Geography

• USDA plant hardiness zones run in bands across the country. The coldest, Zone 2, touches parts of Minnesota, and has annual average lows of minus 45 to minus 40 degrees Fahrenheit. Heading south, zone numbers get higher and warmer. The warmest is Zone 10, which includes parts of Florida and Texas, with annual average lows of 35 to 40 F.
  
  

Significance

• Each zone tells the average annual low that each area can expect. For example, areas such as Kentucky, northern Oklahoma, and parts of Nevada are in Zone 6, which has average lows between minus 10 and 0 degrees Fahrenheit.
  
  

Function

• Most plants and trees have a USDA plant hardiness zone rating that tells for which climate range they are best suited. Some plants cannot handle freezing northern temperatures, while others will wilt in the sun of the south. Japanese holly cannot tolerate extremes, and is therefore rated for Zones 5 though 8.
  
  

History

• The current USDA Plant Hardiness Zone Map was created in 1990, and replaced a 1965 version. It is based on data collected from areas across North America between 1974 and 1986. Changes were made to the map that reflect additional environmental stresses, pollution, and climate change. These changes are always in process, and always being measured.