The USDA divides the United States into 13 planting zones based on the 10-degree Fahrenheit differences in the average annual minimum temperature. It publishes guidelines to help gardeners determine which plants grow best in their regions. A zone with a higher number indicates warmer temperatures in that area. Most of the United States is located in zones four through eight. Alaska is in zones one and two, and Hawaii is in zones 10 through 12.Continue Reading
The USDA maintains a map, last updated in 2012, on its website that shows the planting zones and the average winter temperature for each region. Users can find their planting zones either by state or ZIP code and can print images of the entire map or of their state or region.
The USDA intends for the growing zones to be used only as a guide since they only measure an area's extreme winter temperatures. A plant's ability to grow is also affected by factors such as an area's micro climates, summer heat levels and soil moisture, so the USDA supplements the planting zone guide with a list of common plants, called indicator plants, with known limits to their ranges.
Sunset Books supplements the USDA growing zone information in a series of books that incorporates temperature ranges for all seasons, precipitation, wind patterns and elevation, and it breaks the growing zones into 45 smaller areas.Learn more about Botany