North Carolina State University Head lettuce is the most important salad vegetable grown in the United States. Per-capita consumption exceeds 25 lb annually. Repeated exposure to subfreezing temperatures, however, can seriously injure or kill the crop. Lettuce has a relatively high water requirement. Soil moisture shortage rainfall will seriously stunt growth and head quality.
Irrigation greatly reduces risk of crop failure.
There are considerable differences among lettuce varieties in heat tolerance. These differences are the primary reasons some lettuce varieties can be grown in warmer climates. In North Carolina, the crop can be grown in both the spring and fall in eastern N.C. and even during midsummer in western N.C. at elevations over 3,000 ft. In the Piedmont, lettuce is intermediate in season and probably is best as a late spring and early fall crop. Romaine has requirements similar to head lettuce except it can stand more heat.