Lettuce (Lactuca sativa L.) is a popular, cool-season vegetable with a total U.S. production value of nearly $1.5 billion in 2013 (AgMRC, 2015). From 2005 to 2011, the amount of U.S. farmland allocated to the organic lettuce production increased from 4% to 12%, was worth $264 million in sales, and was the number one organic crop commodity (ERS, 2013; USDA, 2015). Currently, most of the organic lettuce is produced in California and Arizona (Toland and Lucier, 2011). Georgia can grow lettuce; however, as is typical of the Southeast region, unpredictable weather patterns, heat, and humidity can make crop production challenging. Alternative production techniques such as high tunnels may help growers mitigate unfavorable climate and weather conditions leading to increased lettuce production in the region. This would help meet growing demands for local produce, organic produce, or both (USDA, 2015).
The optimum temperatures for growing lettuce range from ≈7 °C/16 to 21 °C (nighttime/daytime) (AgMRC, 2015; Sanders, 2001). In addition, lettuce requires a minimum of 15 mol·m−2·d−1 of light (Korczynski et al., 2002; Runkle, 2011; Waycott, 1995). Given these criteria, Georgia is conducive to growing lettuce about 9 months out of the year (e.g., fall through spring). Lettuce production during late spring and early summer can be difficult as average daily temperatures may quickly or unpredictably rise above the preferred range. Warm temperatures may result in the induction of physiological disorders, such as bolting, bitterness, and tipburn (Prohens-Tomás and Nuez, 2008). In addition, it is predicted that the region will experience a growing number of days with temperatures greater than 35 °C and a steady increase in extreme precipitation events (EPA, 2016; Kunkel et al., 2013).
Precipitation and related periods of high RH may increase the incidence of fungal diseases, soilborne diseases or both, whereas strong winds can tear and abrade lettuce leaves. Precipitation events before or during the crop season can also delay field preparation activities. Management techniques such as high tunnels that increased crop protection and the ability to manipulate the crop microenvironment have the potential to increase yield and quality of lettuce production in Georgia.
High tunnels (i.e., hoop houses) are unheated, passively ventilated greenhouse-like structures which can provide some protection to crops from adverse weather events (i.e., cold, precipitation, wind, soil splash back, etc.), selected pests and diseases, or season extension (Alves et al., 2014; Borrelli et al., 2013; Carey et al., 2009; O’Connell et al., 2012; Rogers and Wszelaki, 2012). Early or late season extension may help growers receive premium prices (Alves et al., 2014; Sydorovych et al., 2013) and attract new customers. Furthermore, farmers that use high tunnels may be able to obtain a greater yield or higher quality product by manipulating the microenvironment compared with the field.
High tunnel benefits and management practices are often regionally specific because of local climate characteristics and market preferences. The following research efforts have been executed by others and provided a basis for our project goals which focus on challenges for high tunnel lettuce production in warm, humid regions. An organic high tunnel lettuce study conducted in Tennessee (TN), Texas, and Washington (WA) evaluated season extension; they observed greater bolting incidence in the field compared with under high tunnels in the regions where temperature fluctuations were more frequent (Wallace et al., 2012). A summer lettuce study conducted in Kansas found lower bolting rates when shadecloth was used in conjunction with high tunnels compared with the field but recommended further investigations (Zhao and Carey, 2009). A lettuce study conducted in South Carolina (SC) evaluated the best PDs for yield and quality in the field (Dufault et al., 2006). These researchers observed increased bolting when lettuce was planted in September, October, February, and March, but bolting rates were not different with PDs from November through January. Cultivar choice also influenced days to harvest and yield in this SC study (Dufault et al., 2006). A study conducted in WA determined that winter high tunnel production of Asian greens, spinach, and lettuce was possible, but could be optimized with more informed cultivar selection, seeding dates, and planting densities (Borrelli et al., 2013). A study conducted in North Carolina to evaluate the performance of organic tomatoes in high tunnels suggested that with proper management, one can achieve better yields, increased fruit quality, and provide season extension opportunities (i.e., early spring fruit) for high-value horticultural crops (O’Connell et al., 2012). Another high tunnel tomato study in TN found that high tunnel tomatoes had increased marketability and size and benefitted from earlier PDs compared with the field (Rogers and Wszelaki, 2012).
The Southeastern region’s mild winters present opportunities to grow crops from fall through spring seasons under high tunnels as they may help protect crops from abiotic stressors including cold temperatures, precipitation, wind, etc. However, managing excessive heat during late spring through early fall presents a challenge for growing cool-season crops such as lettuce under high tunnels. Therefore, the goals of this study were to evaluate the effect of high tunnels and PD on early to late spring organic lettuce production in Georgia. Objectives included a comparison of 1) butterhead and romaine lettuce yields grown under high tunnels and the field, 2) butterhead and romaine lettuce yields among three spring PDs, and 3) microenvironmental data for both types of production systems.
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