2017 ASHS Annual Conference

Tolerance to heat is an important characteristic of lettuce (Lactuca sativa), a crop in which exposure to high temperatures causes bolting and associated bitterness. Hot summer temperatures in Maryland make lettuce production difficult, especially during the peak of farmers’ market season. Defining the specific level of heat-tolerance associated with the general label is needed to improve East Coast lettuce production. Additionally, finding growing practices that can reduce the chance of bolting could aid in lettuce production during hot weather. In this experiment, four lettuce cultivars, all advertised as heat-tolerant, were grown on different mulches to investigate how surface reflection affects immediate air temperature. Germination temperatures of 20°C and 30°C were investigated as a priming technique to inhibit bolting. The four different heat tolerant cultivars of lettuce, ‘Coastal Star,' ‘Auvona,’ ‘Salvius’ and ‘Dov,’ were transplanted on June 28, 2016 into raised rows covered with white plastic, black plastic, straw or bare ground. Vermicompost was also trialed as a separate amendment under the white plastic mulch. Lettuce heads were harvested after seven weeks in the field. Heads were compared for evidence of bolting. Lettuce that was germinated at 20°C was generally less likely to bolt than those germinated at 30°C; however, the chance of bolting depended on the cultivar. In those heads that did bolt, a lower germination temperature delayed bolting. Of the four cultivars, ‘Dov’ appeared to be the most heat-tolerant, having the lowest average height. Across most cultivars, straw mulch treatments and the control ground treatment produced the best heads of lettuce. The amendment of vermicompost under the white plastic mulch did not delay bolting or improve head weight compared to white plastic mulch alone.