Lettuce germinates and grows best at low temperates and will withstand a moderate freeze. Because of this, it should be planted as soon as the ground can be worked in the spring. It also does well as a fall crop and can be planted in August. However, high soil temperatures in the summer can inhibit germination. You can improve germination of cool-season plants in summer by shading newly planted seed with a board or light covering of mulch.
Hot weather causes the milky sap to become bitter.
Butterhead and crisphead varieties are especially sensitive to bolting. Conditions in western Nebraska are more favorable for growing lettuce than in eastern Nebraska due to cooler night temperatures throughout the growing season. One problem many gardeners have is too much lettuce at one time. Production time can be extended by making a series of small plantings 10 to 15 days apart. Crisphead and Romaine lettuces take much longer to reach maturity than leaf lettuce.