Features

• In 1960, the U.S. Department of Agriculture created a map dividing North American into 10 zones based on the average minimum temperatures for each area. Zones occur in 10-degree increments with high-number zones occurring in warm southern regions. A 1990 revision introduced Zone 11, with an average minimum temperature of 40 degrees F, as well as "a" and "b" designations that represent 5-degree differences in minimum temperature.
  
  

Using the Map

• Use the USDA Plant Hardiness Zone Map by finding your location on the map and noting the zone number. Nurseries, seed catalogs and other gardening resources label plants according to the hardiness zones they can tolerate. If a plant falls into your hardiness zone, it can survive the low temperatures in your area.
  
  

Considerations

• Areas with significant elevation changes can change hardiness zones every few miles, according to the Utah State University Extension. Check with your local extension office for detailed hardiness zone maps for your area and use your own observations of the weather to attenuate your judgments. The U.S. National Arboretum reminds gardeners that factors such as wind exposure can affect a plant's ability to withstand a particular climate.