Lettuce has an internal counter that keeps track of the number of daylight hours the plant receives. Once a critical number of hours are received, the plant sends up its flower stalk. The exact number of hours varies from cultivar to cultivar. The plant can handle environmental stresses quite well when vegetative. However, once the intermediate stage is reached, environmental stresses, such as high temperatures or drought, will cause the plant to bolt.
Gardeners can reduce the tendency for bolting in various ways.
Lettuce can be started indoors under lights to give them an early start and then planted outdoors while temperatures are still cool. When growing lettuce seedlings under lights, do not leave the lights on for more than 12 hours because lettuce needs short days to grow. Row covers, such as Reemay or plastic tunnels, can be used to extend the growing season both in the spring and fall. Late planted lettuce benefits from a shade cloth placed over the row to reduce heat and light intensity. Soil moisture must be kept in constant supply.