The U.S. Department of Agriculture, Agricultural Research Service's hardiness zones are temperature zones that aid growers in deciding which kinds of plants survive the winter in certain locations. The department determines the zones based off of minimum average annual winter temperatures across the United States. It divides these into 13 zones between -60 and 70 degrees Fahrenheit, each representing an interval of 10 degrees. As of September 2015, the USDA calculated hardiness zones using data from 1976 to 2005.
The USDA cautions growers that hardiness zones are broad guidelines, and that many factors may affect the success of plants grown in a particular area. While quite precise, the hardiness zones cannot account for microclimates, which are small, localized climate variations. For example, an area next to a lot of blacktop may be warmer or an area shaded by a hill may be colder than the greater region's hardiness zone indicates. Additionally, localized factors such as soil moisture, air humidity, the duration of cold temperatures and light exposure may temper or exacerbate the effect of hardiness zones.
The American Horticultural Society developed its heat zone map as a complement to the hardiness zone map, to help growers determine which plants best survive the summer in certain locations. The society divides the United States into 12 zones based on the average amount of days per year that a particular region experiences temperatures above 86 degrees Fahrenheit.