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.
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
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
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.
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.
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…
Is there a READABLE heat map in an 8.5×11 size that my garden club could hand out to new members?
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!
Hi! I’m new in this area, Nevada, MO, I spent many years in the California area. I want to plant flowers, shrubs, all over the back ,front yard and also on both sides of our home. I think I have the amount of rain & sun in the places I’m planting. How ever, the Nevada, MO area is expecting freezing
for several days, How can I keep my plants & seeds from freezing .
Lastly, Is it true that white, distilled vineger kills weeds. If not what is the best natural weed killer.
I live near Newcastle in Australia which is 150 kilometres north of Sydney.
We have the same climate zone as Miami which is 11a which just means that we rarely see an overnight low of below 4°C during the winter time,the Koppen Geiger climate at Miami is Am which is tropical monsoonal whereas we have a Cfa which is humid subtropical in our area.
So far,in late Autumn,we’ve seen an overnight low of 5°C ,one of the 4 jackfruit seedlings that I’m growing,its leaves died,I think it was the weaker one but the other 3 are still alive,I’ve repotted them into bigger pots & placed them out in the sunlight a bit more.
My papaya & mango plants are growing well since the daytime temperature has been at 20-21°C on the main,a more established mango tree will do very well during the summer time when it can get up to 44°C during the day or on the rare occasion 47°C,the mango trees love the heat !
Miamis climate I believe is more like what Darwins summer climate is which is wet & humid, Darwin is different in the way that it dry during the winter & wet during the summer,there is no difference between the winter & summer temperatures meaning there are only 2 seasons – dry & wet !
In Miami, they still have lower winter overnight minimum temperatures than Darwin !
Hello,
i was reading a lot about this topic and actually your article totally stands out and is different compared to 99% of all other content about hardiness.
It is very rare someone talking about AHS Zones within this topic.
i have been working on a tool to research hardiness data:
https://www.asklepios-seeds.de/gb/usda-plant-hardiness-testing-tool
maybe you can have a look at it and if you like it, then please feel free to share it.
I am implementing new features from time to time and AHS data will follow too
best regards
Steffen
Could you please have more pop-ups on your website?
I love how they distract me from all of that content you worked to create.
Oh and the cookie reminder and the email sign up…. great ways to annoy the hell out of someone.
Dumb move – don’t make it hard for people to enjoy your work.
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