High tunnels are large, framed structures covered with a single or double layer of greenhouse-grade plastic with no electrical or ventilation systems and are typically used to produce high-value specialty crops including lettuce and other leafy greens (Knewtson et al., 2010; Lamont, 2009). High tunnels are employed as crop growth enhancers, providing climate protection during severe weather conditions and enabling season extension (Lamont et al., 2002; Reeves and Drost, 2012). In the United States, high tunnels have not previously been as widely used in horticulture production compared with many other countries (Carey et al., 2009; Rader and Karlsson, 2006; Wittwer, 1993). Recently however, improved technology, interest in local food production, and the availability of federal cost-sharing funds have stimulated interest in high tunnels among specialty crop growers. Corresponding research and extension efforts have similarly increased throughout the United States (Carey et al., 2009; U.S. Department of Agriculture, 2011).
High-value crops including fall/winter or early spring-grown spinach (Spinacia oleracea) and brassica greens (Brassica sp.), leafy and head lettuce (Lactuca sativa), and various herbs are commonly grown in high tunnels, especially in regions where climate protection is needed for season extension. Tomato (Solanum lycopersicum) and pepper (Capsicum annuum), small fruit, tree fruit, and ornamental and cut flower production in high tunnels is also expanding in the United States (Demchak, 2009; Lang, 2009; Wien, 2009a). High tunnel production has been popular for several decades in the north-central and northeastern United States, while other regions including the southeastern, midsouthern, and northwestern United States have only recently adopted this technology (Carey et al., 2009). The moderate climates during the fall and winter months in the southern and maritime Pacific northwestern United States may have initially contributed to the slow adoption of high tunnels in those regions. However, adverse weather has propelled grower interest in the adoption of tunnels as a means of added insurance for successful production of specialty crops.
Lettuce is harvested year-round within the United States, occurring in the early spring and late fall in northern climates, and during the winter months in southern latitudes (Dufault et al., 2006). Specific regional production cycles may vary depending on day/night temperatures, sunlight intensity, daylength, humidity, wind speeds, and elevation among other factors. During the winter months, high tunnels provide microclimates suitable for season extension allowing growers to plant lettuce earlier in the season, later in the season, or both. Optimizing the utilization of these microclimates can improve crop yield and quality (Zhao and Carey, 2009).
Depending on location, lettuce production may be limited during early spring and summer months because of unfavorable temperatures that increase the risk of bolting, tip burn, and leaf bitterness (Simonne et al., 2002; Zhao and Carey, 2009). Optimal growing temperature for lettuce is 18.5 °C, though heat-tolerant cultivars may tolerate higher temperatures provided that night-time temperatures are cooler (Dufault et al., 2006; Zhao and Carey, 2009). However, Dufault et al. (2006) reported that as planting dates progressed toward warmer temperatures and longer daylength, lettuce yield and quality were negatively affected. Flower initiation generally occurs between 21 and 27 °C; however, higher temperatures coupled with warm nights may initiate premature bolting and increase the potential for physiological tipburn (Maynard and Hochmuth, 1997; Simonne et al., 2002). Both disorders will decrease lettuce quality and marketability.
Winter and early spring lettuce production using high tunnels in southern regions may potentially mitigate physical/adverse risks associated with planting early open-field crops, and the cooler temperatures typical of earlier plantings in high tunnels allows for improved lettuce quality and yield (Zhao and Carey, 2005). In regions similar to the Texas High Plains, lettuce production is extremely limited because of severe climatic factors, including high temperatures, hail, high winds, and blowing dust, which are common from January through May. In contrast, shelter from excessive rainfall and soil moisture may be needed for lettuce protection in the maritime Pacific Northwest. Although enhanced high tunnel crop production offers growers the potential to increase sales and marketing, microclimate differences in tunnels may influence yield by decreasing potential for tipburn or bolting (Zhao and Carey, 2009). Adverse high tunnel microclimates may trigger both genetic and internal physiological mechanisms within individual cultivars, influencing overall productivity (Dufault et al., 2006). High tunnel microclimates may also create environments that are also unintentionally favorable to plant pathogens, insects, and weeds. Given these concerns, the objective of this research project was to employ a systems approach for comparing the production of six spring-planted lettuce cultivars for yield and quality when grown in high tunnels or open-field production systems located under three contrasting regions within the United States where high tunnel lettuce production has until now been less common.
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