The planting zones in the United States are a set of zones established by the United States Department of Agriculture as "plant hardiness zones." As of 2015, there are 13 main zones, each with two sub-zones. A zone is defined as the area in which a plant is capable of growing, including its ability to handle the minimum temperatures in that zone.
U.S. gardeners and growers use the Plant Hardiness Zone Map as the standard by which they determine which plants can grow in their area. The map is divided into zones of 10 degrees Fahrenheit based on the average minimum temperature in winter. For example, zone 8 includes areas with an average minimum temperature of 10 to 20 degrees Fahrenheit, divided into sub-zones 8a and 8b. This zone is primarily located in the south and parts of the Pacific Northwest. The USDA revised the map in 2012 to reflect the warmer average temperatures that have been observed over the years.
Several factors that may affect the survival of plants are not incorporated into the Plant Hardiness Zone Map. This includes, maximum summer temperatures, amount of snow cover, average amount of daylight hours and soil moisture.
Examples of cities and their corresponding planting zones include Boston in zones 6b and 7a, New Orleans in zone 9b, Chicago in zone 6a and Los Angeles in zone 10b.