Early Season Extension Using June-Bearing "Chandler" Strawberry in High-Elevation High Tunnels

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High tunnels have been used successfully in many areas of the world to extend the growing season for numerous crops. However, very little research has been conducted to evaluate the season extension benefits offered by high tunnels for small fruit crops in high-elevation growing areas such as the Intermountain West region of the United States. The use of high tunnels was investigated in North Logan, UT (lat. 41.766 N, elev. 1405 m, 119 freeze-free days) to extend the growing season for June-bearing strawberries. Growing systems included a fall-planted annual hill system and vertical growing systems in two different orientations. Optimum planting date for each system was determined by transplanting ‘Chandler’ plugs at 2-week intervals over 10 weeks. For the second year of the study, a field planting was also included. Over two seasons, the optimum planting dates were approximately the first week of September. The vertical systems were more susceptible to winter injury likely resulting from the temperature extremes in the root zone. Where winter injury was prevented, the vertical systems had higher yields per tunnel area than the in-ground system, but yield increases did not compensate for higher construction and management costs. The production window for the in-ground high tunnel planting was ≈4 weeks earlier than the field-grown plants and increased profitability by $13/m2 of tunnel area. The fresh market strawberry industry in the United States is dominated by production regions in California and Florida, where ideal growing conditions exist for long seasons (Sjulin, 2003). Ideal conditions for strawberries occur when temperatures are between 20 and 26 °C. Suboptimal temperatures (less than 20 °C) slow the growth and development of both the strawberry plant and fruit, whereas superoptimal temperatures above 35 °C cause the strawberry plant to stop growing (Galletta and Bringhurst, 1990). Coastal regions in southern California provide these conditions from spring to fall, and central Florida has mild conditions throughout the winter. Despite a lack of ideal growing conditions, small-scale strawberry production continues throughout North America, particularly in proximity to urban centers where fresh local produce commands premium prices. This local demand has recently been increasing as a result of the expanding local food movement. Conditions in the high-elevation valleys of the Intermountain West region of the United States are particularly challenging for strawberry production. Early spring temperatures are suboptimal, transitioning rapidly to summer temperatures that are typically superoptimal (Moller and Gillies, 2008). Wide diurnal temperature fluctuations in early spring also limit strawberry production in the region. The production window for June-bearing strawberries also coincides with peak national production and depressed wholesale prices (Pollack and Perez, 2008). The short growing season and depressed prices often make strawberry production in the Intermountain West only marginally profitable. The use of high tunnels offers an alternative approach. High tunnels have been successfully used to manipulate temperature and extend the growing season for many crops, including flowers (Rasmussen and White, 2006), vegetables (Orzolek et al., 2006), and small fruits, including strawberries (Demchak, 2009; Demchak et al., 2006). By extending the growing season earlier into the spring, direct market-oriented producers are better able to attract new customers, maintain current customers, and take advantage of higher out-of-season prices. June-bearing strawberries in an annual hill production system are particularly efficient for maximizing early-season yields (Sjulin, 2003). This annual system has since been adapted to colder climates (Black et al., 2002; Poling, 1993). Optimum fruit production in the fall-planted annual hill system requires balanced vegetative and reproductive growth. The principle environmental factors that condition physiological response in strawberries are growing temperature, photoperiod, and chilling (Galletta and Bringhurst, 1990). High tunnels can effectively manipulate temperature; however, chilling and photoperiod need to be manipulated in other ways such as selecting the proper nursery plant type and optimizing the planting date (Albregts and Chandler, 1994; Maurer and Umeda, 1999). Long days favor vegetative growth (crowns and runners), whereas short days favor reproductive (flower initiation) development (Galletta and Bringhurst, 1990). Ideally, plants will partition most of the energy to crown production in the fall and to fruit development the next spring. Runner production competes with crown growth and fruit development. Planting too early results in excessive runnering, whereas late planting results in inadequate plant establishment and crown formation. Fall planting dates for annual hill production have been optimized for production areas in Florida, California (Galletta and Bringhurst, 1990), and the mid-Atlantic region (Poling, 1993). However, developing annual hill systems for other growing systems and environments requires planting date optimization. The cost of constructing a high tunnel may justify the use of more intensive growing systems. For strawberry production in greenhouses, suspended containers and soilless media are often justified by the higher cost of maintaining greenhouse space (Takeda, 1999). Suspended systems offer means of maximizing the use of available sunlight, thereby increasing yields per production area. This study compared vertical and in-ground high tunnel strawberry production when planting date was optimized for each system. The objective was to develop methods for farmers to maximize yields of top-quality strawberries that could be sold for a premium price at local markets.

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