First developed by the United States Department of Agriculture (USDA) as a rough guide to landscaping and gardening, a hardiness zone is a geographically defined area in which a plant is capable of growing and withstanding the minimum temperatures of the zone. The map is based on the average annual minimum winter temperature, divided into 5-degree F zones. Keep in mind that local variations such as moisture, soil, winds, and other conditions might affect the viability of individual plants.
