USDA Hardiness Zones
Based on the minimum ten-year average winter temperatures, plant hardiness zone maps have been progressively developed, first by the Department of Agriculture (USDA) for the United States and then more or less applied to the rest of the planet. The purpose of these hardiness zones is to identify how well plants will withstand the cold winter temperatures of these zones.
- The USDA Hardiness Zone Map divides North America into 13 zones of 10°F each, ranging from -60°F (-51°C) to 70°F (21°C). If you are planning to buy a shrub, perennial or tree, you need to make sure that this new plant will tolerate year-round conditions in your area. Therefore, to ensure your new acquisition will survive and grow year after year, you will need to compare the hardiness zone of your area with the hardiness zone of your plant (included in most American reference books, nursery catalogs and gardening magazines describing plants).
- Keep in mind, however, that this USDA map is mostly a guideline. While the USDA map reflects pretty well the garden climates of the eastern half of North America, it does not factor in any elevation or precipitation which have quite an impact in western climates. It does not include micro climates, humidity, summer heat tolerance either.