This massive AVA , the world's largest , covers 29,914 square miles and portions of southeast Minnesota, southwest Wisconsin, northwest Illinois, and northeast Iowa. This AVA has a cool continental climate which lead many wineries to prune after harvest and bury the vines in mulch to protect them from the cold winter weather. The Mississippi River and tributaries provide sloping riverbanks that allow sun exposure and soil drainage. Limestone is found underneath mainly silt loam and clay soils.The application for the UMRV AVA was based upon evidence of a glacial retreat 15,000 years ago. Hybrid grapes flourish in this area, including Marechal Foch, Frontenac, Saint Croix, Chardonel and Edelweiss.Many new wineries rely on these new cold-climate grape varieties developed by legendary horticulturist Elmer Swenson of Osceola, Wis., and the University of Minnesota. They resist disease, withstand winter temperatures down to –20 or even –40 degrees Fahrenheit, and ripen early.