Summer Cover Crops and Plastic Mulch Affect Fall Lettuce (Lactuca sativa L.) Production

Cover crops have generated wide interests among fruit and vegetable growers in the country. In vegetable cropping systems, in addition to traditionally grown cover crops such as cereal rye and hairy vetch, there are wide choices available. Short duration summer cover crops such as cowpea, sorghum sudangrass, buckwheat, etc. are being increasingly utilized to add organic matter, suppress weeds, and enhance soil quality and health. Another product that has become ubiquitous in vegetable production is black plastic mulch. Black plastic mulch provides benefits such as weed suppression, moisture retention, and reduced nutrient leaching, but it can also increase soil temperature to a level that could hurt plant roots. Vegetables such as lettuce could be affected due to high soil temperatures. This study investigated effects of three summer cover crops (buckwheat, cowpea, sorghum sudangrass, or no-cover crop) and four mulch treatments (black, blue, red, and white plastic mulch) on fall lettuce production.  The study was conducted at the Horticulture Research Station, Ames, IA. Experimental design was a split-plot design with four replications. Cover crops were the whole plot and mulch treatments were the split plot factors. Cover crops were seeded on 20 June, and 10 July in 2012 and 2013, respectively. Cover crops were terminated 60 days after seeding and lettuce ‘Nancy’ was transplanted on raised bed with plastic mulch treatments. Plants were fertigated using drip irrigation. Cover crops significantly affected marketable lettuce yield. Lettuce growing in the cowpea treatment produced the highest marketable weight followed by no-cover crop and buckwheat treatment. Sorghum sudangrass did not produce any marketable lettuce. Lettuce in the sorghum sudangrass treatment showed poor growth and severe stunting. There was no effect of mulch treatments on marketable weight; however, lettuce plants growing on red plastic mulch had the highest total leaf area. Lettuce in cowpea or no-cover crop treatment had higher number of leaves than buckwheat or sorghum sudangrass.  Results from this study indicate a detrimental effect of buckwheat and sorghum sudangrass on lettuce yield and quality. This could be due to allelopathic properties of those cover crops. Growers could minimize this risk by planting lettuce 10–14 days after cover crop termination.  Although plastic mulch did not affect marketable yield, they have the potential to alter leaf number and leaf area.