For years, people have been talking about changes in weather; particularly how our winters seem to be getting shorter and colder for shorter periods of time. Of my friends in Montreal, I don’t know anyone who prepares an outdoor skating rink in the backyard as we did every year growing up. Even with last year’s snowy winter, winters are simply not as cold as they were when I was a kid.
As greenhouse folks, this may be a good energy-saving thing, but winter temperatures are the basis for the highly popular USDA Plant Hardiness Zone Map.
This map is used by everyone planting perennials, shrubs and trees. It is as well known by gardeners as it is by professionals, used in every catalog, heard in every talk and the subject of endless conjecture. The map consists of 11 hardiness zones, based on minimum winter temperatures, and everyone knows his or her USDA hardiness zone.
The current map was constructed in 1990. However, a new, much anticipated USDA Plant Hardiness Zone Map was constructed in 2012. The USDA’s Agricultural Research Service and Oregon State University developed the new map.
The New Map
The new version features13 zones that include the first-time additions of zones 12 (50 to 60°F) and 13 (60 to 70°F). Each zone is a 10°F band and, similar to the old map, is divided into 5°F zones A and B.
Zone boundaries have shifted in many areas. The new map is generally one 5°F half-zone warmer than the previous map throughout much of the United States. That is, Zone 6a is likely now 6b; Zone 7b is probably 8a, etc.
In some cases, no changes have occurred. Changes result partly because the new map uses data measured at weather stations from 1976 to 2005. In contrast, the 1990 map was based on temperature data from a 13-year period from 1974 to 1986.
USDA claims that more sophisticated methods for mapping zones have resulted in some of these changes. The map looks at factors such as changes in elevation, nearness to large bodies of water and position on the terrain, such as in valley bottoms and on ridge tops.
This version also used more weather stations, so the accuracy and detail of the map, especially in mountainous regions of the western United States, should see a marked improvement. Nobody mentions global weather changes, but they would be difficult to ignore, and in some cases, they resulted in changes to cooler, rather than warmer, zones.
Another major improvement is the map’s online availability. Visit PlantHardiness.ars.usda.gov/PHZMWeb/, put in your zip code and you’ll have your zone.
This map will be with us for many years, and I expect to see it as the standard in all plant discussions, from catalogs to lectures.