Emmalea Ernest, Extension Fruit & Vegetable Specialist at University of Delaware
emmalea@udel.edu
The spring lettuce season is underway with outdoor planting beginning in March and continuing through April. In Delaware, lettuce production is typically constrained to the spring and fall since high summer temperatures can cause premature flowering (bolting) and development of bitter flavor in lettuce. Our Mid-Atlantic Commercial Vegetable Production Recommendations advise against May lettuce plantings in the southern part of the region, which includes Delaware. Over several seasons, I have tested three strategies for maintaining quality lettuce during summer production: use of heat tolerant varieties, use of shade cloth and use of white and silver plastic mulch. I have also tested using these strategies in combination. All the trials were conducted at University of Delaware’s research farm in Georgetown, Delaware.
Heat Tolerant Varieties
In all the trials I have done, variety has been the most significant factor determining premature bolting and development of bitter flavor. Some varieties are slow to bolt but still develop bitter flavor. In 2023 I tested 14 lettuce varieties that were reported to be heat tolerant and in 2024 I trialed the six most heat tolerant varieties from the 2023 trial along with 22 additional varieties not previously tested. The 2023 trials were transplanted into white plastic mulch and covered with 30% black shade cloth on low tunnels. The 2024 trials were transplanted onto white and silver mulch with no shade. Transplanting dates were June 5 and June 22 in 2023 and June 6 and July 10 in 2024. The early June plantings were harvested in early July. The late June planting was harvested in late July and early August. The July planting was harvested in mid-August. Even in the plantings exposed to very high temperatures, there were some varieties that were slow to bolt and did not develop bitter flavor. The most heat tolerant varieties in the trials were Muir and Nevada. Additional varieties with notable heat tolerance are Skyphos, Salanova Red Tango, Sunland, Chalupa, Kalura, Mikola, and Magenta.
The most heat tolerant varieties from the 2023 and 2024 trials include various types and colors. Top row: Muir, Nevada, Skyphos. Middle row: Salanova Red Tango, Sunland, Chalupa. Bottom row: Kalura, Mikola, Magenta
Light Colored Mulch
In 2023 I tested the effect of silver and white plastic mulch on lettuce yield and quality. A bare ground treatment and a black plastic mulch treatment were included in the trial as controls. Silver mulch produced the largest lettuce plants and the least bitter lettuce. White mulch also improved lettuce flavor, but not as much as silver. In 2024, I compared 28 varieties on white and silver mulch. In the early June planted trial, there was not a significant difference in bolting or bitterness between the white and silver mulch. For the July planting, most varieties (75%) had less bolting on the white mulch. In both years’ trials, silver or white mulch did not overcome the effect of variety. Heat sensitive varieties still bolted and became bitter on silver and white mulch. Under hot conditions, even with white or silver mulch you cannot produce marketable lettuce without also using a heat tolerant variety.
First harvest of the shade cloth and mulch color trial on July 5, 2023
Shade Cloth
In trials I planted in 2018 and 2019, 30% black shade cloth reduced bitterness in lettuce, however these trials were planted in early April and harvested in early June. In the 2023 trials, which were planted in June and harvested in July and August, 30% black shade cloth decreased lettuce plant size and had no effect on bitterness. In all these trials, shade cloth was applied over low tunnels, which may have a heat trapping effect. Shade cloth might be more effectively used to reduce temperatures for lettuce crops grown in high tunnels. I do not recommend using shade cloth on low tunnels for summer lettuce production.
Conclusions
If you want to try producing lettuce for harvest in July and August, it is vital to use an extremely heat tolerant variety. Nevada and Muir are the two varieties that have had the most consistent performance in my summer trials and other the varieties previously noted have also produced marketable lettuce in heat-stressed plantings. Using white or silver mulch can also help reduce bolting and bitterness in summer lettuce. It is unclear whether there is a benefit of one color over the other. Shade cloth (30% black) on low tunnels did not increase marketable yield in summer lettuce and is not recommended. In 2025, I will be testing the best performing varieties from past trials, along with new ones, hoping to identify additional varieties that are very heat tolerant.
This research is supported by a Specialty Crop Block Grant administered by the Delaware Department of Agriculture titled: Identifying Varieties and Management Practices for Summer Production of Lettuce and Other Greens.