WASHINGTON - The color-coded map of planting zones often seen on the back of seed packets is being updated to reflect a hotter 21st century.
It's the first time since 1990 that the U.S. Department of Agriculture has revised the official guide for the nation's 80 million gardeners, and much has changed. Nearly entire states, such as Ohio, Nebraska and Texas, are in warmer zones.
The new guide, unveiled Wednesday at the National Arboretum, arrives just as many home gardeners are receiving their seed catalogs and dreaming of lush flower beds in the spring.
It reflects a new reality: Some plants and trees can now survive farther north.
"People who grow plants are well aware of the fact that temperatures have gotten more mild ... particularly in the wintertime," said Boston University biology professor Richard Primack.
"There's a lot of things you can grow now that you couldn't grow before."
His giant fig tree stands as an example: "People don't think of figs as a crop you can grow in the Boston area. You can do it now."
The new guide also uses better weather data and offers more interactive technology.
For example, gardeners using the online version can enter their ZIP code and get the exact average coldest temperature.
The map carves up the U.S. into 26 zones based on five-degree temperature increments. The 1990 map mentions 34 U.S. cities. On the 2012 map, 18 of those, including Honolulu, St. Louis, Des Moines, Iowa, St. Paul, Minn., and even Fairbanks, Alaska, are in warmer zones.
The changes come too late to make this year's seed packets, but they will be in next year's, said George Ball, chairman and CEO of the seed company W. Atlee Burpee, which puts the maps on packages of perennials, not annuals.
But Ball said many of his customers already know how it has warmed.
"Climate change, which has been in the air for a long time, is not big news to gardeners," he said.
Mark Kaplan, a New York meteorologist who helped create the 1990 map, said the latest version clearly shows warmer zones migrating north. Other experts agreed.
The 1990 map was based on temperatures from 1974 to 1986, the new map from 1976 to 2005. The nation's average temperature from 1976 to 2005 was two-thirds of a degree higher than it was during the old time period, according to the National Climatic Data Center.
USDA spokeswoman Kim Kaplan, who was part of the map team, said that while much of the country is in warmer zones, 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 W. Wolfe, a professor of plant and soil ecology at Cornell University, said the map plainly reflects warming.
The revised 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 said in an email.