Plant Hardiness-Zone Maps Donald Wyman Harrison L

Plant Hardiness-Zone Maps Donald Wyman Harrison L. Flint

Existing hardiness-zone maps are valuable, but using them requires knowing how they differ one from the other, and how "hardiness" has been defined

Plant hardiness-zone maps have been a valu- differences in lowest monthly mean temper- able aid to those interested in predicting the ature. adaptability of plants to specific climatic A few years later, the prototype of the pres- areas. Most are isotherm maps of geograph- ent Arnold Arboretum hardiness map was ical regions, based upon average annual min- prepared by Donald Wyman. It included the imum temperatures experienced at certain entire United States and was first published weather stations over some period of years. in his book Hedges, Screens and Wind- Many hardiness-zone maps are available- breaks, in 1938. This map was based on some cover small areas such as individual average annual minimum temperatures for states, while others encompass entire coun- the years 1895 to 1935, as published in the tries. Unfortunately, many do not agree in Atlas of American Agriculture, USDA, in their numbering schemes-so zone numbers 1936. A modification appeared in the second assigned to individual plant species cannot edition of Rehder’s Manual of Cultivated be used in referring to all of the existing Trees and Shrubs, in 1940. In that book, the maps. In most cases they can be related only southernmost part of the United States was to the map used in assigning them. The two not included and much of Canada was added, most widely used in this country are the in keeping with the manual’s scope. Arnold Arboretum hardiness map and the The Arnold Arboretum Hardiness Zone "Plant Hardiness Zone Map" prepared by the Map in use since 1949 mcludes the entire Agricultural Research Service, United States United States (except for Alaska and Hawaii) Department of Agriculture (USDA). and southern Canada. This was first published in Wyman’s Shrubs and Vines for American and in his Arnold Arboretum Hardiness Gardens, republished Maps books Trees for American Gardens (1951 and The original map prepared at the Arnold 1965), The Arnold Arboretum Garden Book Arboretum was published in the first edition (1954), Ground Cover Plants (1956), and The (1927) of Manual of Cultivated Trees and Saturday Morning Gardener (1962). Shrubs, by Alfred Rehder. In this map, the The Arnold Arboretum map was revised in United States and southern Canada (except 1967 and again in 1971. The 1967 version was for southern Florida) were divided into eight published in Arnoldia, while the 1971 ver- zones characterized by 5-Fahrenheit-degree sion, which is the one published on the mside front cover of this issue of Amoldia, Reprmted, with slight modifications, from Amoldia, appeared in Wyman’s Gardening Encyclo- Vol. 27, No. 6, pages 53-56 (June 30, 1967). pedia. 33

The 1967 map differed from the previous ble. Unfortunately for the casual user, these version only in that hardiness-zone lines inconsistencies are small enough to be over- were redrawn to conform to more recent looked, and in several instances, writers have weather data. The zone-numbering system erroneously applied the hardiness-zone des- was unchanged-so zone numbers applied to ignations of Rehder to the USDA map. The specific plants in Rehder’s manual and other table shows the relationship between the two publications by the Arnold Arboretum staff numbering systems. could be used with the newer map just as well as with its predecessor. Local Hardiness-Zone Maps More-detailed plant-hardiness-zone maps have been for certain states and locah- USDA Plant Hardiness-Zone Map prepared ties. The total area covered by such maps is This map (Figure 1) was issued in 1960 as still rather small. Fortunately some detailed Miscellaneous Publication No. 814 of the maps use the same zone-numbering system Agricultural Research Service, USDA. It con- as the larger, more general maps. A good tains uniform zones of 10 Fahrenheit degrees, example is a hardiness-zone map of the state and sub-zones of 5 Fahrenheit degrees. Since of Vermont (Hopp and Lautzenheiser, 1966). the Arnold Arboretum map uses zones of dif- This map uses the same zone-numbering ferent ranges (5, 10, or 15 Fahrenheit degrees), system as the USDA map, but is based upon discrepancies between the two are inevita- a larger number of weather stations in Ver-

Figure 1. The U S. Department of Agnculture’s plant hardmess zones m the contermmous Umted States and southern Canada. 34

mont, so zone lines have been drawn in more Relationship Between the Arnold Arboretum and detail than in the larger map. As more areas USDA Hardiness-Zone Numbering Systems are mapped in greater detail in this way, hardiness-zone maps will become increasingly useful.

Canadian Plant Hardiness Map The Canadian Plant Hardiness Map, released in 1967 by the Canada Department of Agri- culture, covers all but the far-northern parts of Canada. This map represented a new approach in that it was an attempt to describe hardiness zones in terms of the whole com- plex of environmental factors that contrib- ute to severity of climate, rather than in terms of a single factor such as average annual minimum temperature. To as great an extent as possible, actual observations of plant adaptability played a part in describing hardiness zones. Direct comparisons between this map and those prepared in the United States are not valid, because of the different criteria used in describing hardiness zones.

The Future

It appears that the business of preparing hardiness-zone maps and assigning plants to their proper zones is still in the experimental stage. Hopefully, we may eventually see wide adoption of a single hardiness-zone map for the United States, for North America, or Reference even for the northern hemisphere. If this is someday accomplished, the problem of Hopp, R.J., and R.E. Lautzenheiser, 1966. Extreme Wm- assigning realistic zone numbers to specific ter Temperatures m Vermont. Umversity of Ver- plants will still remain. The ability to match mont Agricultural Experiment Station Bulletin 648, 19 pages. plants with zones over wide regions will be the ultimate test of any map, and many more careful observations will to have be made Donald Wyman was horticulturist for the Arnold Arbor- before this can be done with most of our etum from 1936 to 1970, during which period he was present trees and shrubs. Meanwhile, the editor of Amoldia. His Gardemng Encyclopedia is a standard reference for and horticulturists. existing maps will continue to be useful. But gardeners Harnson L. Flint, a former staff member of the Arnold to use them most we must recog- effectively Arboretum, is professor of horticulture at Purdue Um- nize their differences and use zone published versity. Among his best-known publications is Land- references only with the right map. scape Plants for Eastern North Amenca ~Wiley, 1983).