Row Cover Management in High Tunnel Winter Spinach Affects Yield, Light Availability, Relative Humidity, and Heat Unit Accumulation

In the northeastern United States, spinach ( Spinacia oleracea L.) is an important winter crop for high tunnel growers. Supplemental row covers are used on crops inside unheated tunnels to allow for more favorable temperatures than those in high tunnels alone. We investigated tradeoffs of “active” row cover management, whereby row covers are removed daily to optimize light exposure and replaced nightly, compared with “passive” management, whereby row covers are placed seasonally and removed only for harvest and maintenance. Additionally, we compared conditions under row covers resting on wire hoops at two heights above the crop. We conducted an experiment during two years in an unheated tunnel in Durham, NH, USA. We hypothesized that passive management would be associated with greater growing degree hour (GDH) accumulation, higher relative humidity (RH), and lower light availability compared with active management. We expected greater yield in plots with active row cover removal because of the light integral bonus. In one winter, active row cover management increased spinach yields by approximately 20% compared with passive management. The GDH accumulation was significantly greater in passive than in active management in some months. Actively managed plots had 36% and 51% increases in cumulative light integral (CLI) compared with passive plots in 2022–23 and 2023–24. Season-average RH was approximately 3% greater under passive management compared with active management. Hoop height had an effect on RH in one year and an interaction effect with management on GDH accumulation in February of both years. We compared our experimental observations of GDH accumulation with three commercial tunnels and found that our ranges were representative of tunnels managed by experienced growers in our region. We also found that row cover management approaches differentially affect abiotic conditions, and that active management can provide a yield increase compared with passive management.