History repeats itself. According to this article (in pdf format) by Peter Del Tredici* on the history of plant hardiness zone maps, there used to be two hardiness maps floating around, the Arnold Arboretum map and the map put out by the USDA. Donald Wyman first created what would come to be called the Arnold Arboretum map in 1938, using forty years’ worth of data taken from a U.S. Weather Bureau map. The AA map was updated in 1951, 1967, and 1971.
The USDA got into the act in 1960 when they issued their first map. This is where the confusion began, because the zones didn’t coincide, and you could be in Zone 5 on one map and Zone 4 on the other (for example). And then if you found a plant hardy to Zone 5, you had to know which map was being referred to. The USDA map issued in 1990 (the current “official” map) was based on data from more than twice as many weather stations (14,500 of them) as the old USDA map, covering a twelve-year period from 1974 to 1986. Del Tredici enthuses,
The publication of the latest 1990 USDA map, based as it is on more abundant and more accurate data than the Arnold Arboretum map, provides the perfect opportunity to resolve this confusion. At long last, the United States has a single, standardized zone map. There can be little doubt that this new USDA map is superior to any, and all, previous efforts.
I remember when this map came out. Fine Gardening included it as an insert in an issue of the magazine, and it was widely hailed as an improvement over the previous hardiness maps. Obviously the National Arbor Day Foundation (NADF) thought it needed further improving, because they went and developed their own map, which is causing a brouhaha among garden bloggers. It’s based on 15 years’ worth of data from 5,000 National Climatic Data Center cooperative stations. I don’t know if an NCDC cooperative station is different from what Del Tredici calls a weather station, but it sounds like the NADF used data from far fewer locations over a longer time period. So is it more accurate, or less accurate? More to the point, did it tell you anything you didn’t already know?
Is it unfair to say that any gardener who’s been gardening in the same location for more than a year or two has a pretty good idea of how cold it gets in winter, how hot it gets in summer, how long between the last frost of spring and the first frost of autumn, and how much precipitation will fall and when it’s most likely to come down? Hardiness maps have always been about giving you a ballpark figure, a starting point from which to make better judgments based on experience. In my case, the USDA map has me in zone 5, and the NADF map has me in zone 6. According to my own data, the temperatures I’ve recorded right here in my own front yard, I was in zone 4 in 1990 and in the last 6 to 10 years it’s been more like zone 5. I say “more like” because in some winters it’s barely gotten below 0F (-18C)(Zone 7) but the date of my last spring frost has scarcely budged. (It was May 19th in both 1992 and 2005, for example, though it came as late as June 12th in 2004.) The fact that the winters have been warmer hasn’t really changed when tomatoes get planted in the ground.
I happen to have an old gardening book (1951) with the Arnold Arboretum hardiness map in it, and it has me in the -10F to -20F (-23C to -29C) zone, which they call Zone 4 but would be Zone 5 by the current maps. Perhaps this would explain the presence of established Rose of Sharon shrubs when we moved in. They are supposedly hardy to Zone 5, and I wondered how the previous owner ever got them growing in our Zone 4 winters. Answer: we didn’t have Zone 4 winters forty years before I moved in. The winters were warmer back then.
As far as I’m concerned, a hardiness map does you the most good when dealing with unfamiliar locations or unfamiliar plants, and is only a stopgap measure until you can develop your own informed opinion. Anyone can draw up a hardiness map, and the only thing that makes it the official one is the consensus that it is the most accurate one for the purpose. While we’re talking about accuracy, let me say that the hardiness map that most accurately portrays my little vegetable kingdom is neither the USDA map nor the NADF map. It’s one developed by Sunset magazine, of all things. This will be no surprise to our gardening friends in the west, who swear by this map, but it was news to me that Sunset had extended their reach across the entire continent, and even more astonishing to discover it was dead-on. (I’m Zone 42.)
If, as Mark Twain is reputed to have said, “Climate is what we expect, weather is what we get,” it can be unnerving when we no longer know what to expect. If one’s climate is getting consistently warmer, that can be anticipated and planned for. Furthermore, you’ll have noticed it getting warmer before any map comes out to tell you about it. But if the climate is getting more erratic and inconsistent, that is more difficult for both the gardener and the plants he or she tends. And in that case, no hardiness map can tell you what you need to know.
*”The New USDA Plant Hardiness Zone Map” by Peter Del Tredici in Arnoldia Summer 1990 (Vol. 50, No. 3). Accessed at http://arnoldia.arboretum.harvard.edu/pdf/articles/807.pdf on 26 Dec 2006.