Gardening Map Changes for Global Warming

The Plant Hardiness Zone Map has changed to show as much as a 10-degree increase in some U.S. zones

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With the release of a new gardening map Wednesday, the U.S. Department of Agriculture essentially announced what most people, especially gardeners, already knew: America is getting warmer.

The Plant Hardiness Zone Map divides the United States into 13 zones by the region's lowest average annual temperature to help gardeners and farmers determine which plants and crops can survive the winter. On average, each region stepped up about half a zone since the previous map was released in 1990, representing a five-degree increase in the average minimum temperature. Some regions have been changed to reflect as much as a 10-degree increase.

While the USDA does attribute some of the changes to global warming, the agency insists some of the zones changed because the new map uses more years of climate data, so it's more accurate than the 1990 version of the map. Also new—the online version of the map is interactive. Plug in your ZIP code and the site will let you know which zone you live in.

The new map design comes as no surprise to experts.

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"A lot of folks who garden have noticed over several years that certain plants are blooming earlier in the season," says Janice Stillman, editor of the Old Farmer's Almanac, which has been around since 1792. "People are noticing there's a change in the weather."

Gardening enthusiasts might recognize the map from the back of seed packets and on plant tags at nurseries, but its usefulness extends beyond the backyard. Community planners use the map to determine which trees and plants to grow in public parks, and large national retailers, such as Home Depot, use the map to help determine which stores they should ship certain plants to.

Of course, you still won't be able to grow pineapples in Vermont or orange trees in Alaska, but the shift expands the possibilities for gardeners in many northern states, explains Susan Littlefield, horticultural editor at the National Gardening Association.

"It broadens the plant palette for a lot of people," she says. "Gardeners always want to plant something that they've seen somewhere else—we'll want to try to grow it in our zone."

According to Stillman, the USDA might have to update the map again in a few years. The Farmer's Almanac predicts a couple years of cooling coming up.

"Based on solar activity, especially the sun spots, we think we're moving into a period that's cooler than normal," she says. "Whether that's offset by greenhouse gases in the atmosphere remains to be seen."