Ministry Of ControversyScience Says

Hardiness Disinformation

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Successful gardeners are firmly rooted in what Karl Rove in 2004 famously disparaged as “the reality-based community.” That is, we study what we find around us and base our actions on that. I mention this because we are still, in a significant way, suffering from the Bush administration’s determination to “create [its] own reality”.  You will notice the effects, in fact, whenever you shop for plants this spring.

You’ll notice because of an intentional defect of the current USDA Plant Hardiness Zone Map. Throughout most of the United States, this is regarded as the ultimate criterion of whether or not a given plant will prove winter hardy in your garden. The current version of the map was published in 2012, but it was created in reaction to a version of the same map prepared by the American Horticultural Society in 2003, which showed dramatic evidence of climate change, an unacceptable lapse during George W. Bush’s presidency.

Even though the USDA had commissioned the American Horticultural Society’s map, the USDA rejected it, and insisted on creating its own map. Instead of basing its map on a dozen or so years of weather history as it had done with earlier versions of the plant hardiness zone map, the USDA chose this time to use 30 years’-worth of history. In this way, by averaging the record heat of more recent years with the unusually cool winters of the early 1980’s, the map’s creators were able to minimize evidence of climate change.

If you doubt what I say, simply check out the National Arbor Day Foundation website. This organization has chosen to draw up its own version of the plant hardiness zone map. (This link is broken—if you copy and paste the following address, it will take you to the map: Its zones correspond to those of the USDA map, in that they are based on the same average winter low temperatures, and it used the same weather records as the USDA, except that it only tracked 16 years and followed the records up to June of 2015. As a result, the Arbor Day map depicts a significantly warmer United States, with the hardiness zones all moved northward. Most interesting is the comparison (link broken — copy and paste this address: for the changes in zones since 1990of its map with an earlier (1990) version of the USDA map – the newer Arbor Day map shows most of the United States as a full zone warmer than in the older USDA map.

Why does this matter? If you have been using the current USDA map as a plant selection guide, you have been misled by manipulated data and have been too conservative in your estimations of what will grow in your garden. Before you go plant shopping this spring, check out the zone of your garden on the Arbor Day map – you are likely to discover that your climate is warmer than you thought. And warmer than the USDA will admit.

Posted by

Thomas Christopher
on March 19, 2016 at 6:43 pm, in the category Gardening on the Planet, Ministry of Controversy, Science Says.


  1. Most nurseries and plant wholesalers/retailers don’t want to sell the customer a plant that won’t prove hardy in their gardens; but opinions do vary about the hardiness of specific plants. Deliberately misinforming the public to satisfy a political agenda is something entirely different, however.
    How is your magnolia doing?

  2. I will have to disagree with the Arbor Day Foundation’s 2015 map based on the reality of my winter low temperatures over the last eight winters. The zone 6 dip into the spine of the Southern Appalachians is missing. I am not a zone 7. No way no how.

  3. Well put. At best, the climate zone maps based only on average winter lows are only a rough gauge of local climates, given factors such as those you mention and microclimates deriving from peculiarities of topography and exposure. Personal experimentation is always the most reliable guide.

  4. Christopher, you and many disagree with this Arbor Day map. It is terribly misleading to the consumer. We are very close to NC here in Norfolk and I talk to NC folks weekly and they are not a zone warmer. This is beyond odd that a map like this would be pushed forth. This new map almost shows all of NC as zone 8, truly ridiculous.

  5. Living as I do in a mountainous border-zone, where elevation or aspect can change everything, I’ve always taken the hardiness maps with a grain of salt. It seems like every year here for the last 25 has been known for some weather uniqueness. But the last few years have been extraordinary, no doubt in my mind that things are changing; I think the map-makers may have to come out with an annual version from now on.

  6. If I’m not mistaken, Bush was not the president in 2012 when the current map was created…? Not sure what the president has to do with these planting recommendations anyway, and why you would try to p— off roughly half of readers by insinuating that the USDA map is some version of “Bush Lied, Plants Died” or some such ridiculousness?

  7. The committee that created the map was set up during the Bush administration and the outlines of the new map, including the deceptive averaging of the weather records, were established then. I covered the story then and interviewed the relevant people at the American Horticultural Society and the USDA. Incidentally, this was also the time when the Bush administration put an end to climate change research at the USDA Beltsville laboratories and tried to muzzle James Hansen, the leading climate change researcher at NASA. As for the links, the glitch is unfortunate, but anyone who really wanted to see those sites could easily cut and paste as I suggested.

  8. Beth, the you are right. The pointless political mentions are sloppy and frankly make this article sillier than it needs to be. This is why folks do not take climate seriously nor will make better decisions in their landscapes. Planting for 1-2 zones warmer will not help anything either, but cause trouble and the use of plants way out of their range. The last 3 winters have proven that the zones have not changed, period. And many folks forget that the USDA maps in the 1940′-1960’s were warmer than the 1990’s map. Historically, our temps in Norfolk from the late 1930’s through the 19’50’s showed a marked warm phase, where ‘typical’ winter activity was not evident, correlating to the warm 1940’s-1960’s maps.. I get the climate issues. I am not agreeing to the BS fluff and silliness, like yourself Beth, associated with the serious discussions.

  9. My concern is that climate change is creating erratic weather conditions and we can’t really predict anything at all. This winter in Atlanta saw a brief dip to the low 20s but last winter it was 10 and the one before was 5! We lost tons of plants that had been hardy for us for 2 decades! This year of course the roses are blooming with the peaches and the tulips are done and we’ve had lightening bugs for weeks.
    I’m thinking the maps may be pointless.

  10. My wife, who is an environmental scientist studying climate change, doesn’t speak of “global warming” — she refers to the process as “climate chaos.”

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