All agriculturists, horticulturists and home gardeners have one central question about any plant they wish to grow: Will it flourish? The ability to predict whether a species can be successfully grown in a location affects everything from agricultural productivity to your own backyard garden. Since 1960, the United States Department of Agriculture (USDA) has compiled the range of temperatures across North America and created maps that indicate zones of plant hardiness. Today, the USDA map — updated in 1990 based on weather records from 1974 to 1986 — is the standard measure of plant hardiness throughout most of the U.S.
The USDA divides North America into 11 hardiness zones, each of which represents an area of winter plant hardiness. Zone 1 is the coldest, found in Canada and the far northern U.S.; zone 11 is the warmest, a tropical area that is essentially frost-free and found only in Hawaii and southernmost Florida. The temperatures listed indicate the average annual minimum temperatures — the lowest temperatures that can be expected each year.
How to Use the New Map
This map supercedes U.S. Department of Agriculture Miscellaneous Publication 814, “Plant Hardiness Zone Map,” which was revised in 1965. This 1990 version shows in detail the lowest temperatures that can be expected each year in the United States, Canada, and Mexico. These temperatures are referred to as “average annual minimum temperatures” and are based on the lowest temperatures recorded for each of the years 1974 to 1986 in the United States and Canada and 1971 to 1984 in Mexico. The map shows 10 different zones, each of which represents an area of winter hardiness for the plants of agriculture and our natural landscape. It also introduces zone 11 to represent areas that have average annual minimum temperatures above 40 F (4.4 C) and that are therefore essentially frost free.
Zones 2-10 in the map have been subdivided into light- and dark-colored sections (a and b) that represent 5 F (2.8 C) differences within the 10 F (5.6 C) zone. The light color of each zone represents the colder section; the dark color, the warmer section. Zone 11 represents any area where the average annual minimum temperature is above 40 F (4.4 C). The map shows 20 latitude and longitude lines. Areas above an arbitrary elevation are traditionally considered unsuitable for plant cropping and do not bear appropriate zone designations. There are also island zones that, because of elevation differences, are warmer or cooler than the surrounding areas and are given a different zone designation. Note that many large urban areas carry a warmer zone designation than the surrounding countryside. The map-contains as much detail as possible, considering the vast amount of data on which it is based and its size.
In using the map to select a suitable environment for a landscape plant, today’s gardeners should keep in mind the following:
Stress Factors. We became aware of additional stresses to plants during the 1970’s. Acid rain, gaseous and particulate pollution, security lighting, and toxic wastes, among many other stress factors, have significantly increased the potential for unsatisfactory performance of landscape plants. We need to document the tolerances of plants to these factors.
New Plant Management Systems. New techniques of planting, transplanting, watering, fertilizing, and providing pest control measures have done much to increase the vigor of landscape plants. But used unwisely, these same measures can reduce plant hardiness.
Artificial Environments. We have pushed the use of plants into totally artificial environments such as expressways, malls, elevated decks, and buildings where plant roots are totally removed from the ground and its warming influence. The assortment of plants that can adapt to such environments is proving to be very restricted. Hardiness ratings alone are inadequate to guide landscapers in selecting the most successful plants.