Hardiness Zone Maps
Cold hardiness zone ratings are based on the average minimum winter temperature in an area. The corresponding zone maps are tools that show where various landscape plants can survive based on temperature. For best results select plants rated hardy to yours or colder zones.
Plant Hardiness Zones of CanadaThose gardening in Canada should refer to the Plant Hardiness Zone Map created by the Canadian Forest Service and Agriculture and Agri-Food Canada in 2000. The zones are based upon the average climatic conditions of each area and Canadian plant survival data.
Interactive USDA Zone Map
This is truly the coolest USDA zone map on the internet. To use this map, simply click on your state for detailed information.
Using the Map
A map of the U.S. is the beginning key to selecting the hardiness zone to be used in the search. The maps provided include a detailed, color-coded breakdown by hardiness zone for each state.
Zones 2 through 10 have been subdivided into light- and dark- colored sections that represent 5-degree differences WITHIN the 10-degree zone. The lighter color is the colder section; the darker color is the warmer section. These subdivisions are intended to further help the user define the average minimum temperature for his or her area. However, the plant search is conducted on the whole number zone range.
If the precise location of your site borders another hardiness zone, you should search twice: first, on the colder zone (lower number), secondly on the warmer zone (higher number). Because cold tolerance is so important, it is better to select plants which will be MORE cold-tolerant rather than less. For example, for a site in zone 7 which borders zone 6: search first for zone 6 plants, secondly on zone 7 plants.
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.
The 1990 USDA Hardiness Zone Map
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?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.)
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.
Zone map provided by VisonScape