The USDA Hardiness Zone Map divides North America into 11 separate zones; each zone is 10°F warmer (or colder) in an average winter than the adjacent zone. If you see a hardiness zone in a catalog or plant description, chances are it refers to the USDA map.
What are Zone Maps?Gardeners need a way to compare their garden climates with the climate where a plant is known to grow well. That's why climate zone maps were created. Zone maps are tools that show where various permanent landscape plants can adapt. If you want a shrub, perennial, or tree to survive and grow year after year, the plant must tolerate year-round conditions in your area, such as the lowest and highest temperatures and the amount and distribution of rainfall.
Great for the EastThe USDA map does a fine job of delineating the garden climates of the eastern half of North America. That area is comparatively flat, so mapping is mostly a matter of drawing lines approximately parallel to the Gulf Coast every 120 miles or so as you move north. The lines tilt northeast as they approach the Eastern Seaboard. They also demarcate the special climates formed by the Great Lakes and by the Appalachian mountain ranges.
Zone Map DrawbacksBut this map has shortcomings. In the eastern half of the country, the USDA map doesn't account for the beneficial effect of a snow cover over perennial plants, the regularity or absence of freeze-thaw cycles, or soil drainage during cold periods. And in the rest of the country (west of the 100th meridian, which runs roughly through the middle of North and South Dakota and down through Texas west of Laredo), the USDA map fails.