Simple unheated greenhouses covered with clear polyethylene, also known as high tunnels, in which plants are typically grown in the ground have become popular for extending the growing season for high-value horticultural crops. Although they are used principally to produce annual crops such as vegetables and cut flowers, increasing interest has focused on their use for perennial crops such as raspberries, blackberries, and ornamentals. Studies of temperature variation within the tunnel during the growing season have emphasized the rapid rise in air day temperature above ambient during the day and an equally rapid decrease at night. Spatial variation in temperature within the tunnel were much less marked, however, with air temperatures at the edge of a 10-m wide tunnel only ≈2 °C lower than in the center. For perennial crops, tunnel conditions during the off-season are also an important factor. In winter, air temperatures in the tunnel during sunny days rose above freezing even when ambient air temperatures stayed below freezing. Soil temperatures during the day and night fluctuated much less both inside and outside the tunnel and were significantly higher in the tunnel. Studies with nursery plants overwintered in similar structures indicate that spatial variation is again dwarfed by the overall air temperature fluctuation in these structures.