Emmalea Ernest, Extension Fruit & Vegetable Specialist; emmalea@udel.edu
Lettuce is an important crop for direct market and restaurant sales. In Delaware, lettuce production is typically constrained to the spring and fall since high temperatures can induce premature flowering (bolting) and development of bitter flavor in lettuce. However, I have been testing combinations of practices that might allow for lettuce production in the summer also. Over several seasons, I have tested three strategies for maintaining quality lettuce for 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 of the trials I have done, variety is 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 for performance with early and late June planting dates. This trial was transplanted into white plastic mulch and shaded with 30% black shade cloth.
The bitterness ratings from the June 5 planting, which was harvested in early July are in Table 1.
Bitterness was rated on a 1-4 scale with ratings of 1 and 2 considered marketable and ratings of 3 and 4 considered unmarketable. The varieties with the lowest bitterness ratings were Skyphos, Muir, Nevada, Jericho, Salanova Red Tango, and Sunland. In the June 22 planted trial, harvested in early August, only Muir and Nevada produced marketable lettuce.
Table 1. Flavor Ratings for Lettuce Varieties Transplanted on June 5 and Harvested in Early July
| Variety Name | Type | Average Flavor Rating* | |
| Skyphos | Butterhead, Red | 1.04 | a** |
| Muir | Batavia, Green | 1.08 | ab |
| Nevada | Batavia, Green | 1.13 | abc |
| Jericho | Romaine | 1.17 | abc |
| Salanova Red Tango | One-Cut, Red | 1.33 | abc |
| Sunland | Romaine | 1.38 | abc |
| Salanova Red Butter | On-Cut, Red | 1.42 | bcd |
| Verigo | One-Cut, Green | 1.46 | cde |
| Rubygo | One-Cut, Red | 1.75 | def |
| Salanova Green Oakleaf | One-Cut, Green | 1.79 | efg |
| Adriana | Butterhead, Green | 1.92 | fg |
| Starfighter | Leaf, Green |
2.04 | fg |
| Salanova Green Butter | One-Cut, Green | 2.13 | g |
| Red Cross | Butterhead, Red | 3.21 | h |
*Flavor was rated on a 1-4 scale with ratings 1 and 2 having marketable flavor and 3 and 4 being un-marketable.
**Averages followed by the same letter are not statistically different from one another.
The most heat tolerant varieties from the 2023 July harvested trial.
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 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. Silver or white mulch did not completely overcome the effect of variety, however. Heat sensitive varieties still bolted and became bitter on silver and white mulch. You cannot produce marketable lettuce under hot conditions if you are not also using a heat tolerant variety.
Shade Cloth
In trials planted in 2018 and 2019, use of 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 of 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 air temperatures for lettuce crops grown in high tunnels.
Conclusions
If you want to try producing lettuce for harvest in July and August, using an extremely heat tolerant variety is vital. Nevada and Muir are the two varieties I have tested with the most heat tolerance. I am testing additional varieties in 2024, and hoping to identify more that are very heat tolerant. Using white or silver plastic mulch can also help reduce bolting and bitterness in summer lettuce. Silver mulch has some additional bitterness reduction and growth promotion advantages over white mulch. However, silver mulch also costs more and is not fun to work around on sunny days. Shade cloth (30% black) on low tunnels did not increase marketable yield in summer lettuce and is probably not worth the cost and trouble in this production system.
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.