The New USDA Plant Hardiness Zone Map
Peter Del Tredici
The publication of this map provides an opportunity to review the history of hardiness zone maps.
The new Plant Hardiness Zone Map, updated wealth of new data used to create the map also for the first time in twenty-five years, was allows the borders of the zones to be drawn released by the United States Department of in more detail than before. Agriculture this past February. Unlike the According to its makers, the new map does previous edition, the map includes Alaska and not seem to uncover any global warming Hawaii and is detailed enough to show county trend, but it does reveal some regional lines within the scates. In addition, the map changes. On both coasts, but particularly in includes Canada and Mexico for the first time. the Southeast, temperatures are given as five According to Dr. Marc Cathey, Director of the to ten degrees cooler in the winter than on the National Arboretum in Washington, D.C., previous map. Isolated pockets of the North- who oversaw the updating and production of east are slightly warmer; and sections of the the map, such expansion is appropriate given Midwest show some minor changes, as do that "we share many plants, both native and parts of Canada. introduced, with these countries." Since 1960, when the USDA published its The large-format map, measuring four feet first zone map, considerable confusion has by four feet, has eleven color-coded zones arisen from the fact that it used different tem- based on ten-degree (Fahrenheit) differences in perature ranges to define its zones than did the average annual minimum temperatures. its well-established predecessor, the Arnold As in the old map, each zone is divided into Arboretum zone map. The publication of the A and B regions based on five-degree differ- latest 1990 USDA map, based as it is on more ences. One new zone, Zone 11, has been added abundant and more accurate data than the (including parts of Mexico, California, Hawaii, Arnold Arboretum map, provides the perfect and Florida) where the average annual mini- opportunity to resolve this confusion. At long mum temperature is above forty degrees last, the United States has a single, stan- Fahrenheit. dardized zone map. There can be little doubt Data from 14,500 weather stations, gathered that this new USDA map is superior to any, between 1974 and 1986, went into this update and all, previous efforts. -more than twice as many stations as were Brief of used for the maps introduced in 1960 and History Hardiness Zone Maps 1965. With the additional data, small areas of This is perhaps the appropriate time to take microclimates are indicated for the first time. a brief look at the history of hardiness zone These are either cool pockets caused by maps, the first of which was published in mountaintop elevations or hot spots due to 1927, in Alfred Rehder’s ground-breaking the heat of cities or protected valleys. The Manual of Cultivated Trees and Shrubs. This 17
20142014201420142014201420142014201420142014201420142014201420142014201420142014201420142014201420142014201420142014201420142014201420142014J The new 1990 USDA Hardiness Zone Map. early attempt divided the cold temperate the accompanying figure, the lines separating United States into eight zones characterized the zones were very approximate. by uni form five-degree (Fahrenheit) differences Rough as it was, however, this map stood in the lowest mean temperature of the alone until 1938 when Donald Wyman, using coldest month. All the plants listed in the data from a U.S. Weather Bureau map for the Manual were assigned, at least tentatively, to years 1895 through 1935, redrew its contours one of the hardiness zones. As can be seen in based on the average annual minimum tem- 18
The first Hardiness Zone Map from the first edition of Alfred Rehder’s Manual of Cultivated Trees and Shrubs, published in 1927. The contour lines are based on the lowest mean temperature of the coldest month.
The second Hardmess Zone Map from the second edition of Rehder’s Manual, published in 1940. The contour lines shown here are based on average annual minimum temperatures. 19
peratures, and published a new map in his its first hardiness zone map with zones based book Hedges, Screens and Windbreaks. While on uniform ten-degree ranges in average based on a different temperature standard annual minimum temperature. It is interest- than Rehder’s map, the contours of the two ing to note that while both the USDA and the are remarkably close. Wyman’s new map was Arnold Arboretum maps were based on the published in the second edition of Rehder’s same weather station data, their differences Manual (1940), and the hardiness ratings of were only in where the contour lines were the various plants were adjusted accordingly. drawn. From this perspective, it is clear that Wyman, with assistance from various Arnold the uniformity of the USDA zones is prefera- Arboretum staff members, updated his map ble to the arbitrary Arnold Arboretum zones. in 1951, 1967, and, ultimately, in 1971. Limitations Unlike Rehder’s original map, however, Wyman’s hardiness zones were not based on In considering national hardiness zone maps, a uni form number of degree differences. Some one should not overlook the intrinsic limita- of his zones had 15-degree ranges in the aver- tions of the whole concept of hardiness zones. age annual minimum temperature, while Alfred Rehder was the first to point out these others were based on 5- or 10-degree ranges. limitations in his 1927 Manual: In particular, it is worth noting that Boston There are, however, many other factors besides tem- is located in one of the two zones. 5-degree perature in winter which will influence the hardi- This lack of was into uniformity called ness and growth of certain plants, as soil, its physical question in 1960 when the USDA produced as well as chemical composition, exposure, rainfall,
The Arnold Arboretum Hardiness Zone Map from 1967. 20
The USDA Hardmess Zone Map published in 1960.
humidity of the air, shelter from cold wmds. As a the minimum winter temperatures. rule one that stand cold better in a may say plants Taken with a hefty dose of skepticism, drier situation than in a wet one, and that decidu- hardiness zone do ous trees and shrubs prove hardier in a more exposed however, maps provide a to situation and in a climate with higher summer tem- gardeners with useful guide the plants perature, while evergreen plants prefer a sheltered they can grow safely in their area. situation, and like a more humid climate and less extreme summer and winter temperatures. Availability of the new USDA Hardiness Zone Indeed, as most gardeners have learned dur- Copies ing the 1980s, taking "microclimate" varia- Map are available through the Superintendent of Government tions into account often allows gardeners to Documents, Printing Office, DC for Publ. grow plants that, according to the hardiness Washington, 20402, $6.50 (Misc. or from the Arnold Arboretum Book- maps, should not survive in their area. At the 1475); Arnold Arboretum, for example, it is known shop, 125 Arborway, Jamaica Plain, MA 02130. that the fluctuating temperatures of early spring can be more damaging to some plants (particularly those from northeast Asia) than Peter Del Tredici is Editor of Arnoldia.