May 10, 2024
Plant hardiness zone maps are not created equal
BY JAMIE AALBERS
Being in the plant business means we all need to be very familiar with and cognizant of where the plants we grow, install and sell will survive. Plant hardiness zone maps are an important tool when determining what plants can be included in a landscape. But plant hardiness maps in Canada and the U.S. are not created using the same criteria and as such cannot be used interchangeably. Zone 5 in the U.S. is not the same as Zone 5 here in Canada.
The plant hardiness zone map by the United States Department of Agriculture (USDA), which was updated in 2023, relies solely on average annual minimum temperatures to determine its rankings. The plant hardiness zone map developed by Natural Resources Canada, which is currently under revision, considers seven factors averaged over a 30-year time span. These include:
- Monthly mean of the daily minimum temperature of the coldest month.
- Mean frost-free period above zero C in days.
- Amount of rainfall from June to November.
- Monthly mean of the daily maximum temperatures of the warmest month.
- Rainfall in January (important because freezing temperatures following rainfall can be hard on roots).
- Mean maximum snow depth.
- Maximum wind gust in 30 years.
Natural Resources Canada also has a map that shows plant hardiness zones for Canada based on the USDA extreme minimum temperature approach. Both maps can be found at planthardiness.gc.ca.
There is about a one zone difference between the two systems. For example, a plant labelled with the USDA hardiness Zone 5 is a Zone 6 in the Canadian seven factor system. Challenges result when plants are labelled in Canada with the U.S. hardiness zone. Obviously that USDA Zone 5 plant will not thrive — or even survive — in the Canadian Zone 5.
As our climate continues to warm, another challenge starting to surface is the assumption that it’s worth experimenting with using plants from a warmer zone in one they historically were not meant to be planted in. For example, Kousa dogwood (Cornus kousa) is Zone 4-7 in the Canadian hardiness zones. It may be tempting to experiment and plant this shrub in Zone 3, especially after a warm winter like we recently experienced. Until the Canadian hardiness zones are revised, this type of decision remains an experimental one and you need to be aware this could result in that plant succumbing to winter kill.
In Canada, the extreme minimum temperatures have been trending upward. But closer to the East Coast there are actually some areas of the country where there’s been a decrease in extreme minimum temperatures over time. Part of this is due to things like icebergs spawning and traveling further down the Eastern Seaboard.
Natural Resources Canada will publish an updated version of the Canadian plant hardiness map sometime in 2024. Regardless, it is very important to label plants (at the farm or at the garden centre) with the current and correct Canadian hardiness zone. And until we know better, remember that planting in a zone outside of the labelled zone remains an experiment without a guarantee of success.