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. 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’s Heat Tolerance Map.
The USDA Plant Hardiness Zone Map divides North America into 11 zones based on average annual minimum winter temperatures. Each zone represents a 10°F temperature difference, further subdivided into “a” and “b” for 5°F variations. To use this map, simply locate your area and identify your zone number.
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 Zone 7 and warmer.
Once you know the cold, hardiness zone you live in, you can choose plants that will survive the winter in your area.
Finding Your Hardiness Zone
Use the below chart and the color coding on the map to determine your hardiness zone.
USDA Hardiness Zone Map
The map below shows the zone ranges in color codes. The blue and purple regions (zone 1 and zone 2) have the longest periods of cold temperatures. On the other hand, the lighter blue and green zones (zone 5 and zone 6) make up most of the Midwest, which has more heat days per year and a longer growing season.
| Zone | Temperature | Example Cities |
|---|---|---|
| 1 | Below -50 F | Fairbanks, Alaska; 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 can be found at USDA Plant Hardiness Zone Map. You can download the map in various file formats and sizes at USDA Plant Hardiness Zone Map Downloads.
Finding Your Heat Zone
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 has published a map defining 12 zones based on summer temperatures. The zones are defined based on how many days the temperature typically goes above 86º F.
You can find your zone by consulting the AHS Plant Heat Zone Map
How Do I Know Which Plants Are Right for My Hardiness and Heat Zone?
To determine which plants are suitable for your hardiness zone, start by checking plant labels or descriptions for zone recommendations. 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.
However, if the plants you’re considering don’t have a label, consult with a local grower or garden center. They’ll be able to tell you the best new plants for your climate zone and soil type.
These maps are an invaluable tool for gardeners, especially when creating a planting schedule, 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, can injure or kill growing plants.
- Overall, plant care and health 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.
Final Thoughts
The USDA Plant Hardiness Zone Map and AHS Heat Zone Map are useful tools to help you select plants that can withstand your local winter and summer temperatures, respectively. By choosing plants suited to your zone and considering other environmental factors, you’ll increase your chances of gardening success.
Frequently Asked Questions
What if a plant is borderline hardy in my zone?
You can try planting it in a protected area or use winter protection methods like mulching or covering. However, it still may not survive severe winter weather.
Can I grow plants from warmer zones indoors?
Yes, you can grow many plants from warmer zones as houseplants or in containers that you can move indoors during cold weather.
Do hardiness zones change over time?
Yes, zones can shift due to climate change. The USDA catalogs the average temperatures and periodically updates the map to reflect these changes.
