Hardiness Disinformation by Thomas Christopher

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 — I’m working on fixing it; in the meantime, go to this address:  https://www.arborday.org/media/zones.cfm). 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 — go to this address:  https://www.arborday.org/media/map_change.cfm) 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.

Hardiness Disinformation originally appeared on Garden Rant on March 19, 2016.


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