USDA Zone Maps
Climate zone maps 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.
Gardeners need a way to compare their garden climates with the climate where a plant is known to grow well. Zone maps provide this critical climate information.
The 1990 USDA Hardiness Zone Map
The USDA Hardiness Zone Map (above) divides North America into 11 separate zones. Each zone is 10F 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 is the one most gardeners in the eastern United States rely on, and the one that National Gardening, most national garden magazines, catalogs, books, and many nurseries currently use.
Great for the East
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.
Zone Map Drawbacks
But 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.
Problems in the West
Many factors beside winter lows, such as elevation and precipitation, determine western 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 than in high elevation, inland Tucson, Arizona, even though they're in the same zone USDA zone 8.