The updated "Plant Hardiness Zone Map" includes state-by-state details like this one of Iowa. Des Moines, for example, used to be in zone 5a, meaning the lowest temperature on average was between minus 15 and minus 20 degrees Fahrenheit. Now it's 5b, which has a coldest temperature of 10 to 15 degrees below zero.
A federal "plant hardiness" map used by millions of gardeners was updated Wednesday for the first time in two decades, revealing many areas where the coldest day of the year isn't as cold anymore.
"Compared to the 1990 version, zone boundaries in this edition of the map have shifted in many areas," the U.S. Department of Agriculture said in a statement announcing the update.
The map, which is online and allows users to type in zip codes, uses 26 temperature zones, each in 5 degree Fahrenheit increments, to show planting conditions across the U.S.
For example, Des Moines, Iowa, used to be in zone 5a, meaning the lowest temperature on average was between minus 15 and minus 20 degrees Fahrenheit. Now it's 5b, which has a coldest temperature of 10 to 15 degrees below zero.
A key change in the update is using average temperature records from 1976-2005 instead of the earlier 1974-1986 span. The nation's average temperature from 1976 to 2005 was two-thirds of a degree warmer than for the old time period, according to statistics at the National Climatic Data Center.
"It's important to recognize that the plant hardiness map represents the extreme coldest temperature a location experiences each year, averaged over a period of years," Chris Daly, an Oregon State University researcher whose climate group created the map for the USDA, told msnbc.com. "So, it is really an extreme cold map, if you will, and doesn't necessarily track average temperatures.
"That said, the more recent averaging period has generally caused a one 5-degree half-zone shift to the warmer side over much of the U.S., especially the central and eastern U.S."
"The higher quality of the map is reflected most in hilly and mountainous areas, where we have both better measurements and a better mapping technology," Daly added.
USDA Undersecretary Catherine Woteki called it "the most sophisticated Plant Hardiness Zone Map yet for the United States" but made no mention of climate change.
USDA spokeswoman Kim Kaplan distanced the map from global warming issues. Even though much of the country is in warmer zones, she said, the map "is simply not a good instrument" to demonstrate climate change because it is based on just the coldest days of the year.
David Wolfe, professor of plant and soil ecology in Cornell University's Department of Horticulture, said the USDA was being too cautious.
"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,' and will be an essential tool for gardeners, farmers, and natural resource managers as they begin to cope with rapid climate change," Wolfe told The Associated Press.
Another and even more dramatic sign of global warming in the plant world is that spring is arriving earlier in the year, Wolfe said.
In 2003, the last time an update was attempted, the USDA was embroiled in controversy and shelved the map, which it had contracted the American Horticultural Society to prepare.
The society went ahead with its own map, and emphasized what it felt was a connection to global warming.
The Arbor Day Foundation also issued its own hardiness guide, which also had warmer climate zones and was based on more recent temperature data. The new map is very similar to the one the group adopted six years ago, said Arbor Day Foundation Vice President Woodrow Nelson.
"We got a lot of comments that the 1990 map wasn't accurate anymore," Nelson said. "It's been a long time coming," he said of the new map.
Nelson, who lives in Lincoln, Neb., where the zone warmed to a 5b. Nelson said he used to "a solid 4" but now he's got Japanese maples and fraser firs in his yard — trees that shouldn't survive in a zone 4.
Msnbc.com's Miguel Llanos and the Associated Press contributed to this report.
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