The southern portion of York County and the stretch of Maine coast from the Sheepscot River to Penobscot Bay are now in the same planting zone as central Massachusetts and northern Connecticut, according to a new planting guide published by the U.S. Department of Agriculture.

The color-coded map of planting zones often seen on the back of seed packets is being updated by the government, reflecting a hotter 21st century. It’s the first time since 1990 that the government has revised the official guide for the nation’s 80 million gardeners,

Much has changed. Nearly entire states, such as Ohio, Nebraska and Texas, are in warmer zones. In Maine, the biggest shifts occur in the midcoast and York County.

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.

The zone changes reflect a new reality: The coldest day of the year isn’t as cold as it used to be, so 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 throughout the year, 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.”

The giant fig tree in his suburban Boston yard 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,” Primack said.

On the new map, the midcoast of Maine and the southern portion of York County have moved from zone 5b to zone 6a. The rest of the area along the Maine coast, including parts between the new 6a zones, remain at 5b.

William Cullina, executive director of the Coastal Maine Botanical Gardens in Boothbay, said the new map confirms his own observations: that the midcoast is the “banana belt” of Maine.

“The map is validating that in a shocking way,” he said.

The average annual extreme cold temperature in zone 5b is minus 10 to minus 15 degrees. The average annual extreme cold temperate in zone 6a is minus 5 to minus 10 degrees.

Cullina said the shift to a warmer zone means that homeowners will have a significantly greater variety of plants to choose from. He said plants like Japanese maple and Japanese dogwood might survive for a while in 5b but would become stressed in a hard winter. They’d do better in a zone 6a, he said.

Warmer zones also equate to a longer growing season, he said.

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.

Also, for the first time, calculations include more detailed factors, such as prevailing winds, the presence of nearby bodies of water, the slope of the land, and the way cities are hotter than suburbs and rural areas.

The map carves up the U.S. into 26 zones based on five-degree temperature increments. The old 1990 map mentions 34 U.S. cities in its key. On the 2012 map, 18 of those, including Honolulu, St. Louis, Des Moines, Iowa, St. Paul, Minn., and even Fairbanks, Alaska, are in newer, 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 Bell said many of his customers already know what can grow in their own climate and 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 during the previous time period, according to the National Climatic Data Center.

USDA spokeswoman Kim Kaplan, who was part of the map team, repeatedly tried to distance the new zones on the map from global warming. She 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 in Ithaca, N.Y., said the USDA is being too cautious and that 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.

The Arbor Day Foundation issued its own hardiness guide six years ago, and the new government map is very similar, said Woodrow Nelson, a vice president at the plant-loving organization.

“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 in a “solid 4,” but now he has Japanese maples and Fraser firs in his yard — trees that shouldn’t survive in a zone 4.

Vaughn Speer, an 87-year-old master gardener in Ames, Iowa, said he has seen redbud trees, one of the earliest blooming trees, a little farther north in recent years.

“They always said redbuds don’t go beyond U.S. Highway 30,” he said, “but I’m seeing them near Roland,” 10 miles to the north. 

Staff Writer Tom Bell contributed to this story.