Planting Fall Plant Bulbs? Consult New Hardiness Zone Maps

Most gardeners know that the United States Department of Agriculture (USDA) publishes a handy map that separates the country into eleven different temperature regions. These eleven "Hardiness Zones" help gardeners select the kinds of plants that are sturdy enough to survive and thrive in their gardens. Last spring The National Arbor Day Foundation released a new and different Hardiness Zone Map indicating that the country has heated up measurably in the 15 years since the USDA version was last updated.

Hardiness zones are determined by a region's average annual low temperature. For example, the annual low temperatures in zone 3 range from -40F to -30F, while the average annual low temperature in zone 10 is +30F to +40F. Whole swaths of many states have shifted at least one full hardiness zone warmer. Much of Illinois, Indiana, and Ohio, for example, has shifted from Zone 5 to a warmer Zone 6. Large swaths of the South have moved into a warmer Zone 8.

The USDA's own update could be months away, according to a recent story in the Arizona Republic -- so until it is updated, consider the recommendations of the Arbor Day Foundation. Revised Hardiness Zones, Courtesy of the Arbor Day Foundation. This animated graphic shows the change between 1990-2006.

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