If you’re like most people, you missed the release in late January of the USDA’s new plant hardiness zone map. You probably didn’t even know an update was in the works. This new version replaces the 1990 release, and that one replaced the 1965 and 1960 releases.
The plant hardiness zone map is probably most familiar to people from perennial seed and plant catalogs. I remember seeing it for years without thinking much about it or taking the time to figure out what it was good for—if you’re mostly growing vegetables or annual bedding plants, who cares? As a review, what is it and what are the basic assumptions behind it, and is this new version an improvement?
The intent of the hardiness zone map has always been as a guide to gardeners. In a general way, it allows us to standardize how we think and talk about plant hardiness. So it’s also useful to other plant growers and researchers, and it serves as a basis for the USDA Risk Management Agency’s crop insurance standards program. This most recent version was jointly developed by USDA’s Agricultural Research Service and Oregon State University’s PRISM Climate Group. It’s available at  for access.
The map is characterized by a series of colorful bands that cross the country from east to west. These represent temperature “zones” that differ in10° F increments; each zone is further subdivided into two 5° F temperature increments (designated “a” and “b”, with “a” being cooler than “b”). These incremental temperature zones represent average annual minimum temperatures. They do not indicate the coldest temperature ever recorded or that will ever occur in an area.
Survival of perennial plants over winter is considered, in this scheme, to be the most critical factor in plant adaptation to an environment, and plants are empirically assigned to a hardiness zone largely on that basis. But survival alone is not the whole story. Lots of things affect plant adaptability. Wind, soil type, soil moisture, humidity, snow, and winter sunshine are all important, as are cultural factors such as plant size and health. Under the plant hardiness zone scheme, zone ratings are intended to indicate “excellent” adaptability. A plant assigned to Zone 5a, for instance, should do more than just survive in that zone; it ought to thrive.
In the new map, zone average annual minimum temperature is computed from 30 years of data (1976–2005), whereas the previous version used only 13 years of data (1974–1986). Here, more is surely better. USDA says that the new map is generally a half-zone warmer (that is 5° F) than the previous map throughout much of the USA. But in the same paragraph, USDA says that the additional years of data do not make a significant difference in defining the zones. Oh? Maybe I’m wrong, but that half-zone temperature shift sure sounds significant to me.
What else is new? The mapping technique algorithms are more sophisticated, allowing more precision in interpolating temperature readings between weather stations. More weather stations were used; so this too adds greater precision and detail to the new map. And the vicinity of large bodies of water and variation in terrain and elevation were taken into account for the first time. USDA notes that these additions are improvements and may be most important for people in mountainous areas. No doubt that’s true, but my guess is that you if you live on a mountain, you already know that it’s generally cooler up there than in the valley below.
Two new zones have been added: Zones 12 (50–60° F) and 13 (60–70° F). These are included on the maps of Puerto Rico and Hawaii only.
In the past, a large printed zone map could be ordered from USDA. This option is no longer available. The new map is strictly internet based, but high resolution versions (of the entire USA and individual states) can be downloaded and printed. In addition to the printable “static” map(s), an interactive, GIS-based map is available. This map allows you to zoom in and see roads and other land features and click on them to determine the hardiness zones and where they’re located. If you’ve ever used Google Maps or weather.com maps (which may be the same technology) to check the condition of the shingles on your roof or to see if your neighbor down the street ever moved his boat, you may be underwhelmed by the resolution of the interactive zone map. It’s good enough. You can infer where your neighborhood is from the larger roads that show up, but you won’t see your garden plot unless it’s a section or more. I don’t think for most people that this is any better than the hardiness zone–zip code function that is still available and was part of the 1990 map.
You say you ordered seeds and plants in December before the release of this new hardiness zone map? No need to worry. This may be “the most sophisticated Plant Hardiness Zone Map yet for the United States”, as the USDA press release says, but in a practical sense, the improvements over the 1990 version are insignificant. And the resolution on the interactive map is a big disappointment.