In the northeastern US, cold-tolerant crops such as spinach are an important source of winter income for high tunnel growers. Supplemental rowcovers are used on crops inside unheated high tunnels to expose plants to optimal temperature ranges for longer periods of each day and to provide greater protection from injurious low temperatures. Rowcover management impacts temperature, light availability, humidity, pest populations, crop yield, and labor requiremenst. Some growers actively manage rowcovers, removing them daily to optimize light exposure and then reapplying them in the evening and on cloudy or very cold days. This is thought to provide a yield benefit due to increased crop light availability, but it is labor intensive and experienced growers often make decisions about removal based on a combination of current conditions and forecasted weather. Therefore, some growers opt for a “set it and forget it” approach, where layers are applied once and then only removed as needed, rather than daily. Here we describe the tradeoffs of “active” rowcover management, where rowcovers are removed during the day and replaced nightly, compared to “passive” management, where rowcover is removed only for harvest and regular maintenance. We conducted a replicated trial in two experimental years in an unheated high tunnel to investigate the effects of rowcover management on winter spinach yield, pest abundance, and abiotic environmental factors including growing degree hour accumulation. We also compared our observations with those from commercial tunnels managed by three experienced growers in the northeast. In both experimental years, growing-degree hour accumulation and the cumulative amount of time below freezing did not differ significantly between active and passive management. However, active rowcover management did result in greater light accumulation, lower relative humidity, and increased spinach yields in one experimental year compared with passive management.
