Lettuce is a cool weather vegetable best planted in early spring or late summer to avoid killing freezes or very hot weather. Most lettuce subjected to hot weather has a bitter taste and bolts, though some varieties, such as the oak leaf lettuce, do well in the heat.
However, even lettuce that grows in hot weather can stand some shading. It is a good idea to plant lettuce under taller crops such as corn or squash.
Lettuce that forms a head, such as the familiar iceberg variety, needs more care than other kinds and does well started indoors from seed. When the seedlings reach a certain size, harden them off, and then transplant them in the garden. Hardening off means placing the seedling outdoors for an increasing number of hours until they're tough enough to spend nights outside.
Plant lettuce seed from 1/4 to 1/2 inch deep in rows that are 12 to 18 inches apart. As the seedlings sprout, thin them out to about 4 inches apart. Fortunately, the removed seedlings are transplantable elsewhere. Unlike other vegetables, which do best with deep but infrequent watering, lettuce thrives when watered briefly but frequently. When planting lettuce, mulch the planting area to help retain moisture and keep the ground temperature cool.