Hardiness Ratings and the USDA Zone Map
In the U.S., most garden-worthy trees, shrubs, perennials, bulbs and other plants have been tested for their cold tolerance or “hardiness” and are rated using a numbering system developed by the U.S. Department of Agriculture. The USDA Hardiness Zone Map divides the country into 13 climatic zones. These range from zone 1, where the lowest average winter temperature is -50°F, to zone 13, where winter temperatures rarely dip below 65°F.
If you garden in zone 6, for example, and purchase a plant that is rated as hardy in zone 6, you can be quite confident the plant will survive winter in your yard without any special attention.
Find Your Location and Climate Zone