HALF THE NATION’S 80 million gardeners will find themselves officially declared a half-zone warmer today, when the United States Department of Agriculture launches the first update since 1990 to its Plant Hardiness Zone Map. No, the new map is not technically a confirmation of a trend toward global warming, the agency says—a different set of data is used in it than in those longer-range calculations—but it is a more accurate picture of growing conditions across America, particularly in tricky areas such as mountainous ones that may have been rated too cold or too warm in previous versions. We have ever-more sophisticated computers to thank for enabling scientists to capture the more accurate take on things.

“The increase in our computing power today allows the research team to build into their algorithms things they knew were important factors in 1990, but couldn’t include,” said Catherine Woteki, Chief Scientist and Under Secretary for Research, Education and Economics for the USDA, in an interview yesterday. Factors such as elevation, the slope of land, or how close to a body of water a location is, can cause sharp variation despite close adjacencies.

“Taking those into account now provides a lot more detail,” Woteki said, “and people will be able to see islands of heat, and also cool ones, on the new map. As a scientist and a home gardener, I love seeing this so much more clearly.”

The new map is built using digital Geographic Information System technology and you’ll notice the sharper boundaries and better resolution right away when you visit its interactive website. To find out whether your zone has shifted, start here. Though I’ve always thought of myself as a Zone 5B, I’m now officially there, no longer in Zone 5A—so I’m among the half who are officially warmer. My new New York State map is above; I also looked right away at Texas, for instance, where I could see a similar shift to the warmer 50 percent (new East Texas map below).

The new Plant Zone Hardiness Map draws on data from 1976-2005—in initial calculations, adding more recent years didn’t affect the outcome—specifically 30-year averages of what are essentially extreme events (the coldest temperature of the year). Climatologists studying climate change, by comparison, look for trends in overall average temperatures over a 50- to 100-year period. The 1990 map drew on data from 1974-1986.
“This is fun,” said Woteki, “and a really a good time to have it because I just got my first plant catalog in the mail this week.” The “new math,” she says, will make for some good shopping–especially for those who find themselves in a different zone and are feeling adventurous. Indeed.

Highlights of the New Map

• Data from a total of 7,983 stations were incorporated into the calculations.
• Two new Zones—12 and 13, with extreme minimum temperatures of 50 and 60 degrees F, respectively—have been added to the former 10-Zone structure
• Puerto Rico is on the map for the first time.
• You can instantly search by your state or by zip code for your zone.
• Though the USDA will no longer print maps for sale, you can download and print at different DPI, resulting in an instant detailed color image of the area(s) desired.
• Completed drafts were reviewed by a team of climatologists, agricultural meteorologists, and horticultural experts—in a process Woteki charmingly calls “ground truthing”–to find errors.
• More details on what’s new are covered here, on the map site.

Who Uses the Map?

IN ADDITION to gardeners, nurseries and plant breeders are among those who use the Plant Hardiness Zone Map most, as the data also figures into research models that include for crop-risk insurance assessment, and the spread of exotic weeds and insects.

For Canadian Gardeners

For Canadian gardeners, use the map at this site instead.