Tuesday, February 7, 2012

Does the new USDA Plant Hardiness Zone Map hold any changes for your garden?

Whether or not you are a gardener, you’ve probably heard of the USDA Plant Hardiness Zone Map.  This map divides the country into zones according to winter temperatures and is designed as a tool to assist in plant selection for different regions of the country.  Each zone differs from the next by 10 degrees Fahrenheit and the larger the zone number, the warmer the winters in that area. 
Just a few weeks ago, a new hardiness zone map was released.  Scientists at the USDA looked at weather data from the 30-year period of 1976 to 2005 and, as I’m sure I don’t have to tell you, the trend is toward warmer weather.  If you are a gardener, you’ve most likely already adjusted your plant selections to account for the changing weather.  The new USDA map, which is now interactive and highly specific for each area, just makes it official.

Many areas of the country are now in a warmer USDA zone than they were on the old map, but most of Central Texas remains unchanged.  We’re still zone 8b, which means that our average lowest winter temperature is 15 to 20 F.  This doesn’t mean that our average temperature for the entire winter is that low, it simply means that each winter we usually see at least one cold snap fall within that range.  Some winters we may not see temperatures that cold, but we could just as easily experience an extreme weather event where temperatures might fall well below our normal range.  

While most of Austin remains in zone 8b, where we were in the old map, if you zoom in on our region you’ll see some slight differences.  One of the cool things about the new map is that you can go to the USDA website, enter your zip code and get a very precise reading.  In fact, you can even get down to a particular location of longitude and latitude and see specific weather data for your exact location.  If you look at a close-up of the Austin region, you’ll notice that quite a large area west of MoPac and north of Bee Caves Rd is actually zone 9a, which means that it’s warmer than the surrounding area.

Most of Austin in still zone 8b. But the darker shaded area represents a slightly warmer zone 9a.

The new system lets you pinpoint a precise area, seeing data for that exact spot.

Even within small areas, temperatures will differ due to elevation, the urban heat island effect, and many other factors.  These areas are known as microclimates, and they may allow you to successfully grow plants that wouldn’t normally be recommended for your area.  Or, they might cause plants to do poorly in your yard that might do well across town or even at your neighbor’s house.  

While the USDA map is helpful, it’s still best to consult with local experts and nurseries about what particular plants will do well for you.  A plant’s hardiness isn’t, by far, the only characteristic that’s important.  You also need to know water requirements, tolerance to different soil types, fertilization needs, and heat tolerance, among many other details.  As we try to recover from the harsh summer of last year, it’s best to be armed with as much information and knowledge as possible.  We can’t change the weather, but we can change our reaction to it.

Instead of printing maps, the USDA chose to put the hardiness map only on the web.  If you’d like to print a copy, you can find high resolution images at the following website:  http://planthardiness.ars.usda.gov  There you can also find more information on plant hardiness zones and an interactive map where you can put in your zip code and zoom in on your exact location.  

And the Arbor Day Foundation has created an animated map that shows the changes between the old hardiness zones and the new ones.  It’s fascinating to watch as the colors move, virtually illustrating the colder temperatures receding northward.  It reminds me a little of a melting glacier.   http://www.arborday.org/media/mapchanges.cfm

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