Northern Minnesota is typical of many high-latitude and high-elevation areas of the United States in having cold winters and short, cool growing seasons. In the north central region of Minnesota, the Grand Rapids area averaged a growing season of 115 d from 1916 to 2009 and is classified as the USDA plant hardiness zone 3b (Cathy, 1990; National Weather Service, 2006). The harsh weather combined with the short growing season significantly increases the risk of freezing before a crop can be harvested for primocane-fruiting raspberries. In warm years with later killing frosts, growers can expect to have a reasonable crop of fall raspberries; but in other years, growers might have a minimal crop or none at all (D. Wildung, personal communication). For these reasons, there is very limited primocane raspberry production in northern Minnesota (USDA, 2009a), although the crop is one of high value.
High tunnels have the potential to extend the growing season by several weeks in the spring and fall, which would be a benefit for fall-bearing raspberries (Demchak, 2009; Heidenreich et al., 2009). In high tunnels with an additional 3 to 4 weeks of growing season (Wells and Loy, 1993), the majority of the primocane-fruiting raspberry fruit would be harvestable. Fall-bearing raspberries have been successfully grown in high tunnels in New York, Pennsylvania, and other cold-climate states, but the upper midwestern United States presents unique climatic limitations. Tomato (Solanum lycopersicum), pepper (Capsicum annuum), lettuce (Lactuca sativa), and other annual vegetables have been successfully grown in high tunnels at Grand Rapids, MN, since 2003 (Wildung et al., 2007). One of our research goals was to evaluate the potential for perennial crops in high tunnels and primocane-fruiting raspberry is an ideal crop to investigate. This high tunnel primocane raspberry experiment was initiated in 2008. The objectives were to investigate the potential of high tunnel season extension for fall raspberries, compare commercially available cultivars, monitor crop nutrition, pests, and diseases, and assess possible winter damage in a USDA plant hardiness zone 3b area.
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