Can't always get what we want! Finding and creating cold for hardiness screening at the University of Minnesota

Cold temperature is a major environmental factor limiting the diversity of woody plants grown in northern landscapes. Consequently, cold hardiness has been the primary selection criterion throughout the 57 year history of the University of Minnesota's Woody Landscape Plant Breeding Program. Specifically, the program focuses on developing woody landscape plants capable of thriving in USDA Plant Hardiness Zone 3 and 4 [average minimum temperatures of -28.9 to -40.0°C (-20 to -40°F)]. The majority of cold hardiness screening has been accomplished in field trials at the Horticulture Research Center (HRC) and Minnesota Landscape Arboretum which are located at 44°52"N, 93°38"W. However, in recent decades, these trial sites have rarely experienced true Zone 3/4 winter temperatures, causing us to consider alternative screening methodologies. Our most reliable alternative involved developing field plots at the University's North Central Research and Outreach Center (NCROC) located in Grand Rapids, Minnesota, 47°14″N, 93°29″W. To maximize screening efficiency and reduce labor costs associated with field trials, containerized seedlings are now transported from the HRC to Grand Rapids in late-October and installed in a high-density configuration within a snow-excluding polyhouse. Following exposure to ambient winter conditions, the plants are returned to the HRC in April and evaluated for cold injury following bud break. Additional hardiness screening is conducted in January at the HRC on potted bundles of cold-hardened seedlings that are placed in a programmable freezer and exposed to a single freeze/thaw cycle with a minimum temperature of -35 to -37°C (-30 to -35°F). Following treatment, seedlings are returned to cold storage and then evaluated for injury following satisfaction of their chilling requirement. The programmable freezer is also utilized to more fully characterize midwinter hardiness, acclimation and de-acclimation characteristics of seedlings selected from screening trials as well as commercially available cultivars. Combining outdoor freezing trials with controlled laboratory freezing tests provides both an effective system for identifying cold tolerant germplasm for our breeding program and a means of assessing the hardiness of new cultivars appearing in the marketplace.