Last Updated November 18, 2023
The United States and Canada each use plant hardiness zones that are similarly labelled with numbers and letters, but the two systems are not the same and they cannot be used interchangeably. The Canadian plant hardiness zones, which range from 0a to 9a, are determined by a variety of climatic conditions, including rainfall, frost-free periods, maximum snow depth, and more. The American zones published by the United States Department of Agriculture (USDA) range from 0a to 13b and are based on the average lowest temperatures experienced in a region each year.
This post will answer commonly asked questions about the two plant hardiness zone systems and share a detailed comparison with examples for specific locations.
What are plant hardiness zones?
Plant hardiness zones are geographic areas that are divided based on their climate conditions for the purpose of easily identifying which perennial plants will thrive there. For example, an apple tree that is hardy to zone 3a can be expected to survive an average winter in zones as low as (but also higher than) 3a.
In regions where heat is the limiting factor rather than cold, heat zone maps are a useful alternate tool.
Canada’s Plant Hardiness Zones
Canada’s plant hardiness map is maintained by National Resources Canada. First developed in the 1960s for trees and shrubs, it has been updated several times and now combines a wide range of climatic information for a sophisticated measure of climate suitability for plants in general. The seven variables used to calculate a Canadian plant hardiness zone are:
- For the coldest month of the year, the monthly average of the daily minimum temperatures
- For the warmest month of the year, the monthly average of the daily maximum temperatures
- The average frost-free period (that is, the number of days for which temperatures stay above 0°C)
- The amount of rainfall from June to November
- “Winter factor,” a calculation concerned with temperature and precipitation in January
- Maximum snow depth
- Maximum wind gust in 30 years
Plant Hardiness Zones in the United States
The USDA system of plant hardiness zones is based on extreme minimum temperature. That is, it is based on the lowest temperature each region experiences in an average winter. The straightforward scale climbs by 10°F per number, with the letters a and b representing 5° subdivisions within that increment. For example, the coldest winter temperatures experienced in an average year in zone 4a range from -30°F to -25°F, whereas in zone 4b they range from -25°F to -20°F.
What USDA zones are in Canada?
Since the USDA’s system is so simple, it can be easily extrapolated for other countries. Canada’s government has done so, which simplifies conversion for Canadians dealing with resources from the United States.
I’ve taken data for the following chart of temperature conversions from the USDA and Natural Resources Canada websites respectively. In order to show at a glance where the two countries have overlapping climates, I’ve left blank rows in the Celcius (Canada) or Fahrenheit (United States) column for zones that each government did not deem useful to include for its citizens.
Canadian and USDA zones cannot be meaningfully compared except by specific locations, so I have included examples of Canadian locations. Note that two Canadian cities with the same USDA zone might have different Canadian zones based on other climate factors. I have illustrated that by showing two different locations wherever I could confidently identify them using the maps Natural Resources Canada has made available.
| USDA Zone | Extreme min. temp., °F | Extreme min. Temp., °C | Sample Canadian locations in that USDA zone | Location’s Canadian zone |
| 0 | NA | <-51.1 | extreme north, inland | 0a |
| 1a | -60 to -55 | -51.1 to -48.3 | Alert, Nunavut | 0a |
| 1b | -55 to -50 | -48.3 to -45.5 | Gjoa Haven, Nunavut | 0a |
| 2a | -50 to -45 | -45.5 to -42.8 | Whitehorse, Yukon Cambridge Bay, Nunavut | 0b 0a |
| 2b | -45 to -40 | -42.7 to -40.0 | La Loche, Saskatchewan Hay River, Northwest Territories | 2b 1a |
| 3a | -40 to -35 | -39.9 to -37.3 | Fort McMurray, Alberta Kuujjuaq, Quebec | 3a 0a |
| 3b | -35 to -30 | -37.2 to -34.5 | Edmonton, Alberta Winnipeg, Manitoba | 3b 3b |
| 4a | -30 to -25 | -34.4 to -31.7 | Prince George, British Columbia Calgary, Alberta | 4b 4a |
| 4b | -25 to -20 | -31.6 to -28.9 | Fredericton, New Brunswick Ottawa, Ontario | 5a 5b |
| 5a | -20 to -15 | -28.8 to -26.2 | Corner Brook, Newfoundland Barrie, Ontario | 5a 5b |
| 5b | -15 to -10 | -26.1 to -23.4 | Toronto, Ontario Charlottetown, Prince Edward Island | 6a 5b |
| 6a | -10 to -5 | -23.3 to -20.6 | Halifax, Nova Scotia London, Ontario | 6b 6a |
| 6b | -5 to 0 | -23.3 to -20.6 | Yarmouth, Nova Scotia Windsor, Ontario | 6b 7a |
| 7a | 0 to 5 | -17.7 to -15.0 | Kelowna, British Columbia | 6a |
| 7b | 5 to 10 | -14.9 to -12.3 | Penticton, British Columbia | 6a |
| 8a | 10 to 15 | -12.2 to -9.5 | Prince Rupert, British Columbia | 8a |
| 8b | 15 to 20 | -9.4 to -6.7 | Vancouver, British Colunbia | 8b |
| 9a | 20 to 25 | -6.6 to -3.9 | Victoria, British Columbia | 9a |
| 9b | 25 to 30 | NA | NA | |
| 10a | 30 to 35 | NA | NA | |
| 10b | 35 to 40 | NA | NA | |
| 11a | 40 to 45 | NA | NA | |
| 11b | 45 to 50 | NA | NA | |
| 12a | 50 to 55 | NA | NA | |
| 12b | 55 to 60 | NA | NA | |
| 13a | 60 to 65 | NA | NA | |
| 13 b | 65 to 70 | NA | NA |
Which is colder, a or b?
In both plant hardiness scales, a is colder than b. Since the numbers begin at 0 for the coldest zone and increase from there, 5a is colder than 5b. In the US system, which uses a simple scale based on ten degrees Fahrenheit per number, the letters each represent five degrees of difference.
How do I use plant hardiness zones?
You can often find out your zone by typing your location into Google with the words “plant hardiness zone.” If not, you can visit your country’s official map at National Resources Canada or the USDA.
When plant companies sell perennial or biennial plants—anything that needs to survive at least one winter—they include information about what zone that plant is hardy to. If you are in zone 5b, you can grow plants that are hardy to zone 5b, or to any number smaller than that. That’s because a plant that can survive a zone 2a winter, for example, can definitely survive yours in warmer zone 5b.
Often, growers in cold climates grow plants as annuals (plants that only live one year) when they are not hardy enough to survive the winter. As long as the plant matures enough for you to enjoy it within one growing season, you can still plant it for the summer and either move it indoors for winter or let it die and replace it the next year.
Sources
National Resources Canada. “Plant Hardiness of Canada.” Accessed on January 30, 2022 at http://planthardiness.gc.ca/
United States Department of Agriculture. “USDA Plant Hardiness Zone Map.” Accessed on January 30, 2022 at https://planthardiness.ars.usda.gov/
Updated on November 18, 2023 when a revised map was released with zones shifted due to climate change
Plantmaps. “Ontario Plant Hardiness Zone.” Accessed on January 30, 2022 at https://www.plantmaps.com/interactive-ontario-plant-zone-hardiness-map.php