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About Hardiness Zones

Below is a link to the 2003 US National Arboretum "Web Version" of the USDA Plant Hardiness Zone Map and some information about the map and its usefulness.

This map 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.

Go To the Map

Find Plant Examples listed by Zone

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.

Go To the Map

Find Plant Examples listed by Zone

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.