Originally created in 1926 and modernized in 1960, the United States Department of Agriculture (USDA) plant hardiness zones is a geographical map that divides the country into growing zones based on temperature. Its purpose is to help gardeners and landscapers determine what plants will thrive in their area of the country. Zones are determined by the average lowest winter temperature and separated by increments of 10 degrees F with Alaska as Zone 1 and Florida as Zones 9 and 10.