It tolerates a moderate freeze and does best in cool seasons, so in Nebraska lettuce should be planted as an early spring or a fall crop. Hot weather causes lettuce sap to become milky and bitter. It also causes seed stalks to form, and stops continued leaf growth. Butterhead and crisp-head varieties are particularly sensitive in this regard. Bolting and bitter flavor that result from high temperatures can be delayed by shading the crop.
Row covers, cheesecloth or other shading material can be effective.
Selecting a planting site that will be shaded in the afternoon is another alternative. Conditions in western Nebraska are more favorable for lettuce production than conditions in eastern Nebraska. A series of plantings 10 to 15 days apart will supply fresh lettuce for an extended period. Leaf lettuce should be thinned to a stand of 3 inches between plants. Allow 6 to 8 inches between Butterhead varieties and 10 to 12 inches between crisp-head varieties.