Heat Tolerant Lettuce Varieties and Preparing for Fall Lettuce Planting

Emmalea Ernest, Extension Fruit & Vegetable Specialist; emmalea@udel.edu

Head lettuce for fall harvest can be direct seeded in Delaware from August 1 through August 20 and transplanted from August 10 through August 30. Getting good lettuce germination for either transplants or direct seeded plantings can be challenging when temperatures are high. The optimum soil temperature range for lettuce germination is 60-68°F and high soil temperatures can induce thermal dormancy which prevents germination. In southern Delaware, average daily soil temperatures are typically above 80°F through the third week of August. For both transplant and direct seeded production, primed seed can help reduce the risk of thermal dormancy. When starting transplants, trays can be kept in a shaded location, a cool building or germination chamber until seedlings emerge. After germination, grow out transplants in a cool location, such as a porch or other partially shaded area.

In 2012 I conducted lettuce trials with two spring and two fall planting dates. Forty-three varieties were tested. The fall harvest transplanting dates were July 27, which is earlier than the recommended window, and August 15, which is within the recommended window. Even when planted within the recommended planting window (August 15), 83% of the varieties fully or partially bolted. Therefore, Delaware growers should use heat tolerant lettuce varieties for fall plantings. (As a side note, nearly all the varieties I tested produced marketable heads when transplanted on March 30).

The varieties that performed well in the 2012 fall trials, which are still available are: Skyphos and Starfighter. This year I am testing fourteen lettuce varieties (all with reported heat tolerance) in two summer trials on white mulch with shade. On July 17 we completed harvest of the first planting, which was transplanted on June 5. The varieties with no bolting and bitter flavor in that trial were: Muir, Nevada, Skyphos, Jericho and Sunland.

Lettuce varieties that have shown heat tolerance in Delaware trials

Lettuce varieties that have shown heat tolerance in Delaware trials.