The USDA’s New Plant Hardiness Zone Map – Evidence of Climate Change?

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The new USDA Plant Hardiness Zone map is more detailed than ever before, and through the USDA’s website, users can click to county-by-county higher-resolution maps. Image from

Just in time for you to order your plants and seeds for this growing season!

This month the U.S. Department of Agriculture publicized their new plant hardiness zone map.  This is the first update to the map since 1990, but thanks to incredible technological advances in weather measurements since then, users can expect a much higher resolution and more accurate product.  Click through to check out the map’s interactive features!

Stick to native plants and your local ecosystem will thank you. GeekMom Kathy uses her stalks of corn to allow beans to climb. Photo by Kathy Ceceri, used with permission.

Calculating the hardiness zones isn’t rocket science.  The zone is based on the average lowest temperature range a particular area reaches.  This doesn’t necessarily mean it’s the coldest it’s ever been.  Unlike past maps that used average minimum temperatures from the past 13 years, this time the map is averaging the past 30 years.  In addition, with more temperature measurement capabilities than ever before, users can expect more detailed data, with more refined delineations between zones, particularly along coastlines and near significant changes in elevation.

This map also introduces two new zones, 12 and 13, which are expressly for tropical and sub-tropical regions (namely, the Hawaii and Puerto Rico areas of the map).

On average, most of the U.S. has shifted by one-half zone on the warmer side.  Does this mean climate change is forthcoming?  Well, don’t base your conclusions on this map; there isn’t enough data.  The USDA wants to make it clear that their map is not indicative of climate change:

“Climate changes are usually based on trends in overall average temperatures recorded over 50-100 years. Because the USDA PHZM represents 30-year averages of what are essentially extreme weather events (the coldest temperature of the year), changes in zones are not reliable evidence of whether there has been global warming.

The new (map) is generally one half-zone warmer than the previous (map) throughout much of the United States, as a result of a more recent averaging period (1974-1986 vs. 1976-2005). However, some of the changes in the zones are the results of… more sophisticated mapping methods…which has greatly improved accuracy”.

Another thing to keep in mind.  Shifts in these zones will mean growers will be tempted to introduce plants that aren’t native to their homes.  While this may make a great conversation piece in your front yard*, I ask you to exercise extreme caution (and fellow GeekMom Laura wants to caution you also).  Native plants are best for your home landscaping.  Introducing new plants can introduce imbalances in local ecosystems.

*This statement is coming from a girl who lives in a rental house decked out in front with Chinese fringe flower shrubs and hibiscus plants.

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2 thoughts on “The USDA’s New Plant Hardiness Zone Map – Evidence of Climate Change?

  1. As someone who grew up in an environmentally-conscious home in the 80’s, I am entirely convinced that climate change is not only occurring, but is accelerating even faster than originally expected. That being said, I don’t believe that the USDA’s map is any sort of evidence in favour of that.

    The map uses average temperatures from the last 30 years (which, I believe, overlaps the time period used in the last map), and as most credible scientists who study climate change will tell you, that’s not really a significant enough sample. As well, the map only talks about average low temperatures; it doesn’t take in to account the wider fluctuation at the top and bottom of the scale that we’ve been seeing.

    With regards to the advice about native plants, I agree that caution is warranted when bringing in a plant species that is completely new to a particular area/climate. However, there can be a BIG difference between non-native and invasive – the two aren’t inextricably linked.

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