Hardiness and Heat Tolerance: Understanding Planting Zones

By: Julie Day

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 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 Heat Zone Map

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

Print


Comments

Please Leave a Comment

7 Comments on “Hardiness and Heat Tolerance: Understanding Planting Zones”

You can follow comments to this article by subscribing to the RSS news feed with your favorite feed reader.

  • Official Comment:


    Ben Erickson Says:
    January 8th, 2016 at 12:48 pm

    Judie,
    I tried adding a printable 8.5×11 of the map, but the text was too small to read. I added a link to the article above where you can download the map in various sizes at http://planthardiness.ars.usda.gov/PHZMWeb/Downloads.aspx

    Hope that helps!



  • Judie Kraemer Says:
    January 8th, 2016 at 11:35 am

    Is there a READABLE heat map in an 8.5×11 size that my garden club could hand out to new members?



  • Kathryn Brill Says:
    October 22nd, 2015 at 4:35 pm

    Kudos for compiling the Heat Zone Map! I see that a poster-sized hard copy can be purchased for $10.00. Is there any way I could access an on-line version? I would be willing to pay for it if necessary…



  • Roberto Diamanti Says:
    October 19th, 2015 at 2:36 pm

    Dear Sir, I live in Dominican Republic and the average temperature for most part of the year is around 27 celcius. I would love to be able to buy seeds of different flowers to start a project and eventually perhaps start my own business. I am ignorant when it comes to determining zones and a few of the seeds I am considering mostly refer to Hardy zone 5-10. Would I have any chance of germinating these and for the plant to survive? I would very much appreciate an answer to this and an advice if you were to provide it.Thank you in advance and give you my regards.



  • Susan Says:
    February 7th, 2015 at 6:39 pm

    Why can’t I enter my zip code to find my heat zone? It works for hardiness. And the heat zone map is too small to see my zone.



  • Rohit Says:
    March 18th, 2013 at 4:01 am

    Hi,I’m doing a analysis on hardiness zone.I have a data which has zip codes of canada.Can you please tell me from where can I get the list of all the zip codes in canada and their corresponding hardiness zone

    Thanks!!

    Regards,
    Rohit



  • Bob Says:
    January 25th, 2012 at 8:00 pm

    Hi Julie,

    I live in zone 9b (new hardiness map just released). Would it be best not to buy a plant rated for zones 3-8?

    Thanks,
    Bob


We want to hear from you! In addition to posting comments on articles and videos, you can also send your comments and questions to us on our contact page or at (800) 946-4420. While we can't answer them all, we may use your question on our Today's Homeowner radio or TV show, or online at todayshomeowner.com.