Plant hardiness zones, an indicator of the coldest winter temperature, have shifted north and northwest since 1990 around Washington, D.C. (USDA; Patterson Clark and Laris Karklis - The Washington Post)

On Wednesday, the USDA released a new plant hardiness zone map, which contours the nation according to average annual lowest winter temperatures. The new zones analyze these temperatures for the period 1976-2005, updating a 1990 version of the map, which covered 1974-1986.

Interactive map: USDA upgrades Plant Hardiness Zone Map

Although these zones, which serve as a guide to the kinds of plants that can grow, have shifted north in most areas, USDA shied away from making a climate change connection.

“The map is not a good instrument for determining climate change,” said Kim Kaplan, a spokeswoman for the USDA’s Agricultural Research Service. “It’s not that there hasn’t been global climate change it’s that the map isn’t a good (vehicle) for demonstrating it.”

USDA’s line of reasoning in perplexing. Climate data are used in USDA’s analysis and the northward jog in planting zones is fully consistent with other data and indicators that establish warming of the coldest temperatures in the U.S. (and most locations globally).

Percent of U.S. affected by extremely warm and cold minimum temperatures from 1910 to present. (NOAA)

Reporting for the Associated Press, Seth Borenstein spoke with David Wolfe, a professor of plant and soil ecology at Cornell, who agreed USDA is being “too cautious” in laying off the climate change connection.

“At a time when the ‘normal’ climate has become a moving target, this revision of the hardiness zone map gives us a clear picture of the ‘new normal,’” Wolfe said.

Related: Warm is the new norm

I’m not arguing USDA should use the hardiness zones to quantify the degree of climate change, or draw conclusions about its causes. But it’s truly bizarre that it would stop short of making a simple connection between what’s happening to planting zones and the climate when it is so apparent and so societally relevant.

Additional links: New plant map shifts area to warmer zone | USDA’s Plant Hardiness Zone website (with interactive, searchable maps)