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Over the Fence

Where gardeners come to find out what's happening out in the yard.

New USDA Plant Hardiness Zone Map

Posted by Sarah Navrotski -

Down the Garden Path

Richard Hentschel, Extension Educator

New USDA Plant Hardiness Zone Map

USDA recently released a new Plant Hardiness Zone Map that really changes what Extension Educators have taught and what gardeners have come accustomed to when shopping for hardy landscape plants. The first question most gardeners have when looking for new landscape plants is "Is it hardy in our area?" The question remains very valid as you would not want to purchase a plant that will not survive our winter weather in the home landscape. Gardeners are willing to take a chance once in awhile with some of the broadleaved evergreens like Holly, Rhododendron, Boxwood as long as they can plant them early in the season and provide winter protection for the first several years and IF they have a protected location .

The old zone map used the data from decades past that created localized pockets within larger zones. This new hardiness zone map uses data from just 1976 through 2005 to create the current zones. This map, just like the old one uses average annual minimum winter temperatures to create the new zones broken out into 5-degree divisions. Gardeners will see that those smaller colder or warmer pockets are now gone. The small band of Lake Affect is still immediately west and south of Lake Michigan in Chicago.

In Illinois the zones range from 0 to 5 degrees at the very southern portion of Illinois to -15 to -20 degrees in the Northwest portion of Illinois. Kendall, Kane, and Dupage counties are in zone 5b with temperatures of -15 to -10 degrees for our winter range. We used to be in 5a, having temperatures 5 to 10 degrees colder during the winter. These new zones reflect the trends we are experiencing the last several winters, not as cold or the cold does not stick around and for extended periods limited or no snow cover.

This change in the zone map will mean it will take awhile for the horticulture industry to catch up with accurate hardiness zone map information on the plant tags. There have always been some differences in the industry, based on which zone map you were looking at. There has been the map from USDA and the other from Arnold Arboretum out east. Any plant rated in a hardier zone than us will survive our winters easily.

The new site is interactive, allowing you to put in your zip code to reveal the hardiness zone you live in and you can also print out individual state maps. Besides the 48 states you can check out Hawaii, Puerto Rico and Alaska. The web site is: www.planthardiness.ars.usda.gov



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