High tunnels, also called hoophouses, are used worldwide to provide environmental control for raising horticultural crops (Lamont, 2009; Waterer, 2003; Wells and Loy, 1993). Protected cultivation with high tunnels is not as prevalent in the United States as it is in other parts of the world, but interest is growing rapidly for small-scale producers wanting earlier crop maturity and a longer season for sales at local markets (Carey et al., 2009). High tunnels are used primarily to minimize environmental effects on crop production either by extending the growing season in both spring and fall or providing protection from wind and rain in warmer months or climates (Lamont et al., 2003; Lang, 2009; Panter, 2007; Wein, 2006). High tunnels can modify the climate sufficiently to lengthen the growing season from 1 to 4 weeks in the spring and 2 to 8 weeks in the fall (Wells and Loy, 1993). Unlike traditional greenhouses, high tunnels do not have power for heat, lights, or fans, and crops are planted directly into the soil. Giacomelli (2009) describes thoroughly high tunnel design, construction, and environmental control.
Many crops have been successfully cultivated in the United States and throughout the world in high tunnels, including vegetable crops (Carey et al., 2009; Lamont, 2009; Lamont et al., 2003; Waterer, 2003), small fruit (Demchak, 2009; Lamont et al., 2003), cut flowers (Lamont, 2009; Wien, 2009), and small-statured fruit trees (Lang, 2009). Benefits that high tunnels provide to horticultural crops include soil warming, reduced soil compaction, and often reduced diseases and insect pests (Demchak, 2009; Lamont et al., 2003; Lang, 2009). As a result, crop yield and quality can be improved when crops are grown in a high tunnel (Carey et al., 2009; Demchak, 2009; Lang, 2009; Waterer, 2003).
Cultivar selection is an important consideration for winter production because crops must have tolerance to cold temperatures and freezing even in protected structures. Maynard and Hochmuth (2006) have classified many leafy green vegetables as cool season-hardy (cabbage, kale, mustard, spinach) or half-hardy (Chinese cabbage, lettuce). Information on optimum growing conditions is not available for all Asian greens (Brassica rapa L.) but Chinese cabbage grows best in moderate to cool temperatures (13 to 24 °C; Bennett et al., 2010) and can tolerate frost (Cramer et al., 2006). The optimum temperature range for lettuce (Lactuca sativa L.) is 13 to 16 °C but it can tolerate temperatures as low as –2 °C without damage (Mansour and Raab, 1996). The optimum temperature is ≈17 °C for spinach (Spinacia oleracea L.), but it can tolerate temperatures as low as –9 °C without injury (Koike et al., 2011). Spinach may be grown in the open field where winters are mild by seeding in mid to late September (Mansour and Raab, 1996). The cold hardiness of Asian greens, lettuce, and spinach suggest they may be ideal for winter high tunnel cultivation in Washington State and may expand the market for leafy greens in the region and other areas with similar climatic conditions.
The purpose of this study was to assess the yield of leafy green vegetables grown for a salad mix in a high tunnel during the winter in two climatically different locations in Washington and to characterize several environmental factors that may affect their yield. The specific objective was to examine the effects of location, crop type, and time of harvest on yield of cultivars included in this study. This information could provide growers an affordable opportunity to attain year-long local production.
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