The U.S. Department of Agriculture’s Plant Hardiness Zone Map is the most widely used hardiness zone map. The map has been so useful for gardeners in the United States that the system has been extended to Europe and other regions. The USDA zone map is the one that most gardeners in the United States use, and the one that most garden magazines, catalogs, and books currently use.

The USDA zone map divides North America into zones based on the average annual minimum temperature. Each zone is 10°F (–12°C) warmer (or colder) in an average winter than the adjacent zone. North America encompasses 20 zones, numbered 1 through 11. (Zones 2 through 10 are subdivided into a and b regions where average minimum temperatures differ by 5°F, or 15°C. We don’t include the subzones in this book because these designations exist only for North America.) Western Europe has 10 zones (numbered 1 through 10).

These zone maps link regions that share an average winter minimum temperature. For instance, typical winters in Colorado Springs, Colorado; Albany, New York; and Prague, Czechoslovakia, reach –20°F (–29°C), so each city is in USDA Zone 5. The climate in these distant cities isn’t necessarily the same, and the same plants may not grow well in all three cities. But the average winter minimum temperatures are very similar, which is one of the key factors that determine plant survivability. All the plants in this book bear the code for a USDA hardiness zone. If you live in one of the plant’s recommended zones, you have some assurance that the plant is hardy enough to survive winter.

Unfortunately, no zone map — including the USDA map — is perfect. In the eastern half of North America, 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. Other factors that determine plant survival that this zone map can’t accommodate include the amount and distribution of rainfall (or availability of irrigation water), and soil conditions.

The USDA Plant Hardiness Zone Map

Click here to go to the USDA’s Plant Hardiness Zone Map website.