Shade Cloth for Lettuce Production

Emmalea Ernest, Scientist – Vegetable & Fruit Crops; emmalea@udel.edu

In Delaware, spring lettuce is planted in March and April. Head lettuce should be seeded in March and transplanted by mid-April. Leaf lettuce can be direct seeded in March and April. Lettuce is a cool season crop that grows best at temperatures of 59-68 °F. Temperatures of around 85 °F promote bolting (flower stalk formation) and development of bitter flavor. Some lettuce varieties are less prone to bolting and bitterness when exposed to heat. Heat tolerance information is listed in some seed catalogs and varieties that have shown heat tolerance in Mid-Atlantic trials are indicated in lettuce section of the Mid-Atlantic Commercial Vegetable Recommendations. Because of the high likelihood of temperatures in the 80s during the spring lettuce season, heat tolerant varieties are recommended for Delaware production, especially for later season plantings.

Besides heat tolerant varieties, shade cloth is another tool that can be used to protect lettuce crops from quality loss due to heat stress. In 2018 and 2019 shading experiments were conducted in spring-planted lettuce at the University of Delaware’s research farm in Georgetown, DE. The trials were transplanted on April 12, 2018 and April 10, 2019 and shade was applied on May 4 in 2018 and on May 20 in 2019. These experiments were designed to answer several questions:

Can shading reduce bolting and bitterness in spring lettuce?
Overall, shading reduced bitter flavor in both years. Shade cloth effect on bolting was more complicated, with shade cloth increasing bolting in heat sensitive varieties.

Are certain shade cloth colors more effective?
Black 30% shade cloth was the most effective at reducing bitterness. The other shade cloth colors tested were blue 30%, red 30%, silver 30%, white 40%, and white 22%.

How does shading work in combination with heat tolerant varieties?
Four varieties were used for the experiment: two romaine varieties, Arroyo (heat tolerant) and Salvius, and two butterhead varieties, Skyphos (heat tolerant) and Alkindus. Shading was most impactful on the flavor of the heat tolerant romaine variety Arroyo. In both years Arroyo had a marketable bitterness rating with black 30 % shade and an unmarketable rating without shade. The other romaine variety did not have a marketable bitterness rating in any treatment in either year although shading did reduce the bitterness ratings in both years. For the butterhead varieties, Skyphos, the heat tolerant butterhead, had marketable bitterness ratings with and without shade and shade did not reduce the bitterness rating. The heat susceptible butterhead had slightly lower bitterness ratings with black 30% shade compared to no shade and the shade treatment averaged in the marketable range, whereas the no shade treatment averaged unmarketable.

*Bitterness Ratings <2 are considered marketable.

How did shade impact soil and plant temperatures?
The soil and leaf temperatures were lower in the 30% back shade treatment on both hot days and on cooler days. Average soil and leaf temperatures for no shade and the 30% black shade treatments are shown in the table below.

Shade Cloth Implementation
The lettuce shade experiments showed that shading can reduce bitterness and maintain marketable quality, especially in certain varieties. In a field setting, shade cloth can be applied over low tunnels or larger structures to create “shade houses”. In the experiments described the shade cloth was draped over pepper stakes and secured to the ground with landscape staples. Shade cloth can also be used to cover high tunnels where heat sensitive crops are being produced.

The 2018 lettuce shade trial on May 16.