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.

USDA is not describing what the new map will show, but outside experts say that the trend is for zones to shift northward. "Some places have definitely warmed, although others haven't changed at all," says Tony Avent, owner of North Carolina-based Plant Delights Nursery and an advisor for the revision.

They also describe the new map as much more sophisticated than the 1990 version, which was based on a data set covering only 13 years (1974 through 1986 for the U.S.). The revised map draws on 30 years of data and uses a complex algorithm to factor in other variables that affect local temperatures, such as altitude and the presence of water bodies.

"All we could really do earlier was draw a straight line between data points, but now we're trying to input a lot of other information," says USDA spokeswoman Kim Kaplan. "We'll pick up more heat islands and cold zones, and the edges of zones will be defined more clearly."

The new map is being developed by Oregon State University's PRISM Group, a team of modelers that also produces climate maps for other state and federal agencies. Unlike past versions, the 2009 map will be GIS-compatible, storing and linking layers of information in a digital version that can be read with widely available GIS (geographic information system) viewing programs. It will have a resolution of 800 square meters, so users will be able to zoom in on their hometowns or zip codes and see where they lie within zones.

USDA commissioned the revision after a flap in 2003, when the American Horticultural Society released a draft update based on 16 years of temperature data. USDA had funded the project but rejected the update, which was configured differently and showed significant warming over the 1990 version, with many parts of the nation shifted into warmer climate zones. (The Arbor Day Foundation displays a modified version of the rejected map on its web site, along with an animation that shows the foundation's estimate of warming since 1990.)

Some observers suggested that the Bush administration pulled the map because it showed the nation warming, but Kaplan calls that idea an urban myth. "There was no memo from the White House," she says. "The draft was rejected because it wasn't web-friendly and wasn't layered in a standard geographic information systems (GIS) format. The data were never reviewed – formatting and technology issues got it bounced."

Both the 1990 map and the aborted 2003 version are unreliable because they use too little data to show lasting trends, Avent contends. "The first time they got a cold data set and the second time they got a warm data set," he says. For example, Avent points to Chicago, which lies on zone 5b on the 1990 map but shifted into zone 6 on the 2003 draft. "In 2004 Chicago had a -21-degree winter," Avent said. "If Chicago gardeners had planted zone 6 plants, they would all have failed. When plants die customers give up gardening, and that's the nursery business's worst nightmare."

Michael Dosmann, curator of living collections at Harvard University's Arnold Arboretum in Boston and an advisor on the USDA revision, expects that the new map will be much more credible than either the AHS version or the current 1990 map. "I think we can have a lot of faith in it. The data set is very robust, and the modelers have done an amazing job," says Dosmann. (Arnold Arboretum horticulturists produced early climate zone maps from the 1930s through the 1960s.) Like Avent, he predicts that the new map will show "some zone creep, but not the extremes in the Arbor Day map."

Although it's significant when hardiness zones shift, gardening experts emphasize that they are guidelines, not blueprints. "The map minimizes people's odds of planting things that are likely to fail," says Avent. For example, calla lilies are hardy in zones 8 to 10, so there's not much point in planting them in Minneapolis, which lies in zone 4a.

But there's no guarantee that a plant rated hardy for zone 4 will actually thrive in Minnesota backyards. Every garden contains microclimates that influence what can grow there. Fragile plants may thrive in sunny protected corners, even if exposed areas of the same yard are ten degrees (i.e., a full climate zone) colder.

"To help plants succeed, you have to understand how they grow and site them carefully," says Dosmann. "Plants can't read the map, and they don't always respect our zones." Dosmann and his colleagues study the Arnold Arboretum's microclimates intensively and try to work with them – for example, by planting less-hardy plants on warm south-facing slopes.

Once gardeners know what's possible in their own flowerbeds, they can start to push the envelope. Boston lies in zone 6, but the Arboretum constantly tests species that are adapted to warmer areas, such as some hardy types of camellias. It does so not in response to climate change but as part of its mission to keep adding new plants to its collection.

"We always try to test the limits," says Dosmann. "All gardeners are in zone denial."

This article originally ran at The Daily Climate, the climate change news source published by Environmental Health Sciences, a nonprofit media company.