Gardeners need a way to compare their garden climates with the climate their plant is known to grow well. That's why the hardiness zones were created. USDA hardiness zones are used to indicate 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 familiar plant zone map on the back of many seed packet is visual representation of the system. Seed packet maps are based on the USDA Plant Hardiness Zone Map, which was originated by the U.S. Department of Agriculture and overseen by the National Arboretum.
The USDA Plant Hardiness Zone Map is one of several plant zone maps developed to provide this critical climate information. The USDA plant zone 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 plant hardy zone is 10 degrees 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.)
The USDA zone 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.
Problems with the Zones
Although a good guide for many gardeners, the USDA zone map is not perfect. In the eastern half of the country, the USDA zone 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 zone map fails.
Many factors beside winter lows, such as elevation and precipitation, determine 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 gardening 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 from those in high-elevation, inland Tucson, Arizona, even though they're in the same Zone (USDA Zone 8).
Each USDA Zone in the system represents a region of minimum average winter temperatures. The lower the USDA Zone number, the colder the region. Although factors other than temperature affect the ability of a plant to survive, the USDA Zone system is a reasonable starting point for many gardeners.
The chart below shows the temperature ranges associated with the Zone system. In this chart, the USDA garden Zones are divided into A and B regions, which are sometimes used to fine-tune plant recommendations.
Zone Minimum Temperature Example Cities 1 Below -50 F Fairbanks, Alaska; Resolute, Northwest Territories (Canada) 2a -50 to -45 F Prudhoe Bay, Alaska; Flin Flon, Manitoba (Canada) 2b -45 to -40 F Unalakleet, Alaska; Pinecreek, Minnesota 3a -40 to -35 F International Falls, Minnesota; St. Michael, Alaska 3b -35 to -30 F Tomahawk, Wisconsin; Sidney, Montana 4a -30 to -25 F Minneapolis/St.Paul, Minnesota; Lewistown, Montana 4b -25 to -20 F Northwood, Iowa; Nebraska 5a -20 to -15 F Des Moines, Iowa; Illinois 5b -15 to -10 F Columbia, Missouri; Mansfield, Pennsylvania 6a -10 to -5 F St. Louis, Missouri; Lebanon, Pennsylvania 6b -5 to 0 F McMinnville, Tennessee; Branson, Missouri 7a 0 to 5 F Oklahoma City, Oklahoma; South Boston, Virginia 7b 5 to 10 F Little Rock, Arkansas; Griffin, Georgia 8a 10 to 15 F Tifton, Georgia; Dallas, Texas 8b 15 to 20 F Austin, Texas; Gainesville, Florida 9a 20 to 25 F Houston, Texas; St. Augustine, Florida 9b 25 to 30 F Brownsville, Texas; Fort Pierce, Florida 10a 30 to 35 F Naples, Florida; Victorville, California 10b 35 to 40 F Miami, Florida; Coral Gables, Florida 11 above 40 F Honolulu, Hawaii; Mazatlan, Mexico
The plants listed below provide examples of the coldest USDA garden Zones in which specific plants will survive. In this list, only the coldest USDA Zone is considered; some of the plants listed will not thrive in substantially warmer areas. Always check with the source of your plants for information on whether they are well suited to your area.
Zone 1: Below -50 degrees F
- Netleaf willow (Salix reticulata)
- Dwarf birch (Betula glandulosa)
- Crowberry (Empetrum nigrum)
- Quaking aspen (Populus fremuloides)
- Pennsylvania cinquefoil (Potentilla pensylvanica)
- Lapland rhododendron (Rhododendron lapponicum)
Zone 2: -50 to -40 degrees F
Zone 3: -40 to -30 degrees F
Zone 4: -30 to -20 degrees F
Zone 5: -20 to -10 degrees F
Zone 6: -10 to 0 degrees F
Zone 7: 0 to 10 degrees F
Zone 8: 10 to 20 degrees F
- Strawberry tree (Arbutus unedo)
- Mexican orange (Choisya temata)
- New Zealand daisy-bush (Olearia haastii)
- Japanese pittosporum (Pittosporum tobira)
- Cherry-laurel (Prunus laurocerasus)
- Laurustinus (Viburnum tinus)