Climate Change Redraws the Map for Gardeners

By Joel M. Lerner
Saturday, July 21, 2007

If you live in the southern part of Northern Virginia, I've got news for you: You're now in North Carolina. If you live in Maryland, you're now in Virginia. No, your state boundaries haven't been redrawn. But something else has -- a map of plant hardiness zones in the United States.

As a gardener, you know that plants grow best in their comfort zone -- not too hot or too cold. For decades, landscape professionals, gardeners, foresters, and nursery and garden-center staff have relied on the Agriculture Department's hardiness zone map to determine which plants are appropriate for a given area. The map, developed by the National Arboretum, the American Horticultural Society and plant scientists across the country, was designed to help expand the range of plant materials that could be cultivated by predicting which flora would survive in specific locations.

However, many horticultural professionals think the map is out of date. The USDA last revised it in 1990, based on data from 1974 to 1986. But the climate has changed since the mid-1980s. The agency rejected a proposed update in 2003 and plans to release a new map as soon as this year.

Enter the National Arbor Day Foundation, whose members include arborists, urban and rural foresters, and homeowners who care about trees.

"The USDA map just doesn't seem right anymore," said Woody Nelson, vice president of communications for the foundation. "So we took it upon ourselves to do our own map." For an updated take on which plants are hardy in your region, look at the foundation's map online at http://www.arborday.org/media/zones.cfm.

The foundation used data from the National Oceanic and Atmospheric Administration, the same source the USDA map uses, but the data are more recent, collected from 1990 to 2004. The foundation's map, like the USDA's, is a compilation of average low temperatures because the lack of cold hardiness is the most profound barrier to plant success. Plants simply can't adapt to regions where they are unable to survive the winter.

The changes were startling. Many areas jumped one or two zones higher. "The climate has changed," Nelson said. "It has warmed."

Washington's northern suburbs used to be Zone 6 and its southernmost suburbs Zone 7. Now Zone 7 extends as far north as some parts of southern Pennsylvania. Nelson lives in Nebraska, which has become warmer by half a zone, putting his region in Zone 5.

The online map includes several clever features. One allows you to see which areas changed and by how much. A color-coded map shows pink and red for warmer and light blue and darker blue for cooler. "Anything that's pink has warmed a full zone."

Some isolated spots, mostly around the Rockies, have gained two zones. And some parts of the Southwest, mostly in Arizona, Nevada and eastern California, have lost a zone. In terms of warming, Nelson said, "the far West has changed far less than the East and Midwest. The West is pretty warm anyway."

The map was created by averaging data from weather-reporting stations. The longitude and latitude of each station are known, so it's a matter of connecting the dots to establish zone boundaries. Another handy feature of the new map is that you can search by Zip code and learn which zone you're in. Nelson said the foundation borrowed that idea from the American Horticultural Society's heat zone map.

You can also watch an animation of the northward-advancing hardiness zones. Zone 10 advances in Florida, Texas and Southern California. Zone 3 virtually disappears into Canada.

If the map seems wrong to you, you might be living in a microclimate, an area that is warmer or colder than the major portion of the region you live in. "No one knows their property better than homeowners themselves," Nelson said. And, as he points out, cold hardiness is not the only criterion for successful plant adaptation. Soil type, drainage, exposure, wind, and eccentric on-site conditions such as low wet spots or hot dry spots also make a difference. Urban areas tend to be warmer than the suburbs.

"We work a lot with urban foresters. Downtown landscapes can average 10 degrees hotter than a suburb 10 miles away," Nelson said.

The foundation's Web site also includes lists of the most popular trees for each zone and trees that should thrive there.

Nelson said the foundation is responsible for up to 10 million trees being planted every year, most of them seedlings, so it's important to match species to climate. The new map helps make sure the best information is available and allows the foundation to take advantage of climate changes.

One of the clearest ways to choose a plant that will do well in your yard is to choose a native species. "We strongly advise using native trees," Nelson said. They are usually planted from native seeds and are more accustomed to the kind of changes that occur within an area over time.

The Arbor Day Foundation doesn't advocate planting only native trees, though. "You also need biodiversity," Nelson said. Diseases, such as Dutch elm disease, and insects, like the emerald ash borer, can become problems for single-species plantings. Monocultures, he noted, are vulnerable to the same issues.

"Diversity would help maintain the canopy," Nelson said. "Start with natives, and add a little diversity." The wildlife likes it, and variety can create a more beautiful landscape.

But stay away from invasive species that will drive out indigenous plants when trying to expand the palette of viable trees, shrubs and perennials in your landscape.

"Many of our members tend to be somewhat adventurous, to push the envelope, to be forward-thinking," Nelson said. They understand that the creation of a green canopy requires a variety of trees, he said.

While the new map reflects recent reality, it's not a predictor of future climate conditions, and not everyone welcomes creeping heat. The Union of Concerned Scientists recently predicted that much warmer temperatures in the Northeast would kill the ski industry, lead to longer and more severe droughts, change the coastline and wipe out some fishing. But the report noted that recent efforts to reduce greenhouse gases in the area are a step toward maintaining a stable climate.

Trees are an inexpensive and simple way to improve climate conditions. They reduce carbon dioxide in the air, remove tons of particulate pollution, filter sulfur dioxide and carbon monoxide from our atmosphere, and provide a cooling effect that reduces utility use and results in less burning of fossil fuels. Nelson says trees can make a difference in minimizing climate change, as can greenery planted on all planes of the landscape.

Joel M. Lerner is president of Environmental Design in Capitol View Park, Md. E-mail or contact him through his Web site, http://www.gardenlerner.com.


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