Effects of High Tunnels and Secondary Row Covers On Temperatures and off-Season Production of Zinnia Cut Flowers

This study was conducted to evaluate the use of secondary row covers for off-season production of zinnia, a warm season cut flower (Zinnia elegans ‘Benary’s Giant Mix’) in high tunnels in central Mississippi. There were four secondary cover treatments in each of three high tunnels: no secondary cover as control, spun-bonded polypropylene, polyethylene, and felted black landscape fabric. The row covers were pulled over 5 ft. tall frames built over the crop rows. A definitive high tunnel venting and row covering protocol was used based on the actual external and internal temperatures for the daytime and forecasted external temperatures for the night. Air temperatures were recorded inside and outside of the tunnels, and under each cover treatment. During December 2009, the maximum, minimum and average air temperatures inside the high tunnels were higher by 17.9, 4.9, and 8.0 ^oF, respectively, compared to the temperatures outside the tunnels. However, there were no significant differences among air temperatures under the four secondary cover treatments inside the high tunnels. High tunnels and high tunnels + secondary row covers added 414 and 444 more base 50 ^oF growing degree days (GDD₅₀), respectively, compared to the outside GDD₅₀ accumulation. However, GDD₅₀ under secondary covers were not significantly different from that in the uncovered control. When the minimum temperature outside of the tunnels decreased to the teens in early January, 2010, no secondary row cover treatment prevented significant plant injury from these severe frost events. We were unable to produce marketable zinnia cut flowers in high tunnels in central Mississippi beyond the frost free period in 2009-2010 even with the secondary covers used in this study.