Cos lettuce has been more difficult to grow in Nebraska than other types of lettuce. It is grown for its fleshy, elongated stem in preference to its leaves. The stem is peeled and may be eaten raw like celery, or it may be cooked. Requirements and Culture Lettuce germinates and grows at low temperature, and is one of the first vegetables to be planted in the spring. 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.