USDA Hardiness Map (cont.)

USDA Hardiness Zone Map is an interactive hardiness map, courtesy of the United States National Arboretum.

This map supersedes 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.

How to Use the New Map
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.

    Additional Helpful Information
    1.  Basic Plant Requirements

    All agriculturists, environmentalists, horticulturists, and home gardeners have one all-abiding question about any plant they wish to introduce into their growing spaces.  Will it flourish?

    The ability to predict whether a newly collected species or cultivar can be successfully grown in a location is fundamental to the continued productivity of America's agriculture and to the survival of our landscapes.  The plants in our urban spaces, farms, fields, and forests consist of native vegetation and exotic plants introduced into our land from all over the world.

    All plants must be placed in an environment that meets their basic requirements:

        Day length.  Day length is usually the most critical factor in regulating vegetative growth, flower initiation and development, and the induction of dormancy.  Plants survive only when the day length promotes their growth and prepares them for the seasonal changes.

        Radiation.  Most plants respond to radiation in the 270- to 3000-nm region.  Cloudy, rainy days coupled with the shade provided by nearby plants and structures can significantly reduce the amount of radiation available.  Plants survive only where the amount is within a specified range.

        Temperature.  Plants grow best within an optimum range of temperatures; and the range may be wide for some species, narrow for others.  Plants survive only where temperatures allow them to metabolize.

        Frost.  Plants differ in ability to survive frost, their responses varying from immediate death to sustained performance.  The previous environmental and cultural conditions of plants can often shift, but not permanently alter, their tolerance to freezing.  Plants survive only when they are adapted to subfreezing weather.

        Heat.  The thermal cutoff temperature varies widely from species to species.  By tradition we group plants into sun, partial sun, and shade types and plant them according to their light and heat tolerances.

        Rainfall.  Gardeners need to know how much water a landscape plant requires in determining its usability in low maintenance landscapes.  Rainfall gardening often greatly limits which species can be used successfully.  Gardeners also need to know how much and how often to water plants in high maintenance landscapes.

        pH.  The ability of plant roots to take up water and nutrients depends on the pH (measure of acidity or alkalinity), presence of soluble and insoluble salts, and aeration of the growing medium.  The successful culture of all plant species requires that they be grown in a medium within a definite pH range and with from 10 to 14 essential nutrients in appropriate balance.  Although plants may tolerate some extraneous elements and compounds, every plant species and cultivar has well prescribed limits.

    2.  How the Map Was Started

    Every plant can adapt to a range of environments.  Gardeners have learned through experience where the great variety of landscape plants can be grown.  Over the years many schemes have been proposed to help gardeners locate those environments when they introduce new species, forms, and cultivars.  The pooling of many of these schemes culminated in the development of the widely used "Plant Hardiness Zone Map," under the supervision of Henry T. Skinner, the second director of the U.S. National Arboretum.  In cooperation with the American Horticultural Society, he worked with horticultural scientists throughout the United States to incorporate pertinent horticultural and meteorological information into the map.

    The elements of that map were-

         Zones.  The contiguous United States and southern Canada were divided into 10 zones based on a 10 F (5.6 C) difference in average annual minimum temperature.

         Winter Hardiness.  Survival of landscape plants over winter was selected as the most critical criterion in their adaptation to the environment.

         Classification.  The zone ratings were intended to indicate excellent adaptability of the plants.   Many plants may survive in warmer or colder zones.  Usually, mere survival does not represent satisfactory performance.

         Interactions With Other Environmental Factors.  Many other factors may come into play in determining satisfactory growth.  Wind, soil type, soil moisture, humidity, snow, and winter sunshine may greatly affect the adaptability of plants.

         Interactions With Cultural Factors.  The way plants are placed in the landscape, how they are planted, and their size and health can greatly influence satisfactory adaptability.

    3.  Why the New Map was Created

    The "Plant Hardiness Zone Map" was published in 1960 and revised in 1965.  Since then, many changes, new interests, and new responsibilities have emerged in North American landscaping:

        Changes in Weather.  We have been losing from our landscapes plants that apparently survived the 1940's to the 1960's.  Many of the hardiness zone classifications of plants are no longer considered valid.  In North America, the ranges of temperature and moisture for the past decade were wider than those recorded for the 1940's through the 1960's.

        Introduction of New Elite Forms.  Our landscape industry has worked with plant explorers and breeders to introduce many new forms of traditional plants that are adapted to a wider range of environments than the older forms.

        Scope.  The continental United States is contiguous with Canada to the north and Mexico to the south.  We share more than a common border-we share indigenous plants and many introduced plants that can be grown successfully.  We needed to expand the scope to include the whole of North America.

    4.  How the New Map Was Created

    The average annual minimum temperature data were analyzed for Mexico, the United States, and Canada.  Of 14,500 stations that measured temperature during the period of interest, almost 8,000 could be identified by latitude and longitude and by a valid average annual minimum temperature (i.e., an average based on at least 10 years of data).  Data from only the latter stations were used in the map.  The data were archived by Servicio Meteorologico Nacional (Tucubaya, D.F., Mexico), the National Climatic Data Center (Asheville, NC), and Environment Canada - Canadian Climate Centre (Downsville, Ontario).  Temperature data were compiled and maps prepared under contract with the Meteorological Evaluation Services Co., Inc., 165 Broadway, Amityville, New York 11701.   The map is an Albers Equal Area Projection.  Standard parallels of 29.5 , and 45.5 , were used to generate the map of the three countries.  The map was computer generated by latitude and longitude.  Because of the large area involved, it is not possible to draw one map that is accurate for all of North America.  The part representing the United States has the least distortion.  The Agricultural Research Service proposes to periodically evaluate weather data and issue updated maps as necessary and appropriate.