USDA Zone Map
Most gardeners are familiar with the U.S. Department of Agriculture's Plant Hardiness Zone Map. First published in 1960, with the current version updated in 2006, this map is based on average annual minimum temperatures recorded throughout North America. By using the map to find the zone in which they live, gardeners are able to determine what plants will "winter over" in their garden because they can withstand these average minimum temperatures.
Although these zones are useful as an indicator of a plant's likelihood for survival in a given area, many factors, including soil type and fertility, soil moisture and drainage, humidity, and exposure to sun and wind determine a plant's growth and success or failure in its enviroment. Today, nearly all American reference books, nursery catalogs, and gardening magazines describe plants using USDA hardiness zones.