Understanding a Hardiness Zone Map
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Understanding a Hardiness Zone Map

By Damian Fagan (D_Fagan)February 3, 2014
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The term hardiness refers to a plantís ability to withstand a yearly minimum temperature average. Average is the key, as plants may experience temperatures lower than the average.

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Gardeners that inquire about whether a certain plant will grow in their yard are often asked in return, "What zone are you in?" To the new gardener this might be an odd response, but experienced gardeners know this question can be answered by referring to one of several zone maps - for U.S. gardeners, one of the most common is the one produced by the U.S. Department of Agriculture (USDA) and Oregon State University.

The newest version of the map was completed in 2012 by the USDA and Oregon State University. OSU developed a newer version of the PRISM software which allows gardeners to upload an interactive version of the GIS-based map (Geographical Information Services). The color coded map incorporated 30 years of weather data (1975-2005) and is based upon the average annual minimum temps.

These are not the lowest of the lows, and there are 13 zones of 10 degree increments.

Each of the 13 zones is further split into an "a" and "b" microzone that represents a 5 degree range. As an example, I live in Central Oregon and my home is zone 6b (-5 to 0 degrees F). Yeah, brrrr and good luck growing tomatoes outside.

Armed with this information, gardeners can select plants that are likely to survive and thrive in their location. Most plants in a nursery have their plant hardiness zone(s) printed on their identification label. As an example, the ‘Autumn Joy' sedum that I planted in my garden is rated for plant hardiness zones 3-9. Or the vine maple that we planted in a shady corner in the yard has a rating of 6a to 9b.

Of course, there may be microclimates, fine-scaled variations of this hardiness zone that exist within an area. Those sites may be influenced by blacktop, concrete, small hills, canyons and valleys, and represent locations that are either above or below the determined plant hardiness zone for your area. Gardeners can take advantage of these microclimatic sites to grow plants not rated for their zone. For Canada there is the Agriculture Canada Plant Hardiness Zone Map which uses a wider range of climatic variables to determine hardiness zones.

Although early zone maps were developed in the 1920s and '30s for the United States, the first USDA plant hardiness zone map was published in 1960. That map had 10 zones that were also based on 10 degree F. increments. This map was redone in 1990 using temperature data over a 12-year span (1974-1986). With the inclusion of Alaska and Canada to this 1990 version, one new zone was added and the zones were split into 5 degree increments with an "a" and "b" designation. But warming changes in the wintertime minimal temperatures over the past thirty years prompted an update to the map.

Wider ranges in annual temperature and precipitation averages were reasons behind the update. It is interesting to note that the changes reflected a northward movement of the plant zones, indicating a warming trend that may be linked to global climate change.

An interactive version of the map is available on the USDA's website. Just select the Interactive Map tab, enter the provided validation code, and then enter your zip code. Zoom in to the street-level scale to see pop-up windows that indicate the hardiness zone for any backyard location.

Though the plant hardiness zone map is not the only tool in a gardener's basket, it is an important one for maintaining success in the garden.


  About Damian Fagan  
Damian FaganDamian is contributing writer to Dave's Garden. He is a freelance writer, hiking guide, gardener and wreath-maker living in Central Oregon. He has published several books including Pacific Northwest Wildflowers and Canyon Country Wildflowers with Globe Pequot Press.

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Discussion about this article:
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Improper link herbella 1 11 Feb 4, 2014 11:01 AM
Understanding a Hardiness Zone Map AlpineBloke 1 28 Feb 3, 2014 4:21 AM
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