The U.S. Department of Agriculture’s newest map, the 2012 Plant Hardiness Zone Map, was introduced last year. It’s the first serious face lift the map has had since 1990.
Researchers at Oregon State University determined the zones by using 30 years of the mean annual extreme low temperatures, 1976 to 2005. They gathered temperatures from 7,983 weather stations and developed a climate-mapping technology using a digital grid; each grid-cell measured a half mile, said Kim Kaplan, public affairs specialist for the Agricultural Research Service of USDA.
In addition, they calculated the effect of geographic factors such as elevation and proximity to large bodies of water or concrete. Each zone represents a 10-degree difference; each sub-zone (a or b) represents a five-degree difference.
The completed map was reviewed by dozens of climatologists, agricultural meteorologists, botanists, and horticulturalists. If any of them questioned a region’s designation, the data were double checked.
Two new zones were added, 12 and 13, for warm climes with mean annual extreme minimum temps of more than 50 and 60 degrees, respectively. That’s Hawaii and Puerto Rico. Growers of tropical plants wanted specifics that would guide northerners who buy the plants for use on summer patios, plants that need to be taken indoors come autumn.
Each zone is assigned two shades of the same color to indicate its sub-zones. Ohio's and southern Michigan's 6a and 6b, for example, are darker and lighter shades of forest green.
Zone maps have been around since at least the 1920s, but this is the first to be done on sophisticated computers.
Ms. Kaplan said the new map does not indicate climate change because its information is different than what was used in previous maps. Climatologists, she said, take a longer view, generally evaluating weather trends in at least 100-year cycles.
Moreover, the 1990 map was done when computers were infants and was based on winter minimum temps of just 13 years, 1974 to 1986. And those results skewed cold because the 1970s included some extremely cold winters.That map also didn't take into account factors such as elevation and proximity to large bodies of water or concrete.