The USDA Hardiness Zone Map divides North America into 11 separate planting zones; each growing zone is 10°F warmer (or colder) in an average winter than the adjacent zone. If you see a hardiness zone in a gardening catalog or plant description, chances are it refers to this 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.
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. For example, take a look at the map below, you’ll notice that The lower region of Georgia is in Zone 7(yellow). Now looking at the map, head west to Oregon- you’ll notice that part of the state is also Zone 7 yet we know that OR has a less intense summer than GA. With this in mind consider if your GA perennial can survive a winter in Oregon or if your favorite plant in Oregon can tolerate the staunch heat of a GA summer.