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Plant hardiness zones in Minnesota.

Minnesota is divided into zones 3 and 4, according to the U.S. Department of Agriculure's Zone Map. (Note on image: A narrow area of zone 4 along Lake Superior, probably no more than half-mile wide in most locations.)

Be wary of new hardiness zones

From eNews, Feb. 22, 2007

In Minnesota, plant hardiness is an important consideration. Gardeners who want to create long-lasting, sustainable yards and gardens should select reliably hardy plants--or plants that are tolerant of cool temperatures, light frost and cold winds--for the majority of their landscapes, say University of Minnesota Extension horticulturists Mary Meyer and Nancy Rose. Minnesota falls primarily within Zone 4 and Zone 3 on the current USDA zone map. According to a revised map developed by the National Arbor Day Foundation (NADF), parts of southern Minnesota, including the Twin Cities, are placed in Zone 5, characterized by average annual minimum temperatures of -10 to -20 degrees Fahrenheit. The NADF map also moves Zone 4 farther north, and leaves a small section of Zone 3 in the northernmost quarter of the state (see the revised map). "The NADF map uses only the most recent 15 years of climate data to determine zone borders," says Meyer. "The results reflect the string of above-normal-temperature winters in recent years, but may not accurately reflect long-term climate conditions." The USDA map has also been criticized for using climate data from a relatively short, though colder, period of time (1974-1986), Meyer points out.

Hardy research at the U

The University's Plant Hardiness Laboratory is renowned for its studies in cold hardiness. The concept of "Deep Supercooling" originated here in the 1960s to explain how--at the cellular level--plants survive winter. Over the years, laboratory-freezing tests plotted for various climate fluctuations have helped U researchers determine a cultivar's ability to withstand harsh winter conditions.

For a selection of the hardiest Minnesota plants, developed at the University of Minnesota, see "Minnesota Hardy."

Source: Minnesota Agricultural Experiment Station

"Both maps are also limited in predicting plant hardiness because they tell us only one thing: the average annual minimum temperature in a given region," says Rose. "A plant's ability to survive winter is affected by many factors, not just the single coldest temperature of the winter." Even in years when the minimum temperature only reaches the Zone 5 range, plants may be damaged or killed by sudden below-normal temperatures, especially in early or late winter, adds Rose. Soil moisture, snow cover and overall plant health are important additional factors that can affect winter survival. Since the exact conditions for next winter--let alone the next five winters--can't be predicted, Meyer and Rose recommend that Minnesota gardeners continue to choose landscape plants carefully. Less-hardy plants may require special care, including the application of winter mulch. Gardeners who want to experiment with less-hardy plants should consider starting with herbaceous perennials or small shrubs. If colder winters return and kill these plants, they can be readily replaced with hardier plants. Larger, long-lived landscape plants such as shade trees and large evergreens are costlier to replace, and their loss can be a serious setback to a maturing landscape. To learn more about cold hardiness, visit University of Minnesota Extension.

University of Minnesota Moment Listen to Extension horticulturist Mary Meyer discuss the changes in the hardiness zone map on U of M Moment.