Abstract: Four microclimate modification methods including spun-bonded and slitted low tunnels both with and without the addition of water-filled plastic tubes were tested for their effect on early and total yields of warm and cool season vegetable crops in Morgantown, West Virginia, USA. Peppers, tomatoes, radishes, and lettuce were organically grown in 2006 and 2007. Early season pepper yields were higher when water tubes were included with low tunnels while early tomato yields did not differ. Total yields for warm season crops in some microclimate modification treatments were higher than the control, and harvests started up to four weeks earlier in the spring. Cool season crop yields in the four treatments showed no increase over the control despite one to three weeks earlier harvests for radish and lettuce. These results show potential for earlier safe planting dates and increased yield, especially in warm season vegetable crops using low tunnels and water tubes. Additionally, economic analysis demonstrated a potential for increased profits over control plots using these microclimate modification techniques.