The USDA has released an updated version of the Plant Hardiness Zone map, shifting many zones one half zone warmer than the previous version of the map, released in 1990. This new edition of the map is based on weather data over a 30 year period, from 1976 to 2005.
So why the changes?
The 1990 map used temperature as the basis for mapping the zones, whereas this version of the map uses data like elevation, slope, wind and proximity to water to determine zone. The new map also uses data from more weather stations than the 1990 map did.
“The USDA was very careful to downplay that this has anything to do with global warming, and I was happy to hear that, because to me it’s comparing apples to oranges,” says Cornell’s Bill Miller. “The mathematical models are different. The ability to handle the data is different. I don’t think you can directly compare this map to the prior map. But I have colleagues that say that taken together, this all means that there is a lot of global warming going on.”
What does this mean to growers and gardening? Miller says hardiness zones have always been guides to gardeners, and plants will not now be able to be grown in drastically different areas because the zones have changed.
“There’s probably more variability in plant survival and performance within 50 gardens in one city than there is across the entire city, because of microclimates, shade, exposure, wind, soil drainage, slope and snow cover,” Miller says. “All these factors will have a huge effect. None of this means we’re going to start growing palm trees in Willoughby, Ohio.”
Garden centers can take this opportunity to challenge their customers, however, to push the boundaries of their gardens slightly.
“What it might do is allow garden centers who are really creative and sophisticated to use this to encourage people to try things that they might not otherwise have been tried,” Miller says.
Click here for the new USDA Plant Hardiness Zone map, including a search by Zip code.