What Are Hardiness Zones?
Gardeners need a way to compare their garden climates with the climate where a plant is known to grow well. That's why the hardiness zone system was created. Zones are used to indicate 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.
The familiar Zone map on the back of many seed packet is visual representation of the system. The Zone Finder tool, located on the left-hand side of most pages in the BHG.com Garden channel, is a more accurate way to determine which Zone your garden is in. Both the seed packet maps and the Zone Finder are based on the USDA Hardiness Zone Map, which was originated by the U.S. Department of Agriculture and overseen by the National Arboretum.
The USDA Hardiness Zone Map is one of several maps developed to provide this critical climate information. The USDA map is the one most gardeners in the eastern United States rely on, and the one that most national garden magazines, catalogs, books, and many nurseries currently use. This map divides North America into 11 separate zones. Each zone is 10 degrees F warmer (or colder) in an average winter than the adjacent zone. (In some versions of the map, each zone is further divided into "a" and "b" regions.)
The 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.
Although a good guide for many gardeners, the USDA map is not perfect. 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.
Problems in the West
Many factors beside winter lows, such as elevation and precipitation, determine growing climates in the West. Weather comes in from the Pacific Ocean and gradually becomes less marine (humid) and more continental (drier) as it moves over and around mountain range after mountain range. While cities in similar zones in the East can have similar climates and grow similar plants, in the West it varies greatly. For example, the weather and plants in low-elevation, coastal Seattle are much different from those in high-elevation, inland Tucson, Arizona, even though they're in the same Zone (USDA Zone 8).