A new spring bloom: U.S. gardeners might have to head back to the drawing board because the flowers that can flourish in their area have likely changed in the past decades due to climate change.
FLICKR/MATT MCGEE

As winter retreats northward across the nation, gardeners are cleaning tools and turning attention to spring planting. But climate change is adding a new wrinkle, and now a standard reference – the U.S. Department of Agriculture's Plant Hardiness Zone Map – is about to make very clear how much rising temperatures have shifted planting zones northward.

The guide, last updated in 1990, shows where various species can be expected to thrive. A revision is expected sometime this year, and while the agency hasn't released details, horticulturalists and experts who have helped with the revision expect the new map to extend plants' northern ranges and paint a sharp picture of the continent's gradual warming over the past few decades. The new version will have a wide audience: the National Gardening Association estimates 82 million U.S. households do some form of gardening, a number expected to increase as more Americans plant vegetable gardens to cut food costs.

"Anyone involved with gardening, especially with perennials, uses the map to pick the right plants for their location," says NGA horticulturist Charlie Nardozzi. "Shifting hardiness zones are a very tangible result of climate change, and people will see that change happening where they live over a short period of time."

Familiar to anyone who has paged through a nursery catalogue, the USDA hardiness map divides North America into 11 latitudinal zones, each representing a 10-degree (Fahrenheit) range of "average annual minimum temperature" – the coldest lows that can be expected in that area. Zones 2 through10 are each subdivided into two sections – "a" and "b" – that represent 5-degree (Fahrenheit) ranges. Zone 11 (southern Mexico and much of Hawaii) is tropical, with winter lows above 40 degrees Fahrenheit.

Reclassifying a gardener's yard into a warmer area opens new options for planting flowers and shrubs that would probably not have survived local winters in the 1970s or 1980s. And the visual impact of a map, with inevitable comparisons to the 1990 version, is likely to make even non-gardeners ask what it means to live in zone 7 instead of 6.

By injecting climate change into one of America's favorite pastimes, the revised USDA map could become an important public education tool. "Hopefully the new map will clear up a lot of confusion about what's happening to the climate," said Nardozzi.

USDA climate zones are based on measurements from the Commerce Department's National Climatic Data Center, plus national sources in Canada and Mexico. Every ten years the data center calculates new U.S. "climate normals," or 30-year average values, for meteorological elements such as temperature, precipitation, and heating and cooling degree days for thousands of U.S. weather stations.

Station locations change and methods evolve, so the climate data center warns that comparing normals between different 30-year periods may lead to "erroneous conclusions" about climate change. Nonetheless, the center released an image in 2003 showing the difference between average minimum winter temperatures throughout the United States for 1961-1990 and 1971-2000. In nearly every part of the continental United States winter lows were warmer during the second period, rising as much as 2.5 degrees (Fahrenheit) in parts of the Rockies, the northern Great Plains, and central and southern California.