New federal map for what to plant reflects warming

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 Jan. 25 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," said USDA chief scientist Catherine Woteki.

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 five degree increments.

For example, Des Moines, Iowa, used to be in zone 5a, meaning the lowest temperature on average was between minus 15 and minus 20 degrees Fahrenheit. Now it's 5b, which has a coldest temperature of 10 to 15 degrees below zero.

"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, including Honolulu, St. Louis, Des Moines, St. Paul, Minn. and even Fairbanks, Alaska, 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.

USDA spokeswoman Kim Kaplan, who was part of the map team, repeatedly tried to distance the new zones in the map from global warming issues. She said even though 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, professor of plant and soil ecology in Cornell University's Department of Horticulture said the USDA is being too cautious and disagrees with Kaplan about whether this reflects warming.

"At a time when the ‘normal' climate has become a moving target, this revision of the hardiness zone 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.

Another and even more dramatic sign of global warming in the plant world is that spring is arriving earlier in the year, Wolfe said.

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. Kaplan 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 very similar to the one the private plant 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."

A tag on a tree on display Jan. 24 at the Earl May Nursery and Garden Center in Des Moines, Iowa.

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'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 have much of an impact on 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.

Vaughn Speer, an 87-year-old master gardener in Ames, Iowa, doubts the change in zones will mean much to him, but he 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, but I'm seeing them near Roland," he said, referring to a small Iowa town about 10 miles north of the highway that spans central Iowa.