Answer: Lettuce is a cool-weather crop. The optimum temperature range for growing it is between 55 and 70°F. The difficulty you are having is due to the lettuce’s physiological response to warm weather, known as summer dormancy. When the soil temperature exceeds 85° F, lettuce seed experiences thermoinhibition and will not germinate.
Unless you can delay your sowing until the soil has cooled in the fall, you will have to take other steps to get your lettuce seed to sprout. One way to do this is to raise transplants by sowing the lettuce seed indoors, or in a shaded location outdoors, where the soil temperatures are cool. Allow three to four weeks for the seedlings to reach a size where they can be transplanted to the main garden.
Alternatively, you can sow seed directly in the garden under a temporary canopy of shade cloth or wooden lattice, or in the shadow of existing crops. Keeping the seed bed evenly moist will also help cool the soil through evaporation.
Successful seedling establishment is only half of the equation, however. It is also important to select varieties adapted to warm conditions. ‘Buttercrunch’ (Bibb type), ‘Ermosa’ (butterhead), ‘Red Sails’ (leaf), and ‘Apollo’ (romaine) are all heat-resistant varieties that should do well.
This post is excerpted from the July/August 2012 issue of Horticulture.
Image Credit: Wikimedia Commons
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