After seeding, cover the area with a floating row cover tunnel . Leave this tunnel in place throughout the growing season, removing it only to thin or harvest. Thin lettuce progressively. That is, keep the lettuce growth dense rather than widely spaced. Thin plants only as they develop and leaves begin to crowd.
Temperature is extremely important when growing lettuce.
Generally, lettuces prefer cool temperatures. Lettuce seed sprouts best at 60-65 degrees Fahrenheit. At 80 degrees and above, lettuce seed will sometimes not germinate at all. Once plants are up and growing, different temperature responses are also seen. On the opposite end of the temperature spectrum, certain varieties do well in warmth while others bolt or become bitter.