Compost and Black Polyethylene Mulches Improve Spring Production of Romaine Lettuce in Southern New England

Romaine lettuce (Lactuca sativa) is the most popular leafy vegetable in the United States. Organic and synthetic mulch materials are applied by farmers to reduce their weeding expenditures, manage soil temperatures, and increase the yield and quality of their crops. This study examined the impacts of using black polyethylene, compost, and shredded leaves as surface mulches in spring open field romaine lettuce by using data regarding the soil temperature, lettuce growth, and yield parameters. Mulch treatments were compared to determine effects on soil temperature, lettuce growth rates, leaf area index (LAI), yield, and plant size. Experiments were conducted in 2017 and 2019. Soil temperatures were slightly (but significantly) warmer in 2019, the plants grew more rapidly, and LAI, yield, and plant size were all significantly greater than they were in 2017. Mean soil temperatures were 18.0 and 18.9 °C in black polyethylene mulched plots, 17.7 and 18.5 °C in compost, 17.5 and 17.0 °C in bare ground, and 16.9 and 17.3 °C in shredded leaf plots in 2017 and 2019, respectively. Changes in canopy size presented a similar trend over the growing season for all treatments; shredded leaf and bare ground treatments on all measuring dates had the smallest canopy size, and compost and black polyethylene mulches had the largest canopy sizes. Black polyethylene and compost mulches had the highest LAI and yields in both years. In 2017, the bare ground treatment was similar to black polyethylene and compost mulch treatments for both parameters, and all three treatments were significantly greater than the shredded leaf mulch treatment. In 2019, the bare ground treatment was similar to the shredded leaf mulch treatment and significantly lower than the black polyethylene and compost mulch treatments. In 2017, black polyethylene and compost mulches had LAI of 7.67 and 7.37 and yields of 6.38 and 6.68 kg·m−2, respectively. Bare ground had an LAI of 7.16 and yielded 5.94 kg·m−2. Shredded leaf mulch plots had the lowest LAI (5.94) and yield (4.96 kg·m−2). In 2019, the bare ground treatment had an LAI of 7.5 and yielded 7.6 kg·m−2. Black polyethylene and compost mulches increased LAI by 1.7. Yield increased by 2.8 kg·m−2 with black polyethylene mulch and by 2.4 kg·m−2 with compost mulch. Shredded leaf mulch produced LAI and yield similar to but slightly lower than bare ground. Dry weight, plant height, and head diameter in bare ground and shredded leaf mulch treatments were significantly smaller than in black polyethylene and compost mulch treatments. The cultivar rank order was consistent across treatments in both years; ‘Ridgeline’ grew significantly faster than ‘Coastal Star’, and plants were significantly larger at harvest. Black polyethylene suppresses weeds and increases soil temperatures, thereby enhancing yields of spring-to-summer romaine lettuce under southern New England conditions. Compost mulch has similar effects on yields. Shredded leaf mulch is effective at suppressing weeds but decreases soil temperatures and yields.