A Guide to USDA Hardiness Zones
A growing zone—or more accurately, a hardiness zone—is an area that is defined geographically according to climate conditions, including minimum temperatures. Gardeners use hardiness zones to know what category of plant is capable of growing in a particular area.
The United States (and all of North America) is divided into 11 zones. Each zone is broadly divided by minimum temperatures in 10 degree increments beginning with Zone 1 with minimum temperatures below -50 degrees Fahrenheit and going to Zone 11 with minimum temperatures over 40 degrees Fahrenheit. More specifically, Zones 2 through 10 are further broken down into a and b (i.e., Zone 2a and Zone 2b), which are defined by five degree differences in minimum temperature.
History of Hardiness Maps
Hardiness maps were first developed in the 1920s and 1930s. Donald Wyman of the Arnold Arboretum published the first map in 1938. It was subsequently updated in 1951, 1967 and 1971. The USDA finally published its first map in 1960 which was then updated in 1965. With two maps in publication came much confusion for gardeners because the maps’ zones did not necessarily coincide although both were developed using weather data. Gardeners could be, for example, in Zone 5 on one map but in Zone 6 on the other. When they purchased plants which were hardy to Zone 5 or Zone 6 they had to know which map was cited in the plant’s information.
In 1990, the USDA issued an updated map using data from 14,500 weather stations spanning a twelve year period. This hardiness zone map has become the official map in North America. Besides updated data, the map added Zone 11 for the inclusion of Mexico on the map, included Canada, and further broke down Zones 2 through 10 into the a and b designations. On the current map, for example, in Zone 5a the minimum temperature range is -20 to -15 degrees Fahrenheit and in Zone 5b the minimum temperature range is -15 to -10 degrees Fahrenheit. On previous maps, the minimum temperature in Zone 5 was -20 to -10 degrees Fahrenheit.
Drawbacks and Problems
Despite the attempt to categorize the country into climate zones and give gardeners easy-to-find, suitable plants for their zones, the hardiness map has several drawbacks and problems. The map works well for the East coast of the US but does not take into consideration the insulating effect of snow cover or drainage. As the map moves west, it does not take into consideration elevation and amount of precipitation. The problem is very evident in places like Seattle, Washington which has a low elevation and high precipitation, and Tucson, Arizona which has a high elevation but low precipitation. Both cities are located in Zone 8 but plants which thrive in Seattle’s climate would not fare well in Tucson’s. Even with these drawbacks, the hardiness zone map is a valuable tool for gardeners.
The Hardiness Zones
Here are the eleven zones with minimum temperatures, example cities, and popular plants hardy to that zone.
Minimum temperatures: below -50 degrees F.
Example cities: Fairbanks, Alaska and Resolute, Northwest Territories (Canada)
Plants: Lapland Rhododendron, Dwarf Birch, Black Crowberry, Quaking Aspen
Minimum temperatures: Zone 2a -50 to -45 degrees F, Zone 2b -45 to -40 degrees F.
Example cities: 2a – Prudhoe Bay, Alaska and Flin Flon, Manitoba. 2b – Unalakleet, Alaska and Pinecreek, Minnesota
Plants: Paper Birch, American Cranberry Bush, Silverberry, King of Hearts, Snowdrop Anemone
Minimum temperatures: Zone 3a -40 to -35 degrees F, Zone 3b -35 to -30 degrees F.
Example cities: 3a – International Falls, Minnesota. 3b – Tomahawk, Wisconsin
Plants: Sugar maple, Panicle Hydrangea, Tatarian Honeysuckle, Virginia Creeper
Minimum temperatures: Zone 4a -30 to -25 degrees F, Zone 4b -25 to -20 degrees F.
Example cities: 4a – Minneapolis/St. Paul, Minnesota. 4b – Nebraska.
Plants: Chinese Juniper, Boston Ivy, Japanese Yew, Aster, Hosta, Sunflower, Violet
Minimum temperatures: Zone 5a -20 to -15 degrees F, Zone 5b -15 to -10 degrees F.
Example cities: 5a – Illinois. 5b – Mansfield, Pennsylvania and Columbia, Missouri.
Plants: Ferns, Roses, Azaleas, Plantain Lillies, Climbing Hydrangea, Ash, Beech, Douglas Fir
Minimum temperatures: Zone 6a -10 to -5 degrees F, Zone 6a -5 to 0 degrees F.
Example cities: 6a – St. Louis, Missouri and Lebanon, Pennsylvania. 6b – McMinnville, Tennessee and Branson, Missouri.
Plants: Most popular plants are hardy to Zone 6.
Minimum temperatures: Zone 7a 0 to 5 degrees F, Zone 7b 5 to 10 degrees F.
Example cities: Oklahoma City, Oklahoma; Little Rock, Arkansas; Griffin, Georgia.
Plants: Most popular plants are hardy in Zone 7.
Minimum temperatures: Zone 8a 10 to 15 degrees F, Zone 8b 15 to 20 degrees F.
Example cities: Tifton, Georgia; Dallas and Austin, Texas; Gainesville, Florida; Seattle, Washington; Tucson, Arizona.
Plants: Lillies, Roses, Azaleas, Ferns, Irises.
Minimum temperatures: Zone 9a 20 to 25 degrees F, Zone 9b 25 to 30 degrees F.
Example cities: St. Augustine, Florida and Houston, Texas.
Plants: Canna, Chrysanthemum, Iris, Lantana, Lily, Jasmine, Gardenia.
Minimum temperatures: Zone 10a 30 to 35 degrees F, Zone 10b 35 to 40 degrees F.
Example cities: Miami, Florida and Victorville, California.
Plants: Caladium, Elephant’s Ear, Hibiscus, Lily, Iris, Liriope, Sage.
Minimum temperatures: Over 40 degrees F.
Example cities: Honolulu, Hawaii and Mazatlan, Mexico.
Plants: King Palm, Mask Flower, Coffee, Mango, Chinese Hibiscus, Weeping Chinese Banyan.
© 2010 Cristina Vanthul