Lettuce is one of the most important fresh market vegetable crops in the United States. In 2017, lettuce was grown on more than 276,000 acres, with a crop value exceeding $3.6 billion [U.S. Department of Agriculture (USDA), 2018]. Most of the U.S. lettuce crop is grown in California and Arizona, but according to the 2012 Census of Agriculture, lettuce was planted on ≈3500 acres in the northeastern U.S. romaine, and leaf lettuces account for 52% of production nationally and 83% of production in the northeastern United States (USDA, 2014). Romaine and leaf lettuces are more popular than head lettuce because of darker green leaves and higher nutrient content (Kim et al., 2016). Demand for local lettuce is growing because of the high cost of shipping, loss of freshness of products because of transit time, and consumer demand for fresh, healthy, and local produce. Along with butterhead lettuce, romaine and leaf lettuce have been grown commercially in the eastern United States for more than 100 years, whereas crisphead lettuce is shipped from the western United States (Ryder, 2002). Many direct-market growers use high tunnels to extend their production season and increase farm income (Bruce et al., 2017; Martinez et al., 2010). Lettuce is the number two crop grown in high tunnels in the United States (Orzolek, 2012). In Rhode Island, lettuce can be harvested from outdoor plantings from May until October; high tunnels allow the harvest season to be extended to include March, November, and December.
Multiple studies have reported success in growing romaine or leaf lettuce in high tunnels, with quality and marketable yields exceeding those of lettuce grown in the open field, particularly in early spring and late fall when weather is unstable (Rader and Karlsson, 2006; Wallace et al., 2012). The rain exclusion benefits of high tunnels in combination with drip irrigation reduce germination of weed seeds relative to the open field (Lamont et al., 2003), but weed control is still necessary in tunnels, particularly during periods of high soil moisture. Plastic mulches are an established practice for weed control in high tunnels, particularly for tomato (Solanum lycopersicum L.) and other heat-loving vegetables (Lamont et al., 2003). However, many growers prefer not to use plastic mulches in high tunnels. The cost of equipment for laying and removing plastic is a barrier to adoption (Goldberger et al., 2015), as are the need to till the soil before laying plastic mulch, and the environmental issues surrounding manufacture and disposal of plastic films (Goldberger et al., 2015). Although paper, straw matting and biodegradable plastic films have been successfully used as alternatives to plastic in high tunnels, equipment and tillage requirements are similar to plastic (Cowan et al., 2014; Sánchez et al., 2008; Wortman et al., 2016).
Many growers apply organic soil amendments, including composts, annually in high tunnels to build soil fertility (Knewtson et al., 2010). Lettuce responds well to organic nutrient sources, and compost amendments have been shown to increase yields and quality of romaine lettuce relative to inorganic fertilizers (Hernández et al., 2016). The use of organic mulches, particularly composts, may be an appropriate weed-control strategy in high tunnels for growers who do not wish to use plastic mulch. High tunnels have been shown to slow decomposition of particulate organic matter relative to the open field (Knewtson et al., 2012) and organic matter levels can become quite high after years of routine addition of composts. However, soil organic matter can become depleted when tunnels are conventionally managed with synthetic fertilizers and frequent tillage (Bonanomi et al., 2014; Knewtson et al., 2012). Incorporation of organic mulches has been shown to improve soil health in high tunnel production systems, where many years of intensive vegetable production had depleted soil organic carbon (Bonanomi et al., 2014).
Organic mulches are known to reduce soil temperatures when used during the summer (Munn, 1992; Sánchez et al., 2008). Soil warming has been shown to increase yields of lettuce grown in high tunnels in early spring and late fall (Bumgarner et al., 2011), and soil-cooling effects of organic mulches could be detrimental when high tunnels are used for season extension. Previous studies of early spring romaine and leaf lettuce production in high tunnels have used black plastic mulch to increase soil temperatures (Wallace et al., 2012). The objective of this study was to determine how using compost as an organic mulch for weed suppression affected yields of romaine lettuce cultivars grown in the late fall and early spring when below-optimal soil temperatures could limit lettuce growth. We chose to test four cultivars of lettuce because cultivars are known to differ in tolerance of environmental conditions expected in high tunnels in Rhode Island in fall and spring, and might differ in response to compost mulch. We chose to use compost because it is an accepted amendment for use in high tunnels, avoiding concerns that raw organic materials could reduce soil nutrient availability or introduce contaminants. Organic mulch was not compared with plastic or paper mulch because farmers in Rhode Island generally do not use plastic or paper mulch in high tunnels, although silage tarps and woven landscape weed barrier are used for weed suppression with tomatoes. In addition to the costs for labor or equipment for laying mulch, plastic mulch has been shown to increase damage from meadow vole (Microtus pennsylvanicus Ord.), a major pest of high tunnel greens production in Rhode Island (Eaton, 2017).
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