The use of HT has successfully extended production time (early- and late-season) of several high value crops (Hunter et al., 2012; Rader and Karlsson, 2006; Rowley et al., 2010; Waterer, 2003) in cold or high elevation climates. By passively trapping heat under plastic covers, HT air temperatures can be 10 to 30 °C higher than ambient air temperatures during the day (Wien et al., 2006) and 3 to 5 °C higher at night (Wien et al., 2006). HTs expose plants to conditions closer to their optimal temperature range for longer period each day, thereby improving growth rates and reducing low temperature–related stresses. However, during the winter, cold air and soil temperatures as well as seasonal changes in light levels become the main limiting factors affecting crop performance in HT.
Plant growth has a predictable response to air temperature. The minimum or base air temperature for spinach growth is 2 to 4 °C (Boswell, 1934; Koike et al., 2011; Maynard and Hochmuth, 2007), although some report values as cold as 0 °C (Marques, 2016). Spinach leaves can withstand temperatures much lower than this, when acclimated, but no growth occurs at very low temperatures (Fennell and Li, 1987). Whereas unacclimated spinach plants are cold tolerant, acclimated plants show decreased susceptibility to both cold and light damage (Ruelland et al., 2009; Schӧner and Krause, 1990). Acclimated spinach plants are capable of surviving air temperatures as low as −17 °C (Schӧner and Krause, 1990), although low temperatures significantly lower photosynthesis rates. At air temperatures below the base temperature for spinach growth, decreased leaf initiation and expansion rates lower final yields. As temperatures rise above the minimum, growth rates accelerate. Maximum growth rates for spinach occur between 15 and 23 °C, the recorded optimal air temperature range (Iwama et al., 1954; Koike et al., 2011; Maynard and Hochmuth, 2007). When exposed to temperatures above the optimum, growth progressively slows down, the risk of bolting increases, and at the maximum growth temperature (24 to 29 °C), growth ceases (Koike et al., 2011; Marques, 2016; Maynard and Hochmuth, 2007). If air temperatures rise further, spinach plants could experience damage or death (Leone et al., 2003).
Root zone temperatures also affect spinach growth. Like air temperature, soil temperatures inside a HT are higher than soils outside during the winter months. However, soil temperatures within a HT during fall or winter months are often below optimal ranges, particularly at night and on cloudy days (Bumgarner et al., 2011; Hunter, 2010). Although soils have the thermal mass to store heat, once soils become cold, they require significant energy inputs to warm them, particularly if they are moist. Recorded optimal soil temperatures for spinach seed germination, root growth, and functionality are between 7 and 23 °C (Maynard and Hochmuth, 2007). Root zone temperatures above or below this alter normal root growth and affect the plants’ ability to uptake water and nutrients from the soil (Bumgarner et al., 2012).
In addition to low temperatures, HT experience large diurnal temperature fluctuations which are quite common in the western United States (www.climate.usu.edu). Within a HT, day-to-night temperatures can fluctuate by 40 °C or more. While spinach is cold hardy and frost tolerant, the effect of extreme diurnal temperature fluctuations on plant production is unknown. Growth could be stunted, and irreversible damage to plant tissues and important physiological processes could occur, resulting in diminished yields. Cloud cover and associated weather conditions (rain/snow) can influence these fluctuations. As autumn transitions into winter and then to spring, there is an increase in cloudy or snowy conditions in the mountain regions of the western United States (www.climate.usu.edu). These events are variable but may have a significant influence on potential crop productivity in fall or winter production periods.
The use of secondary covers (low tunnels and floating rowcovers) within HTs can increase temperatures around plants beyond that of the HT alone. When secondary covers are used in conjunction with a HT, plants with extra protection may spend a longer period each day at temperatures near or within their optimal range (Bumgarner et al., 2011, 2012; Hunter et al., 2012; Wien et al., 2006). Additional layer of plastic or fabric can increase air temperature adjacent to plants by 5 to 10 °C on sunny days and 3 to 5 °C at night (Lamont, 2009; Waterer, 2003; Wells and Loy, 1985; Wien et al., 2006). Borrelli et al. (2013) noted the need to better regulate extreme temperature fluctuations and to maintain consistent growing conditions for crops throughout the winter. They suggested that low tunnels or rowcovers be used inside HT to enhance crop growth when additional heat or insulation is necessary. Thus, the combination of HT and secondary covers not only successfully increases biomass and yields of a range of vegetables (Bumgarner et al., 2011; Hunter et al., 2012), but also exposes plants to temperatures closer to their maximum which may be harmful to productivity.
The addition of soil heating cables generally leads to an increase in the total yield for early spring grown tomatoes, bush beans, peppers, broccoli, and lettuce (Bumgarner et al., 2011; Hunter et al., 2012; Rykbost et al., 1975) and results in increased yield for winter-grown lettuce (Bumgarner et al., 2011, 2012; Hunter, 2010). Additional heat increased total plant biomass and nutrient uptake by maintaining root zone temperatures above freezing and closer to optimal root temperature. Spinach with its greater cold tolerance should see similar increases in production levels during late fall and early winter production periods when provided with supplemental heat.
The objective was to evaluate the use of secondary covers in concert with heating to provide additional protection during HT fall and winter production periods.
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