Zonal Migration

The USDA Plant Hardiness Zone Map gets a tune-up.

October 22, 2013

In January 2012, the USDA sent gardeners rushing to computers when it released a new Plant Hardiness Zone Map, the first since 1990. Many found that their zones had changed, and were surprised to see two new zones, 12 and 13.

Kim Kaplan, a spokesperson for the USDA’s Agricultural Research Service, has been working on the map since 2006. She says that the 1990 map used only 13 years of data; the 2012 version uses 30 years of data from more than 8,000 locations. The USDA’s partner, the PRISM climate group from Oregon State University, fed the data into an algorithm that included slope, elevation changes, and prevailing winds to interpolate between data-collecting points. Then, Kaplan says, “we ground-truthed it. We sent it out to a lot of experts in various fields and said, ‘does this look right to you?’” A few gaps or biases in the data were discovered, which led to further refinements.

While many locations on the map are a half-zone warmer, is this evidence of climate change? Not necessarily, says Kaplan. Many places moved up in zone, but it was sometimes due to shifts as little as a few tenths of a degree. The two new zones were created so tropical plants could be described in the same vocabulary as plants in other climates. The data is also more accurate than the old map, so “comparing the 1990 and the 2012 maps is apples and oranges,” says Kaplan. “You can’t ascribe one particular zone change to any one category of methodology change.” And, of course, the map measures only average low temperatures, not highs.

The online map is a virtual treasure trove for gardeners. The interactive feature allows for zooming down to ZIP code or even street level; offers satellite, terrain, and street views; and can shift in transparency for clearer viewing. It is also available for download so gardeners can print maps.

Being in a new zone doesn’t necessarily mean you’re in new gardening territory. As Kaplan says, “Nothing supplants a gardener’s knowledge. What works in your garden today will work in your garden tomorrow.”

Originally published in Organic Gardening Magazine, April/May 2012