The United States Department of Agriculture (USDA) in cooperation with the National Weather Service (NWS) has identified various regions within North America by their 60-year average minimum winter temperature. These regions, called "hardiness zones," are represented by bands across the continent.
When we say a plant is rated to Zone 6, we mean it will generally not survive the winter below -10F (-23C). Sometimes, however, you'll read where a plant is rated between two zones (e.g., Zone 5 to 9). By this we mean the plant can survive to -20F (-29C) in winter, but the summer temperatures found in Zone 9 will be too warm for the plant to survive. That is, the plant will consume more energy trying to survive than it can create with its normal metabolic processes. The upper limit temperatures are not represented.
Sometimes you'll see a further refinement of the 10F divisions. That is, the divisions are in 5 increments such as Zone 7a or 7b, for example. When associated with the same numbered zone, the "b increment" is always the warmer of the two. Therefore, Zone 7a means the average lowest winter temperature is 0 to 5F (-18 to -15C), and Zone 7b is 5 to 10F (-15 to -12C).
The Dallas-Fort Worth Metroplex in Texas is divided. Dallas is in Zone 8a, while Fort Worth is in Zone 7b. In this case, Zone 8a has an average warmer temperature than Zone 7b.
In 1990, the USDA remapped and revised its chart for the continent.
The charts below are provided for your reference. They are provided as a guide to gardeners, and do not indicate individual microenvironments which may occur in your neighborhood.
Hardiness Zone Table
Plant Hardiness Zones
U.S. Department of Agriculture
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(Map from USDA, Agricultural Research Service)