High tunnels are one way for growers to modify the crop growth environment (Giacomelli, 2009). A high tunnel is defined as a plastic-covered, passively heated, walk-in, semipermanent structure (Jimenez et al., 2005). High tunnels offer some crop protection from insect infestations, rain, snow, and cold weather (Giacomelli, 2009). Generally, no supplemental or artificial heat source is used to create suitable conditions for crop production (referred to as passive-solar heating). The simplest and least-expensive design is a small high tunnel (e.g., 16 × 32 ft) that uses a single layer of translucent 6-mil polyethylene film, often treated with an ultraviolet light inhibitor, and that is constructed by the farmer on-site (Jimenez et al., 2005).
Most research in the United States has focused on crop production in high tunnels in the northeastern and mid-Atlantic states, including Pennsylvania (Lamont et al., 2003; Orzolek et al., 2004), New Jersey (Both et al., 2007), and Connecticut (Gent, 1992). Gent (1992) found that low irradiance limited production during the fall in high tunnels in the northeastern United States. Interest and research has expanded to other areas of the United States, including the Northwest (Borrelli et al., 2013) and Upper Midwest (Flavin Hodge et al., 2019). Research and literature on the use of these structures for winter production in medium- to high-elevation (1250 to 1700 m), semiarid areas of the Southwest (New Mexico, Arizona, and Colorado) is limited (Carey et al., 2009), although recent studies in New Mexico are encouraging (Hecher et al., 2014; Heyduck et al., 2019). Although northern latitude states experience long winters with short days and frequent overcast conditions, the southwestern United States has predominately sunny days. Even during winter, New Mexico receives 70% to 75% of the possible sunshine (New Mexico Climate Center, 2019). Due to geographic location and limited cloud cover, the Southwest receives more sunlight than any other region of the United States.
The use of high tunnels could allow New Mexico growers to extend greens production into the winter months and fill the winter production gap. Partially due to elevation, New Mexico winters are relatively cold and characterized by large annual and diurnal temperature ranges, low humidity, and low precipitation. Therefore, high tunnels could be used as a way of modifying the crop production environment in New Mexico, but detailed temperature information is limited.
The air temperature inside a typical single-layer plastic high tunnel will drop to the same temperature as the outside air at night if there is no heat curtain (a curtain that is used to trap heat inside the high tunnel) or other source of supplemental heat (Both et al., 2007). Additional high-tunnel structural inputs could increase the capacity of the high tunnel to retain heat as the temperature drops during the night, but they will increase the cost of the structure. For example, a double layer of plastic film can insulate the sidewalls and roof of the high tunnel, and a thermal mass (such as water or rocks) can capture, retain, and release heat into the high tunnel at night. In a high-tunnel study comparing different tunnel designs and spring tomato (Solanum lycopersicum) plant sowing dates in Tokat, Turkey, it was found that a double-layer high tunnel had a higher tomato yield than a single-layer high tunnel, likely due to increased temperatures (Saglam and Yazgan, 2000).
Rowcovers typically increase temperatures 2 to 3 °C (Wells and Loy, 1993), and can also moderate temperature fluctuations. Air under rowcovers warms faster on cool mornings than ambient air in a high tunnel, and the heat is retained under rowcovers for longer in the afternoon (Wells and Loy, 1993).
Given the space limitations in high tunnels, extensive replicated testing of different species and varieties of horticultural crops is difficult. Environmental data can provide important baseline information used to describe the potential of high tunnels to support various crops grown during the winter. In this study, three passive-solar high-tunnel designs of different expense and expected heat-retention capacities were evaluated across three winter seasons (2009–12) in southern New Mexico near Las Cruces [U.S. Department of Agriculture (USDA) plant hardiness zone 8a (USDA, 2019a)] and north-central New Mexico at Alcalde [USDA plant hardiness zone 6a (USDA, 2019a)] for their capacity to provide a growing environment to produce a winter (October–March) crop: 1) a high tunnel with a single layer of woven polyethylene covering (SL), 2) a high tunnel with a double layer of woven polyethylene covering (DL), and 3) a high tunnel with a double layer of woven polyethylene covering plus a thermal mass of water barrels (DL+B).
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