The United States Department of Agriculture unveiled a new zone hardiness map this morning. It’s big news in the world of people, who do read maps, but not so much among the world of plants, which do not.
To quote the USDA:
“If your hardiness zone has changed in this edition of the USDA Plant Hardiness Zone Map (PHZM), it does not mean you should start pulling plants out of your garden or change what you are growing. What is thriving in your yard will most likely continue to thrive.”
What IS a zone map? The USDA has a new, well-written description of that, too: “The 2012 USDA Plant Hardiness Zone Map is the standard by which gardeners and growers can determine which plants are most likely to thrive at a location. The map is based on the average annual minimum winter temperature, divided into 10-degree F zones.”
Another thing that’s changed since 1990, the last time the USDA released a PHZM: technology. The new zone map has a cool new interactive feature. Just click on a spot on the map (hoping you’ve aimed your cursor at the right place) and voila! It tells you the zone, the average cold temperature, the low temperature range, AND the latitude.
Here’s what the USDA says about the interactivity: “For the first time, the map is available as an interactive GIS-based map, for which a broadband Internet connection is recommended, and as static images for those with slower Internet access. Users may also simply type in a ZIP Code and find the hardiness zone for that area.”
It’s kind of fun to play with … try it!
Some locations, particularly sections of the northeastern United States, appear to be warmer than they were in 1990.
In Des Moines, where I live, we’ve been shifted from a Zone 5a to a 5b, although counties encircling us appear to be Zone 5a. Perhaps—and this is just a guess—we’re warmer because of all the concrete here. Or maybe I didn’t click in quite the right spot.
A big question is whether this map reflects climate change. Carolyn Pinkard of Briggs Nursery wrote:
“Kim Kaplan of the USDA presented the map during a webinar this morning and fielded questions from the press. Bart Ziegler of the Wall Street Journal and Seth Borenstein from the Associated Press both addressed the question of how the map may demonstrate climate change. The majority of changes made to the map involved warmer vs cooler zones in the northeast region of the U.S. Additionally, of 34 cities mentioned, over half are now listed in a warmer zone. Kaplan was reluctant to associate any connection to climate change and repeatedly stated the USDA does not have evidence showing what caused the changes. She also noted that, while many areas increased a half zone, most represented a minor temperature change with as little as .5°F affecting the change.”
Carolyn and the USDA bring up another good point: Every yard has its own microclimates. In my yard, for example, I can trial Zone 6 plants in a narrow spot between the south side of my house and a rock retaining wall where they are protected and where warmth from the stonework lingers.
The PZHM is just a tool and a guideline. According to the USDA, the map is “based on the average lowest temperatures, not the lowest ever. Growing plants at the extreme of the coldest zone where they are adapted means that they could experience a year with a rare, extreme cold snap that lasts just a day or two, and plants that have thrived happily for several years could be lost. Gardeners need to keep that in mind and understand that past weather records cannot be a guarantee for future variation in weather.”
Seems true enough here in Des Moines, where we’ve had a mostly brown winter, and warmer January temps than I can ever recall.
Finally, here’s the OLD USDA zone map: