How to Use Hardiness Zone Information

The Zone you garden in affects what you can grow -- but it's not the last word on the subject.

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What Are Hardiness Zones?

Gardeners need a way to compare their garden climates with the climate where a plant is known to grow well. That's why the hardiness zone system was created. 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 Zone map on the back of many seed packet is visual representation of the system. The Zone Finder tool, located on the left-hand side of most pages in the Garden channel, is a more accurate way to determine which Zone your garden is in. Both the seed packet maps and the Zone Finder are based on the USDA Hardiness Zone Map, which was originated by the U.S. Department of Agriculture and overseen by the National Arboretum.

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 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 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.

Although a good guide for many gardeners, the USDA map is not perfect. 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.

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Problems in the West

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 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 Zone in the system represents a region of minimum average winter temperatures. The lower the Zone number, the colder the region. Although factors other than temperature affect the ability of a plant to survive, the 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 Zones are divided into A and B regions, which are sometimes used to fine-tune plant recommendations.

Zone Average Minimum Winter Temp. 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 Zone in which specific plants will survive. In this list, only the coldest 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

Betula glandulosa (Dwarf birch)
Empetrum nigrum (Crowberry)
Populus fremuloides (Quaking aspen)
Potentilla pensylvanica (Pennsylvania cinquefoil)
Rhododendron lapponicum (Lapland rhododendron)
Salix reticulata (Netleaf willow)

Zone 2: -50 to -40 degrees F

Betula papyrifera (Paper birch)
Cornus canadensis (Bunchberry dogwood)
Elaeagnus commutata (Silverberry)
Larix laricina (Eastern larch)
Potentilla fruticosa (Bush cinquefoil)
Viburnum trilobum (American cranberry bush)

Zone 3: -40 to -30 degrees F

Berberis thunbergii (Japanese bayberry)
Elaeagnus angustifolia (Russian olive)
Junipercus communis (Common juniper)
Lonicera tatarica (Tatarian honeysuckle)
Malus baccata (Siberian crabapple)
Thuia occidentalis (American arborvitae)

Zone 4: -30 to -20 degrees F

Acer saccharum (Sugar maple)
Hydrangea paniculata (Panicle hydrangea)
Juniperus chinensis (Chinese juniper)
Ligustrum amurense (Amur River privet)
Parthenocissus quinquefolia (Virginia creeper)
Spiraea x vanhouttei (Vanhouffe spirea)

Zone 5: -20 to -10 degrees F

Cornus florida (Flowering dogwood)
Deutzia gracilis (Slender deutzia)
Ligustrum vulgare (Common privet)
Parthenocissus tricuspidata (Boston ivy)
Rosa multiflora (Japanese rose)
Taxus cuspidata (Japanese yew)

Zone 6: -10 to 0 degrees F

Acer palmatum (Japanese maple)
Buxus sempervirens (Common boxwood)
Euonymus follunei (Winter creeper)
Hedera helix (English ivy)
Ilex opaca (American holly)
Ligustrum ovalifolium (California privet)

Zone 7: 0 to 10 degrees F

Acer macrophyllum (Bigleaf maple)
Rhododendron Kurume hybrids (Kurume azalea)
Cedrus atlantica (Atlas cedar)
Cotoneaster microphylla (Small-leaf cotoneaster)
Ilex aquifolium (English holly)
Taxus baccata (English yew)

Zone 8: 10 to 20 degrees F

Arbutus unedo (Strawberry tree)
Choisya temata (Mexican orange)
Olearia haastii (New Zealand daisy-bush)
Pittosporum tobira (Japanese pittosporum)
Prunus laurocerasus (Cherry-laurel)
Viburnum tinus (Laurustinus)

Zone 9: 20 to 30 degrees F

Asparagus setaceous (Asparagus fern)
Eucalyptus globulus (Tasmanian blue gum)
Syzygium paniculatum (Australian bush cherry)
Fuchsia hybrids (Fuchsia)
Grevillea robusta (Silk-oak)
Schinus molle (California pepper tree)

Zone 10: 30 to 40 degrees F

Bougainvillea spectabilis (Bougainvillea)
Cassia fistula (Golden shower)
Eucalyptus citriodora Lemon eucalyptus)
Ficus elastica (Rubber plant)
Ensete ventricosum (Ensete)
Roystonea regia (Royal palm)

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