Four cover crop treatments were tested as winter cover crops in high tunnels in central Mississippi: annual ryegrass (Lolium multiflorum Lam.), annual ryegrass + hairy vetch (Vicia villosa) , the biofumigant mustard blend Caliente (Brassica juncia + Sinapsis alba), and a weed free bare ground treatment.  The tunnels were newly constructed and the cover crops were the first crops in them. The site is maintained as certified organic under the USDA National Organic Program.  Cover crops were sown in mid-November 2010 and grown until mid-February 2011, prior to planting spring vegetable crops.  On January 20, 2011, soil nitrate concentrations levels in the top 20 cm were more than 50% higher in the bare ground plots than in the cropped plots, implying that the cover crops were removing nitrate from the soil.  Additional soil tests did not reveal any significant differences in organic matter content or in concentration of other extractable elements at that time.  At termination of the cover crops on February 15, 2011, the cover crops produced 3.02, 3.74, and 4.95 kg^.m^-2 fresh weight, for annual ryegrass, ryegrass + hairy vetch, and mustard, respectively.