WASHINGTON – Global warming is hitting not just home, but garden. The government’s colorful map of planting zones, most often seen on the back of seed packets, is changing, illustrating a hotter 21st century.

An update of the official guide for 80 million gardeners reflects a new reality: The coldest day of the year isn’t as cold as it used to be. So some plants and trees that once seemed too vulnerable to cold can now survive farther north.

It’s the first time since 1990 that the U.S. Department of Agriculture has updated the map 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, also uses better weather data and offers more interactive technology. For the first time it takes into factors such as how cities are hotter than suburbs and rural areas, nearby large bodies of water, prevailing winds, and the slope of land.

“It truly does reflect state of the art,” USDA chief scientist Catherine Woteki said.

Gardeners can register their ZIP code into the online map and their zone will pop up. It shows the exact average coldest temperature for each ZIP code. The 26 zones, however, are based on 5-degree increments.

The Fort Wayne area used to be in Zone 5 but the city and most of Allen County is now in a triangularly shaped patch of the warmer Zone 6a.

The triangle’s base stretches along the Ohio border north from the southern Jay County line to the southern Dekalb County line. Its point lies at roughly the intersections of Wabash, Whitley and Huntington counties.

Zone 6a patches also exist near Marion, southeast of Warsaw and in a strip in northwest LaGrange County that also covers the width of Elkhart County and stretches to just south of Goshen.

The rest of the region, including northwest Allen County in the Huntertown area, is in Zone 5b.

The average wintertime low temperature in Zone 5b ranges between -10 and -15 degrees. The average wintertime lows for Zone 6a are between -5 and -10.

“People who grow plants are well aware of the fact that temperatures have gotten more mild throughout the year, particularly in the winter time,” said Boston University biology professor Richard Primack. “There’s a lot of things you can grow now that you couldn’t grow before.”

He uses the giant fig tree in his suburban Boston yard as an example.

“People don’t think of figs as a crop you can grow in the Boston area. You can do it now,” he said.

In the old 1990 map, the USDA mentions 34 different U.S. cities on its key. Eighteen of those are in newer warmer zones.

Agriculture officials said they didn’t examine the map to see how much of the map has changed for the hotter. But Mark Kaplan, the New York meteorologist who co-created the 1990 map and a 2003 update that the USDA didn’t use, 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 warmer than for the old time period, according to statistics at the National Climatic Data Center.

Ricky Kemery, horticulture educator for the Purdue Cooperative Extension, says gardeners shouldn’t make any large-scale changes in species or planting habits because of the new map.

“It’s really based on average minimum temperatures over time over the winter months. It certainly doesn’t mean it’s never going to go below that,” he says.

Gardeners, Kemery says, should be especially wary of starting annuals, including vegetables and tender landscape plants, too early in the spring.

“The new map kind of implies you can be out there putting in warm-season plants roughly around April 17 to 20, and I don’t know that that’s going to work,” he says. “We’ve had frosts and hard freezes up until May 20 some years.

“I would err on the side of caution.”

An earlier effort to update the planting map caused a bit of an uproar when the USDA in 2003 decided not to use an updated map that reflected warmer weather. USDA spokeswoman Kim Kaplan, who was part of the map team, said the 2003 map wasn’t interactive enough.

The Arbor Day Foundation later issued its own hardiness guide that had the toastier climate zones.

The new federal map is similar to the one the private 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. “I look forward to (the new map). It’s been a long time coming.”

Nelson lives in Lincoln, Neb., where the zone warmed to a 5b. Nelson said he used to be “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.

In Des Moines, Jerry Holub, a manager for the Earl May Nursery chain, doesn’t think the warmer zone will matter much to gardeners. But he said this may mean residents can even try passion flowers.

“Now you can put them in safely, when you couldn’t before,” he said.