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Area gardeners may have suspected odd weather when their rare palms, gingers and bottle brushes -- which had no business growing this far north -- actually survived a North Carolina winter.

Winters came and went, and the subtropical exotics continue to thrive.

U.S. agriculture officials confirmed last week what plants have been telling us for years: Winters aren't as cold as they used to be. The temperature difference over the past two decades has caused a marked shift in this state's winter-hardiness planting zones, the weather bands on a map that indicate where plants will grow.

According to a new zone map from the U.S. Department of Agriculture, the zones have shifted by several counties in parts of the state, including a total reclassification of Johnston County from Zone 7 to Zone 8.

The difference is most noticeable along North Carolina's southeastern rim, where the dividing line between those two zones has migrated more than a 100 miles, from the Atlantic coast as far inland as Charlotte.

If the trend continues, sometime in this century Raleigh will be reclassified into the same climate zone that today includes Dallas and Montgomery, Ala. Raleigh, technically in Subzone 7B, is now less than 1 degree away from being in Zone 8.

The bands in the plant hardiness zone map represent 10-degree differences in average minimum temperatures, and even a subtle shift registers with leafy beings long before humans catch on.

"There's no question in most of the United States we saw a shift of 2 to 3 degrees," said Tony Avent, owner of Plant Delights Nursery in Raleigh and a technical adviser on the new map. "There's dozens of plants you can grow in 7B that you can't grow in 7A. It makes a huge difference."

But gardeners take heed: Warmer weather doesn't mean the end of those late freezes that can waste a riot of petals overnight. Warmth will confuse plants, forcing them to break dormancy weeks ahead of schedule, said Ted Bilderback, director of the J.C. Raulston Arboretum in Raleigh. These plants will go about their business, budding, blooming and leafing out as if summer were around the corner, only to be flash-frozen by an Alberta Clipper.

Meanwhile, warmer weather could unleash new flowering spurges, tropical spiderworts and other unwanted turf weeds into North Carolina's emerald lawns and flowerbeds.

"You can be the first on your block to have an infestation," said Joe Neal, professor of weed science at N.C. State University. "We will see weeds here that have historically not done well."

The USDA does not explain the cause of the warming trend, but the new map is already sparking debates about man-made global warming versus natural climate change.

The USDA's plant hardiness zone map, the agency's fourth since 1960, is the most accurate yet, developed with the aid of digital technology and based on data collected from some 8,000 weather stations. The data accounts for such temperature factors as terrain and elevation, as well as proximity to lakes and oceans.

In North Carolina, a third of the state is now a solid Zone 8. The western mountains are now a climactic quilt of Zone 7 and Zone 6, whereas on the earlier map they were represented by the unbroken chill of Zone 6.

One reason for the dramatic difference between the new map and its predecessor is that the 2012 map is based on 30 years of data, spanning 1976-2005, while the 1990 map is based on a smaller sample of 13 years. Avent and many others have long believed that the 1990 map shows the country as being colder than it really was.

Still, some plant aficionados, based on their own gardening experience, have more confidence in the older map than in the new one. The new map just looks wrong to Paul Jones, a curator of Asian plants at Duke University's Sarah P. Duke Gardens.

"My experience is they got it wrong in modifying it," Jones said. "I think it is cooler than what they're suggesting by the colors of the map."

Mark Weathington, assistant director of the J.C. Raulston Arboretum, said the proliferation of available plant varieties is not always a sign of warmer weather. In some cases, it's a sign of improved plant breeding and a greater selection of cultivars. In other words, Weathington said, some recently introduced plants would likely have tolerated our winters decades ago.

But if Plant Delights is any guide, the Triangle's average winter lows have warmed up by about 2.5 degrees. The nursery, about 12 miles south of downtown Raleigh, is between Clayton, Holly Springs and Angier.

"In 1996, we recorded minus 1 degree here at the nursery," Avent said. "Since 1996, we have not been below 6 degrees. Based on the plants we grow now, compared to the plants we could grow prior to 1996, we can tell about a quarter-zone shift in our area."