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With gardens a-sprouting, a warm, wet winter behind us,1 and a hotter-than-average summer for much of the country ahead, we decided to look at whether and how climate change was affecting what plants can grow around the country. The easy data solution — or so it seemed — was to look at a series of maps dedicated to showing Americans what plants can survive in their neck of the woods. These are called plant hardiness zone maps, and they’ve been produced since the 1960s by the U.S. Department of Agriculture.
But then we noticed something weird. The USDA’s website specifically asks people not to use these maps to document climate change. Meanwhile, it looked as if other parts of the federal government were doing exactly that in reports such as the National Climate Assessment.
So what gives? It turns out, the government produces two hardiness zone maps — one made by the USDA and one made by the National Oceanographic and Atmospheric Administration. Both divide the country into segments, each of which represents a 10-degree increment of the average annual minimum temperature. But the underlying data used to build out the zones is different. Those differences are driven by the agencies’ goals, and they affect what the different maps are intended to be used for.