In the Garden:
On the USDA Map website, you can download and print the countrywide 2012 USDA Hardiness Zone Map or select particular regions -- the Northeast is shown here-- or individual states.
Now What's My Zone?
With much fanfare, the USDA has released the long-awaited, updated version of its hardiness zone map. Some areas of the country now have new zone designations, and this has lots of folks talking and wondering. Does this mean my climate had changed? What do the shifts in zones mean for me? So it seems like a good time to review just what the map shows and how knowing what zone you're in helps you garden more successfully. Here's the scoop!
What Do Those Zone Numbers Mean?
The hardiness zone map of the U.S. is divided into zones based on the average annual extreme minimum temperature in a particular area -- in other words, the coldest temperature, on average, that occurs over the winter. There are now 13 zones, (two new ones have just been added) further subdivided into "a" and "b" categories. Each zone represents a temperature range of ten degrees; each "a" or "b" subdivision five degrees. The lower the number, the colder the zone; and "a" is colder than "b."
So, for example, my garden in northwestern Vermont is located in Zone 4b, meaning the average lowest temperature of the year is between -20 and -25 degrees F. (Yes, it can get cold up here!) Often in garden references, the zone ratings you see for plants don't bother with the "a" and "b" subdivisions; in this case, Zone 4 temperatures span the range from -20 degrees to -30 degrees F.
A Little History
The first standardized USDA hardiness zone map was put out in 1960. It was revised in 1965; then a major revision was released in 1990, which is the map that has been in use until this new 2012 map was unveiled. The 1990 map included Alaska and Hawaii for the first time, as well as Canada and Mexico, along with a new zone, Zone 11. It drew its data from almost twice as many weather stations as the previous map, so it provided more detailed information, but it was based on only 13 years worth of temperature data, from 1974 through 1986.
What's New in the New Map
For years there have been calls for a new map that better reflects the trend of warmer winters in recent decades. In 2003, a draft revision was made, but rejected for a number of reasons. The new 2012 map is based on temperature data from the thirty year period 1976- 2005, so more recent temperature trends are incorporated into its zone demarcations. It is also based on data from even more weather stations than the previous map, enhancing its accuracy.
How different is this new map from the 1990 version? While a few areas were moved to slightly cooler zones, the majority of the changes reflect a shift to warmer zones, with the zone boundary generally moving up by a half zone, or a five degree change. So for example, an area that was listed as Zone 5a (-15 to -20 F) on the 1990 map may now be in Zone 5b (-10 to -15 F). While many of these changes are due to changes in climate patterns, some are a result of the increased regional accuracy of the map to better reflect the influence of local geography, like changes in elevation or nearness to a body of water, on temperature extremes. Another change is that Canada and Mexico are not included in this new version, but Puerto Rico is.
One of the handiest aspects of the new map is its interactive feature that lets you find your zone by typing in your zip code on the map website. You can also zoom in on the map of your state to see the zone boundaries in detail and download and print maps of various sizes and resolutions. Try it yourself at http://planthardiness.ars.usda.gov/PHZMWeb/.
Many of you are already familiar with National Gardening's similar interactive zip code/hardiness zone finder, which has been a popular feature on our website for years. We are currently working to incorporate the zones changes from the new USDA map into our zone finder software so that it is up to date with the 2012 revision.
What Does It All Mean?
How helpful is it to know what hardiness zone you're in? While this bit of information can be a useful guide to help you select plants that will thrive in your climate, it's important to recognize just what a zone designation tells you and what its limitations are.
As I mentioned before, the zones are based on the average lowest temperature that's likely to occur each year. So some years it may never get that cold; other years the mercury may dip lower. Since winter low temperature is an important factor in plant survival, zone ratings can be useful tool in deciding what plants will succeed in your landscape. But there are many other factors besides the lowest winter temperature that influence a plant's winter hardiness, including temperature fluctuations, the timing of temperature changes, soil conditions, and exposure to sun and wind. The hardiness zone rating is just one factor to consider when selecting plants.
It's also important to keep in mind what the zone map doesn't show. While there may be some correlations, the map does not give you information on winter temperatures apart from extreme lows, frost dates, length of the growing season, summer heat, temperature fluctuations, rain or snow fall, or the likelihood of other extreme weather events, all of which have an impact on the plants in your garden. Your zone rating also does not take into account very localized micro-climatic conditions; this is where your careful observation of your own landscape over time really pays off.
What About Global Warming?
I've been asked if this new zone map shows that we are in the throes of global warming and if it indicates that our climate will continue to get warmer. But taken by itself the map is not designed to answer these questions. Remember that what it shows is the average lowest yearly temperature over the last thirty years, which has indeed moved higher in many areas in recent decades. But the map doesn't provide information about winter temperatures apart from these extremes, or about summer temperatures or any other climatic parameters. And it's not a predictive tool; it offers no information on what might happen in the future.
I think that there is lots of sound scientific evidence supporting climate change and the role of humans worldwide in driving it, and that it is a vital issue to be addressed. But the hardiness zone map does not reflect the spectrum of climatic changes that may be occurring due to global warming. Figuring out if and how much the climate is warming is based on overall climate trends that take lots of factors in account, of which the average of extreme winter lows shown by the zone map is just one.
We New England gardeners tend to pay a lot of attention to hardiness zone ratings because our cold winters often make extreme winter lows the most limiting factor for many plants. (No gardenias and camellias for us, I'm afraid!) So the zone rating for a plant is a good place to start the selection process. But, as I noted above, there are additional factors to consider as well.
How does the new map change things specifically for us northern gardeners? Where there have been changes to warmer zones, gardeners can feel more comfortable about expanding their palette of landscape plants to include choices that in the past may have been considered "iffy" or not hardy. And garden centers and nurseries may feel more confident about offering a wider range of plants for sale.
And did my own hardiness zone change on the new map? Nope -- my garden is still in Zone 4b according to my zip code. But because I'm right on the boundary of Zones 4 and 5, I frequently try Zone 5-rated plants -- and they usually do fine. I garden in very sandy soil, and the good winter soil drainage it provides can help marginally hardy plants survive the cold; if I had soggy clay soil, I might lose more plants. My general philosophy is to stick to zone 4 or lower ratings for the long-lived "backbone" plants in my landscape, such as shade trees and hedges, plants that would leave a costly hole if I lost them. Then I'm willing to take some chances. After all, what fun is gardening without experimentation?
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