The "hardiness" guide, last updated in 1990, lists average minimum temperatures for different latitudinal zones. Each zone is based on 10 degrees Fahrenheit. Two new zones were added in hotter climates this year for a total of 13 zones. 1 is coldest (-60 to -50). 13 is hottest (60 to70) and is found only on Hawaii and Puerto Rico.
The new map, located on the USDA website, uses 30 years of weather data gathered from 1976 to 2005 and is more precise than the 1990 version, showing smaller areas and accounting for higher elevations and bodies of water that can influence temperature. It was designed for the Web, allowing people to enter their ZIP code and see their zone down to half-mile segments. That eliminates the guesswork for the nation's 82 million gardeners.
"Across the country, people will be seeing where there are some changes," says Catherine Woteki, undersecretary of Agriculture. "There is some shifting in zones, but this we attribute to using new average minimum temperatures. We do not think the plant hardiness zone methodology is appropriate for making comments on climate change."
No section of the country changed more than half a zone, according to Woteki. She says scientists used the latest complete data, adding they performed an analysis to see if incomplete data from the past five years would require alterations. "We saw it would not change the map," she says.
The USDA website notes: "Climate changes are usually based on trends in overall average temperatures recorded over 50-100 years. Because the (new map) represents 30-year averages of what are essentially extreme weather events (the coldest temperature of the year), changes in zones are not reliable evidence of whether there has been global warming."
Gardening consultant Charlie Nardozzi lives in northern Vermont, which jumped to a milder zone.
"If you want to look at what might be the most politically correct thing, you can say something's happening," Nardozzi says. "But the climate is changing. Spring is coming sooner and lasting longer. Fall lasts longer, and overall the weather is so much more erratic now."
Among areas of change, a higher number means warmer:
• Much of the Northeast is half a zone warmer. For example, Pennsylvania, which was about equally divided between zone 5 and zone 6 (warmer) in the 1990 map is now about 70 percent zone 6 and 30 percent zone 5.
• Nebraska was mostly zone 4 and is now almost entirely zone 5 (warmer).
• Ohio was mostly zone 5 and now is mostly zone 6 (warmer).
• Both southern Florida and Southern California have new hotter areas around cities, but California also has some colder areas in the mountains.
The zones offer important guidelines for all planters, including tree growers. For example, a sugar maple won't produce maple sugar in a warm climate. Sugar maples thrive in the colder zone 3, dogwoods in zone 5 and rhododendrons in zone 7.
Woodrow Nelson of the Arbor Day Foundation says the group's million-plus members have been waiting for the new map for years. The foundation is the largest non-profit organization dedicated to planting trees. It updated its climate map in 2006, using data from 1991 to 2005. Its map showed a significant northward movement of warming trends across the nation.
"The USDA's new map reflects our map in many ways," says Nelson, director of marketing communications. "We updated ours because we were getting many calls from our members saying the old map wasn't right anymore. The data was pretty clear then. It was saying, why not try new things?"