 to 6.8. Sensitive to low pH. Lime to at least 6.0. To encourage tender and tasty growth, make sure location is rich in organic compost matter. Amend prior to planting if needed.

Sowing

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.

Direct-seeding

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.

Transplants

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.

MAINTAINING

Use row covers to protect very early plantings from cold, to protect young plants from insects, and (supported by hoops) to shade crops when warm weather arrives. 

Make succession plantings every week or two, and grow several varieties with different maturity dates for a continuous supply. Moisture, stress, and high temperatures, particularly at night, encourage bolting. As the season progresses, plant more bolt-resistant varieties. Locate plants where they will be partially shaded by taller nearby plants, latticework or other screen.

Lettuce has a shallow root system. Keep soil moist to keep plants growing continuously. Mulch to retain moisture and suppress weeds (unless slugs are a problem). Fertilizing can be helpful to promote faster growth, especially a fish emulsion type that is not high in nitrogen that can cause greens to become bitter. Water lightly but consistently. 

For fall crops, time maturity around time of first expected frost. Mature plants aren't as tolerant of freezing as seedlings.