High Tunnel Microclimate in Early Spring and Late Fall

High tunnels are unheated structures that modify the microclimate to achieve extended season production of specialty crops.  Microclimate modifications by high tunnels have been reported but conditions have not been characterized for much of the southeast. The objective of this study was to characterize the high tunnel microclimate during fall and winter seasons in the Piedmont of North Carolina (zone 7).  In October 2014, microclimate measurements of air and soil temperature, relative humidity, solar radiation (SR), and photosynthetically active radiation (PAR) were started in a 30’ x 96’ high tunnel located in Greensboro, North Carolina, using Spectrum WatchDog data loggers. In the high tunnel, one season of salad greens were planted and harvested, followed by winter cover crops.  Row covers were laid over the vegetables and the Watchdog when outside temperatures were expected to be below 10°C. Air temperature measurements indicated that the high tunnel would offer at least one hardiness zone of protection.  The lowest air temperatures in November were at -9.7/0°C (outside /inside high tunnel), while in February the numbers were -20.7/-10.6°C, respectively.  Soil temperatures kept relatively high in the high tunnel. The lowest soil temperatures outside/inside high tunnel were 5.9/9.2ºC in November, and -15.1/2.2ºC in February. Growing Degree Days (GDDs) at the 4.4°C and 10ºC base were greater inside the high tunnel. The total GDDs for the month of November were 293.6 and 184.0 inside, and 153.2 and 73.0 outside the high tunnel, respectively.  PAR and SR levels were lower inside the high tunnel. A cloudy day (February 9) had an average hourly reading of PAR at 131.2µmol/m²s and SR at 64.1watt/m² inside the high tunnel; while the readings were 194.4 µmol/m²s for PAR and 90.4 watt/m² for SR outside the high tunnel. This reflects a reduction of 33% in PAR and 30% in SR. On a sunny day (February 11), the average PAR and SR were 611.6 µmol/m²s and 332.4 watt/m² inside the high tunnel, and 866.6 µmol/m²s and 410.5 watt/m² outside the tunnel, which were 30% and 20% reduction, respectively. The preliminary result suggests that, using a high tunnel in zone 7, cool season vegetables can be grown from late fall to early spring, and warm season vegetables can only be grown in late fall and early spring. This research will continue for another year to confirm the preliminary results.