The 2012 ASHS Annual Conference

The objective of our study was to determine the temperature moderating effects of low tunnels for the potential of overwintering hardy vegetable crops in New England. Six (6) experimental sites were selected in New Hampshire, Massachusetts and Rhode Island, spanning USDA hardiness zones 4b through 7a. Low tunnels were constructed of 10 ft lengths of plastic PVC spaced 2.5 ft apart. Each low tunnel was 3 ft wide, 3.5 ft high, and 40 ft. long. Orientation of the tunnels varied depending on the site, but shading was avoided. Temperature data were collected throughout the fall and winter months under three types of coverings: two layers of 1.25 oz/yd² spunbonded polyethylene rowcover (2XRC), one layer of rowcover covered with one layer of 2-ml perforated plastic (RC+perf), and one layer of rowcover covered with one layer of 6 mil IR greenhouse film (RC+GH). In each site, an uncovered control was included. Within each tunnel and adjacent to the tunnels, temperature was recorded every 2 hours. Each temperature logger was shielded from direct sunlight with a ventilated box or radiation shield.  Similar patterns in temperature were observed in all six sites. At some sites, the outdoor logger and/or the low tunnels were covered with snow during some parts of the winter. Snow provided excellent insulation, preventing daily temperature fluctuation almost entirely. The daily maximum and minimum temperatures reached inside the low tunnels were usually, but not always, higher than the temperatures measured outside the low tunnels. The RC+GH tunnel provided the greatest gain in temperature, followed by the RC+perf and 2XRC tunnels. For example, over the 2011–12 season in Durham NH, the daily minimum temperatures were, on average, 5.5 °F, 2.4 °F, and 2.7 °F warmer in RC+GH, RC+perf, and 2XRC tunnels, respectively, than outdoor temperatures. The daily maximum temperatures during the same period were, on average, 19.8 °F, 10.0 °F, and 5.8 °F warmer than outdoors, respectively. The degree of temperature protection offered by RC+GH low tunnels was negatively correlated with the daily minimum outdoor temperature (r = –0.7693); greater temperature gains occurred on coldest days. Our results demonstrated that low tunnels can raise the absolute minimum winter temperature by greater than 20F, offering the potential to effectively modify climate by four or more hardiness zones.