Sunny weekend of plants in Maryland
Researching a daylily

New US hardiness zone map launches soon!!

At last, the promised land is in sight!

USDA Hardiness Zone Map Image ©USDA The USDA (United States Department of Agriculture) is set to launch their new updated hardiness zone map soon. Yippee! We’ve been waiting for quite a few years now, the last update was in 1990 (left). So it’s great news that the new map, based on the latest climate data from more weather stations than before, will soon be with us. British gardeners take note: the Royal Horticultural Society is also currently looking at the whole issue of rating plant hardiness. So read on to get a sense of the issues.

So: “The Agricultural Research Service (ARS) is in the last stages of getting the map ready for release,’ Kim Kaplan of the USDA told me. “The new map is an interactive GIS-based (Geographic Information Systems) map designed to be web friendly. It will also be presented on the web as jpgs so those without broadband access will still be able to use the map.” They’re just sorting out a service to host the new map so that the vast flood of users doesn’t crash the USDA servers. And you’ll see why this has to be a web based service.

It’s going to be far more than just a printed map, with a number of positive changes. In particular, using a web interface will make it possible to see the boundaries between one zone and the next in far greater detail. The ARS has also developed a way of presenting hardiness zones effectively in areas lacking weather stations, especially in rugged parts of the Western United States.

“As a result,” says Kim Kaplan, “the mitigation of extreme low temperatures by nearby, large bodies of water will be visible for the first time, as will the presence of cold sheltered valleys and mountain tops, which are caused by elevation increases and other geographic features. The warming effect of asphalt and concrete of major urban areas will also be visible in many locations.” So, for the first time, the warming effect of cities will be reflected in the hardiness zones map. Isn’t that great?

And all this is based on the most up-to-date weather station data possible. “The 1990 map was based on weather data from 1974 to 1986,” says Kim Kaplan. “The new one will be based on weather data from 1976 through 2005. Also, more weather stations in the mountains of the western United States will be used in the new map.”

There will be no changes to the zones themselves, except an addition at the warm end. “The 1990 map had zones 1-11 a & b,” says Kim. “The new map will have zones 1-14 a & b…  Zones 12-14… have been added to the legend to allow tropical plant breeders and nurseries to provide zone information on when to bring such plants in, such as from the deck.”

The result is a map of twenty-two zones and subzones in steps of just 5F. That really is the sort of detail that all gardeners appreciate.

Musa basjoo,banana,outside Of course, this whole system can never be more than a guide. Other factors like soil drainage, exposure to cold winds are also important and snow cover. But, at last, we’re going to have a zone map that reflects recent changes in our climate. So you may find that you’re now in a warmer zone than you thought – and you can grow far more plants than you thought. Perhaps I can leave my bananas outside all winter in Pennsylvania (on the other hand…)

Have no fear: As soon as the new map goes live I’ll let you know.

This is just so good… I’m so excited… after twenty years we’re going to get a new hardiness zone map... This great… Perhaps I can grow more plants than I thought… etc etc…

(Sorry, I'm a bit lost for images considering the map's not actually out yet...)