High tunnels are plastic-covered, greenhouses where crops are typically grown in the soil and the temperature is passively managed (e.g., ventilation, shadecloth, heat sinks, insulation, etc.) (Wells and Loy, 1993; Wittwer and Castilla, 1995). The popularity of high tunnels has been growing steadily in the United States (Carey et al., 2009; Hochmuth and Toro, 2014). Using high tunnels allows farmers to reduce weather-related risks; improve product quality and consistency; assist with organic growing efforts; and be productive on limited amounts of land, amplify season extension opportunities, or both (Carey et al., 2009; Lamont, 2009; O’Connell et al., 2012). Since 2010, competitive cost-share programs administered by the Natural Resource Conservation Service have provided an additional incentive for growers to invest and explore the benefits of these protected environment systems.
There is a lack of information related to adapting high tunnels to humid, subtropical climates in the southeastern United States including parts of Alabama (AL), Arkansas (AR), Florida (FL), Georgia (GA), Louisiana (LA), Mississippi (MS), North Carolina (NC), South Carolina (SC), Texas (TX), and Virginia (VA) according to the Koppen climate classification system. Currently, the Southeast is at a disadvantage for using high tunnels optimally compared with cooler and less humid areas where a greater body of knowledge exists. Key areas needing additional research include structure design, crop selection, planting dates, cultivar choice, crop rotation strategies, pest and disease management and other cultural practices.
The state of GA, similar to many locations in the Southeast, is subject to short, mild winters interrupted by cold snaps (NCEI, 1982). The average daily winter temperatures range from 4 to 13 °C which is reflected by USDA hardiness zones 6a to 9a (USDA, 2012a). The number of nights subject to freeze warnings has a wide range (e.g., 10 to 110 nights) depending on latitude and elevation. From mid-November to mid-March, the hours of daylight range from ≈10 to 12 h with a daily light integral (DLI) ranging from 15 to 35 mol·m−2·d−1 (Korcyznski et al., 2002; USNO, 2015). There is a lack of information about the minimum average DLI required for many fruit or vegetable crops but the predicted temperatures indicate that many opportunities to grow and/or hold crops under high tunnels in the Southeast during the fall/winter season is possible. This is different from, for example in Maine, where less than 38% of high tunnel growers plant winter crops (Fitzgerald and Hutton, 2012). Typical high tunnel crops grown in the Southeast during the fall/winter season include salad and cooking greens, root crops, alliums, and overwintered flowers. It is unclear, if these choices reflect information from cooler or more light-limited regions which have influenced high tunnel grower crop choices or if they are related to profitability (i.e., customer demand, quick turnover, etc.).
The following studies have evaluated high tunnel use for fruit and vegetable production in the humid, subtropical areas documenting benefits and challenges associated with their use. A strawberry study conducted in FL found 54% greater yields and marketable fruit weight 69% greater under high tunnels compared with open field production (Salame-Donoso et al., 2010). In NC, differences among strawberry cultivars correlated with greater yields under high tunnels indicating the importance of crop and cultivar selection (Gu et al., 2017). An organic tomato study in NC found that heirlooms grown under high tunnels in the spring and summer had equal or greater yields depending on the year, and they were consistently harvested three weeks earlier than the field (O’Connell et al., 2012). In GA, organic spring lettuce yields were equal or greater for a high tunnel system compared with the field depending on the year and were 2–7 d earlier (Jayalath, 2016). An early spring lettuce crop in TN achieved not only greater total yields under high tunnels compared with the field but the crop was also subject to more pest, disease, and physiological damage, reducing marketability of the crop (Wallace et al., 2012). Two studies (GA and NC) found that early blooming of small fruits (i.e., strawberry and Southern highbush blueberry) under high tunnels may increase frost damage and fruit abortion because of early bloom times (Gu et al., 2017; Ogden and van Iersel, 2009). Overall, high tunnels appear to offer a benefit in the region but yearly weather conditions, structure/system characteristics, cultivar selection, and crop sensitivity to fluctuating temperatures may affect potential yield gains.
Informally interviewed high tunnel growers, in the same climate and hardiness zone as our research project, indicated that they would be interested in integrating other crops into their fall/winter portfolio. This would help meet growing year-round and winter holiday market demand for local and organic produce and help expand crop rotation options (Celia Barss, Nicholas Donck and Jay Payne, personal communication, Summer 2015). The National Farmers Market Directory also reports the rapid growth of winter markets (USDA, 2012b).
Open-field broccoli and cauliflower are generally planted in the late summer or early fall and harvested during the months of October and November in this area. The optimal temperatures for crop growth are 15 to 18 °C while avoiding temperatures ≥30 °C (Heather et al., 1992; Maynard and Hochmuth, 2007). These temperature ranges indicate that growing broccoli and cauliflower during late fall and early winter under high tunnels would be possible although it is not typical. This schedule could lead to season extension opportunities from November through January. Therefore, this research project explored the possibility of growing atypical broccoli and cauliflower (Brassica oleracea L.) crops under high tunnels during the fall/winter season in northeast GA (Zone 8a). Specific objectives were to evaluate crop feasibility, planting dates, and cultivar choices.
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