December 27, 2006

Warmer Winters Change Washington Foliage

WASHINGTON -- Fifteen years of warm winter weather is beginning to change the Washington area's landscape - with Southern species like crape myrtles having an easier time and northern types feeling less welcome, according to findings by the National Arbor Day Foundation.

The foundation has revised its map of "hardiness zones" - with each of the nine zones showing a range of average annual low temperatures that help serve as a guide for gardeners and others.

One big change was that the entire Washington area was reclassified in the same zone as parts of Texas and North Carolina. In 1990, according to the U.S. Department of Agriculture, the region sat on the border of the northern and southern zones.

"You could say D.C. is the new North Carolina," said Bill McLaughlin, a curator at the U.S. Botanic Garden on the Mall.

McLaughlin said he was pleased to find that more Southern species like the needle palm or the yaupon, a holly, could be grown more easily. But he added that he believed native plants might eventually find their growing seasons shifted and their life cycles out of sync with pollinating insects.

"It's exciting, in a way," he said. "It's alarming, when you look at native plant communities."

Fifteen years ago, a section of the Washington area fell into Zone 6, an area from Massachusetts to Kansas where the lows were between zero and 10 degrees below. The other part was in Zone 7, which spans the upper South, where temperatures were between zero and 10 degrees.

Now, all the Washington area now lies in Zone 7, which has taken over parts of the District and suburbs in Virginia and Maryland. Zone 7 has crept northward to take in most of Tennessee and Virginia as well as parts of North Carolina, Arkansas and Oklahoma.

The National Arbor Day Foundation uses the same format and the same source of climate data as the U.S. Department of Agriculture, which last put out its hardiness-zone map in 1990. Since then, the foundation has provided its own updates.

The warming trend here was also taking place in other parts of the country.

In sections of Michigan, the weather was warm enough to suit southern magnolia trees, said Arbor Day Foundation spokesman Woodrow L. Nelson. The southern species Arizona cypress also seemed to be better adapted for some parts of the Northeast, he said.

"I mean, who would have thought that an Arizona cypress would be a choice for someone in New Jersey?" Nelson said.

Weather experts there's no single reason why the winter temperatures are higher. However, most scientists say the gradual warming of the planet - due to carbon dioxide and other pollutants preventing heat from escaping the atmosphere - has played a role.

Many of the Washington area's most popular species - including maple and oak - will still tolerate higher temperatures, according to horticultural experts. But species like the Colorado blue spruce, which is native to colder places, could feel the effects.

"The trees are not going to die, and you shouldn't go out and cut anything down," Nelson. "We're saying ... now that we've got a warmer climate, consider different trees."

Some in the region say they have already seen changes.

Horticulture specialist Chuck Schuster at the Montgomery County cooperative extension office, said crape myrtles were doing better than he had ever seen. "It was something that I would never consider planting" years ago, Schuster said. "But now they're thriving."