A hardiness zone (a subcategory of Vertical Zonation) is a geographically defined area in which a specific category of plant life is capable of growing, as defined by climatic conditions, including its ability to withstand the minimum temperatures of the zone (see the scale on the right or the table below). For example, a plant that is described as "hardy to zone 10" means that the plant can withstand a minimum temperature of -1°C. A more resilient plant that is "hardy to zone 9" can tolerate a minimum temperature of -7°C. First developed for the United States by the Department of Agriculture (USDA), the use of the zones has been adopted by other nations.

The hardiness zones are informative: the extremes of winter cold are a major determinant of whether a plant species can be cultivated outdoors at a particular location; however, the USDA hardiness zones have a number of drawbacks if used without supplementary information.

The zones do not incorporate summer heat levels into the zone determination; thus sites which may have the same mean winter minima, but markedly different summer temperatures, will be accorded the same hardiness zone. An extreme example of this phenomenon is seen when comparing the Shetland Islands and southern Alabama, which are both on the boundary of zones 8 and 9 and share the same winter minima, but little else in their climates. In summer, the humid subtropical climate of Alabama is about 20 degrees Celsius hotter than the Maritime Subarctic climate of Shetland, and there are few similar plants that can be grown at both locations. Due to its maritime climate, the UK is in AHS Heat Zone 2 (having 1 to 8 days hotter than 30 degrees Celsius) according to the AHS (American Horticultural Society), whereas Alabama is in Zones 7 to 9 (61 to 150 days hotter than 30 degrees Celsius). Users need to combine the hardiness zone with the heat zone to gain greater understanding of what can be grown in a particular location.