The U. S. Department of Agriculture has issued plant hardiness maps depicting the average minimum winter temperatures across the U.S. and Canada since 1960. The maps provide guidance to gardeners in the selection of plants that will tolerate the winter conditions in their areas. The distribution of hardiness zones is based on records from the numerous weather stations in the U.S. and, to a lesser extent, Canada. The maps divide the country into 10 degree intervals ranging from below -50o (F.) in northern Canada to + 30 to +40 degrees in the Florida Keys. Five degree half-zones also are delineated.
The first map, issued in 1960, used temperatures for the years 1899 through 1938, except for 34 states for which data were “readjusted” based on additional data from 1931 through 1952. Presumably the readjustments improved accuracy. Another level of data adjustment also was incorporated where qualified supplemental data were available.
A revised USDA map was issued in 1990 that relied on nearly double the number of weather stations than the first map. Comparison of the two maps shows that because of unusually cold winters in the 1974-86 period in Canada and the eastern states, some areas were depicted as colder than in the first map. Later, however, recognizing the warmer winters in the 1986 to 2002 period, the American Horticultural Society, undertook a revision of the hardiness zones. The resultant map, released in 2003, more resembled the 1960 map than the 1990 one, in that many spots were depicted as a half-zone warmer. Warming trends prompted the National Arbor Day Foundation to review the data and in 2006, release a map that depicted changes in zonation between the 1990 USDA map and 2006. The frequent shifts to the next highest zone (the pink areas in the map below) produced a dramatic pattern. The USDA, however, continued recommending its 1990 map, until finally issuing an updated one in 2012 based on data for the 1976-2005 period. This map used updated data analysis techniques and should have addressed the bias in the 1990 map brought on by the aforementioned cold interval. Indeed, the USDA states that increased sophistication of mapping methods and better data explain much of the differences in observed patterns. So, presumably, do we now (in 2013) have the most accurate representation of hardiness zones?
Apparently not, according to Dr. Nir Krakauer, of The City College of New York’s Grove School of Engineering. He developed a new method for calculating and mapping the plant hardiness zones. His results recognize the warming trend embedded in the latter portion of the 1976 to 2005 data base). Krakauer found that over a third of the U.S. has shifted by a half-zone, and a fifth of the country, a whole zone compared to the USDA’s latest map. He also notes that winters are warming faster than is summer, a trend consistent with global warming. Changes appear to be occurring at a rate that cannot be well captured by the USDA’s approach.
Weather, is of course notoriously variable and will continue to be, perhaps even more so in a warming world. For example, a run of unusually cold winters could occur in response to volcanic eruptions, sun spot-cycles or shifts in the Arctic Oscillation, to name 3 phenomena known to affect global or regional weather. If so, the value of the current hardiness map could decrease, and it’s possible that, for example, the camellias that have survived recently in Detroit winters might be winter-stressed or even killed back. More timely maps would be the best defense against winter onslaughts and Krakauer has offered a method that would allow for annual updates. See the following for more information: www.phys.org/news/2012-09-warmer-temperatures-usda-zone-obsolete.html (or CUNY Newswire) and Advances in Meteorology Vol. 2012, article ID 404876. The map depicting zone shifts is from http://www.arborday.org/media/map_change.cfm
Note also that depending on your location, your spot may still be within the same zone even though minimum temperatures have somewhat moderated. That’s true for me. My little oasis has remained in zone 9b on all the USDA maps, 1960 to 2012, although the zone 10 boundary has crept closer. Based on my minimum yard temperatures for the period 2004 to 2011, the USDA zone 9b seems correct. The minimum temperature for the period was 26 o and the number of days with below freezing temperatures ranged from 1 to 6 annually. I still have to protect many low temperature-sensitive species in the winter but that is largely because I choose some zone 10 category species for my yard.