There’s nothing more disappointing to a gardener than planting the perfect plant in the perfect spot, only to watch it wither and die during a cold snap or heat wave. While there are many factors that can affect a plant’s health, temperature is pretty decisive – one hard freeze and your petunias are goners. To prevent this, we go to great lengths to try to predict whether or not a plant will survive and thrive in our area.

Thankfully, there are some great resources out there to help, including systems that divide the U.S. into “planting zones,” such as the USDA Plant Hardiness Zone Map and the American Horticultural Society Heat Tolerance Map.

USDA Plant Hardiness Zone Map

If you look at most any plant label, you’ll see a zone designation, such as “Hardy to Zone 7.” These labels refer to the USDA Plant Hardiness Zones, which divide North America into 11 zones based on estimates of the minimum annual temperature. A plant is considered “hardy” if it will survive the winter in that particular zone.

Each zone represents a 10º F. temperature difference and is then further subdivided into “a” and “b” according to 5º differences. Zone 1 is the coldest and is subject to frost year-round while Zone 11 is the warmest and completely frost-free. If a plant is “Hardy to Zone 7,” that means it should survive the winter in zones 7 and warmer.

Once you know the hardiness zone you live in, you can choose plants that will survive the winter in your area.

Finding Your Hardiness Zone

USDA Hardiness Zone Map

Zone	Fahrenheit	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

• An interactive version of this map with much more detailed information is provided by the US National Arboretum.
• Or, you can enter your zip code into the National Gardening Association’s USDA Hardiness Zone Finder.

The first zone map was created by the Arnold Arboretum in 1928 and had eight zones. The USDA Plant Hardiness Zone Map was created in 1960 and revised in 1965 and 1990, with another revision scheduled for 2008.

American Horticultural Society Heat-Zone Map

A plant may survive the winter cold in your area, but what about the summer heat? Using a similar format as the Hardiness Zone Map, the American Horticultural Society published a map in 1997 defining 12 zones based on summer temperatures. The zones are defined based on how many days the temperature typically goes above 86º F.

Finding Your Heat Zone

• You can find your zone by consulting the AHS Heat Zone Map.
• Or, you can enter your zip code in the AHS Heat Zone Finder.

Many plants are now also being labeled with both the USDA Zone and the AHS Zone. If your plant only has one zone label, you can assume is the USDA Hardiness Zone.

These maps are an invaluable tool for gardeners, but keep in mind that they are not set in stone. The maps are based on historical averages and cannot possibly predict the effects of:

• Sudden temperature changes, such as a late frost, that can injure or kill growing plants.
• Overall plant care and health, which can affect a plant’s ability to adapt to and survive tough times.
• “Micro-climates,” which occur in protected areas that may shield plants from cold and rain.
• Winter-long snow cover, which insulates plants and often allows gardeners to grow plants that otherwise wouldn’t be hardy in their zone.
• Other environmental factors, such as plant location, rainfall, sunshine, drainage, soil nutrients, air quality, day vs. night temperatures, elevation, etc.

Nevertheless, knowing your hardiness zone is very important and can save you time and money in the long run by helping you choose the correct plants for your garden. If a plant is not hardy or heat-tolerant in your zone, you may be able to extend its range by bringing it indoors during extreme temperatures.

Further Information

• About the USDA Hardiness Zone Map
• About the AHS Heat Zone Map
• Example of the Arnold Arboretum Hardiness Zone Map

Tags: