U horticulturists urge caution on new hardiness zones

ST. PAUL, Minn. (2/9/2007) - Horticulture experts with the University of Minnesota Extension recommend caution in regard to a recently released map that shows changes in the state's plant hardiness zones.

Extension horticulturists Mary Meyer and Nancy Rose say Minnesota gardeners who want to create long-lasting, sustainable yards and gardens should select reliably hardy plants for the majority of their landscapes.

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.

"The NADF map uses only the most recent 15 years of climate data to determine zone borders," Meyer said. "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.

"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," said 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, Rose said.

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.

The USDA hardiness zones map can be accessed online at http://www.usna.usda.gov/Hardzone/ushzmap.html. To view the revised NADF map, visit http://www.arborday.org/media/zones.cfm.

Sources: Mary Meyer, professor and Extension horticulturist, and Nancy Rose, Extension horticulturist

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URL: http:// www.extension.umn.edu/extensionnews/2005/Hardinesszone07.html  This page was updated Feb. 9, 2007 .
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