Many of us Baby Boomers remember the 1970’s cliche question, “what’s your sign.” Fortunately, that question died out pretty quickly by the end of the ’70s. For home gardeners, the question might be “what’s your zone” — meaning plant temperature zone. Many home gardeners have seen a multicolored temperature zone map of the U.S. in gardening books, magazines, publications, and seed and nursery plant catalogs, in other gardening publications, and on plant tags at the retail garden center. A 1990 full-color edition of the USDA Plant Hardiness Zone Map (PHZM) map helped gardeners determine what perennial and woody plants will grow in what area of the country based on average cold temperatures. Many publications still use this map.
An update of the 1990 map was released by the USDA Agricultural Research Service (USDA — ARS) in 2012. Most home gardeners do not know about the 2012 release and how it works. The 2012 PHZM release is now web-based rather than printed. But before we get into the 2012 map and its changes, there is a fascinating history on how previous plant zone maps came about.
Alfred Rehder, a horticulturist at the Arnold Arboretum/Harvard University in Boston, Massachusetts, printed the first plant zone map in his 1927 book Manual of Cultivated Trees and Shrubs. Rehder’s map divided the U.S. and Canada into eight zones with 5-degree temperature differences. Rehder based his map on the average mean cold temperature each of the 12 months.
Dr. Donald Wyman of the Arnold Arboretum published updates in his 1938 book Hedges, Screens, and Windbreaks using temperature data from 1885 to 1935. Revised maps were found in the 1951, 1967 and 1971 editions of his Wyman’s Gardening Encyclopedia
There was an issue with Wyman’s zones, however. They were not based upon consistent temperature ranges. Some were 5ºF, others 10ºF or 15ºF.
Because of these inconsistencies, the USDA Agricultural Research Service (USDA ARS) undertook a research effort to create a new map. The 1960 edition of the USDA Plant Hardiness Zone Map (PHZM) used temperature data from over 5,000 weather stations and created 10 zones, each zone subdivided by 5 degrees into an “A” and “B” within the zone. In 1990, USDA ARS released an updated map with temperature data from over 8,000 weather stations covering a period between 1974 through 1986. An additional zone, 11, was added to the 1990 map. In 2004, USDA ARS determined that an updated, more accurate PHZM was needed. USDA ARS partnered with the Oregon State University PRISM Climate Group with its climate mapping software to develop a new map. Data from 7,900 weather stations over the period 1976 to 2005 were used in complex statistical algorithms to create an updated map. In 2012, USDA ARS released the new PHZM as a high-resolution interactive web-based map instead of a printed map.
The 2012 map release had 13 temperature zones. Not surprisingly, the 2012 map indicated warming trends in several zones. The 2012 PHZM is generally one-half zone warmer than the previous maps. Part of the warming indication can be attributed to more accurate weather data and more sophisticated computer algorithms and analysis.
However, the 2012 PHZM also showed that some zones, especially in the mountainous western U.S., were cooler. All Cherokee County is in Zone 7B, according to the 2012 PHZM. Atlanta is in Zone 8A, and Blue Ridge is Zone 7A. South Georgia is Zone 8B with a portion of the lower Georgia Atlantic coast 9A. Want to check out the PHZM for other areas of the U.S.? Type in planthardiness.ars.usda.gov/PHZMWeb/Default.aspx in a web browser, click on “Interactive Map,” enter the code given and then your zip code and hit “Locate.” The area of your zip code will come up. You can zoom out on the map for a larger geographical area. If you would like to print a map of the plant zone and zip code, there is a print function on the website.