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["Brungardt, Sam" <Sam.Brungardt@state.mn.us>]

Title: RE: [NAFEX] Maps of change in U.S. hardiness zones

I recently wrote a sidebar about the USDA's and National Arbor Day Foundation's plant hardiness zone maps for the Minnesota Pollution Control Agency's quarterly magazine, Minnesota Environment.   Two knowledgeable people I interviewed, with the University of Minnesota Extension Service and the USDA's ARS, urged caution, especially in regard to planting according to the NADF map.  The USDA map is being revised, taking into account 15 more years of data, will be GIS compatible, and have a much higher resolution that the present one.  They hope to release it "in a matter of months." 

Here's some information from the University of Minnesota's Extension Service. -- Sam Brungardt, St. Paul, Minn.
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Horticulture experts from the University of Minnesota Extension recommend caution in regard to recent proposed changes in Minnesota’s plant hardiness zones. On the current USDA zone map Minnesota falls mainly within Zone 4 (average annual minimum temperature -20 to -30 degrees F) and Zone 3 (average annual minimum temperature -30 to -40 degrees F).  A revised map drawn by the National Arbor Day Foundation (NADF) places parts of southern Minnesota, including the Twin Cities, in Zone 5 (average annual minimum temperature -10 to -20 degrees F). It 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 the most recent 15 years of climate data to determine zone borders.  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).

 
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.  A plant’s ability to survive winter is affected by many more factors than just the single coldest temperature of the winter.  Even in years when the minimum temperature is only in the Zone 5 range (-10 to -20 degrees F) plants may be damaged or killed by sudden below-normal temperatures, especially in early or late winter. Soil moisture, snow cover, and overall plant health are some of the other factors that can affect winter survival.

Since the exact conditions for next winter or winter five years from now can’t be predicted, University of Minnesota Extension horticulturists recommend that Minnesota gardeners continue to choose landscape plants carefully. Less-hardy plants may require special care including application of winter mulches. 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 like shade trees and large evergreens are costlier to replace and their loss can be a serious setback to a maturing landscape. To create long-lasting, sustainable Minnesota yards and gardens, select reliably hardy plants for the majority of your landscape plantings.

-----Original Message-----
From: nafex-bounces@lists.ibiblio.org [mailto:nafex-bounces@lists.ibiblio.org] On Behalf Of Doug Woodard
Sent: Friday, August 17, 2007 10:05 PM
To: North American Fruit Explorers
Subject: [NAFEX] Maps of change in U.S. hardiness zones

See http://arborday.org/media/map_change.cfm

Doug Woodard
St. Catharines, Ontario, Canada
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