Tony Avent has been an adviser on a long-awaited revision of the USDA Plant Hardiness Zone Map. Developed in 1990, it was based on climate data gathered from 1974 to 1986. As it turns out, 1974 to 1986 was a colder period than any other since. Which means the USDA map, that holy writ of hardiness, has made us unwittingly cautious in our choices, growing plants well within our range.

It would seem to be time for a new hardiness map, which Avent says is in the works. “We’ve got our fingers crossed for delivery by the end of the year.” The map would reflect three decades of data instead of one. “The new map will certainly reflect that it’s getting warmer,” Avent says. “I don’t think there’s any question about that. We can grow plants now that we couldn’t in the 80s.” We intend to include factors like the number of days over a certain temperature, humidity, night temperatures and other factors that affect heat adaptation. After all, hardiness is not just about winter lows.