If you’re an avid gardener, you have probably seen that the National Arbor Day Foundation has released an updated version of the U.S Department of Agriculture’s hardiness zone map, which was last updated in 1990. The NADF has used data from 5000 National Climatic Data Center stations around the country from the last 15 years to compile their information and come up with the new zone map. The data for both the USDA and the NADF maps are based on average annual low temperatures.

According to the NADF:

The new 2006 arbor day.org Hardiness Zone Map is consistent with the consensus of climate scientists that global warming is underway. Tree planting is among the positive actions that people can take to reverse the trend.
Of course existing trees should continue to be cared for… Certain species may be more vulnerable to stress with the current warmer climate, but they will continue to provide environmental and economic benefits as they grow. It’s just a good idea to consider more tree species diversity for the future.”

Go to their site to view the updated climate zone map, which will appear similar to the picture to the upper right. You can also look up your own climate zone, or compare the old USDA climate zones to the new NADF ones. (I have also included a place to look up your NADF climate zone on this blog entry). While you’re at it, check out the NADF press release, which will explain the process of compiling the new info in more detail.
[ Click here to look up your arborday.org hardiness zone.]

Just because we may be in a slightly different zone doesn’t mean that we can begin planting landscape plants that were meant for the next warmer climate zone. There are more factors than just the minimum temperatures in a given region. There can be any number of things that effect a plant’s ability to survive a winter in Minnesota.
I have been through winters with average temperatures that were devastating to evergreens because of an early ground freeze and a lack of snow cover. The same is true of of other plants, especially ground covers perennials that were planted too late in the season.

A dry fall can also be fatal to plantings (especially evergreens, which can continue to loose moisture in the winter), or plants that entered late fall in a stressed condition. Be sure you find out more about some of the “new introductions” you may read about in your plant catalogues. You can also find out about reliably hardy plants that were developed in Minnesota by checking out the publication “Minnesota Hardy”.http://www.maes.umn.edu/MNHardy/index.asp

According to an article in UMNews, we should proceed with caution.

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…..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.