USDA climate zones, or precise USDA Plant Hardiness Zones, is a climate classification of areas based on the average lowest annual temperature that the US Department of Agriculture (USDA) has published.
The zones range from 1 (from −60 ° F, approx. –51.1 ° C) for polar regions to 13 (up to 70 ° F, approx. 21 ° C) for the tropics, in steps of 10 ° F ( ≈5.5 ° C). The zones can be divided into half steps a and b each comprising 5 ° F (≈ 2.8 ° C).
They are an international standard for assessing the hardiness of plants. Furthermore, there is a list of indicator plants, which is divided into zones in which these plants can just survive.
The map for the United States was first published in 1990. It was based on an averaging period from 1974 to 1986. In order to do justice to global warming, this map was newly created in 2012 and related to the 30-year period 1976–2005. The individual zones shifted to the north in the range of a few hundred kilometers; in one place, the zoning changed on average by about a 5 ° F half zone.