When choosing plants for your garden, you take all sorts of factors into account: sunlight, soil type, humidity, etc. But the most important factor, especially for northern gardeners, is hardiness: a plant’s ability to survive the winter in your region. Without knowing a plant’s hardiness, you can’t know if it will make it through the winter unscathed or even survive at all. It’s therefore the first criterion to consider when selecting a plant.
In July 2025, Natural Resources Canada updated its Plant Hardiness Zone Maps for the first time since 2014. The old map was based on climate data from 1981 to 2010, while the new one uses data from 1991 to 2020. The very first map, published in the 1960s, covered the period from 1930 to 1960, followed by an update for the period from 1961 to 1990. However, the climate is changing more rapidly, particularly in Canada, making these old data quickly obsolete. Technological advances now make it possible to produce more accurate, reliable, and economical maps.
What Is a Hardiness Zone?
A hardiness zone indicates the minimum climatic conditions that a plant can tolerate in order to survive outdoors, particularly during winter. In Canada, hardiness zones are established using a multivariate index that takes into account not only the minimum extreme temperature, but also other factors such as the length of the growing season, precipitation, snow cover thickness, and wind strength. They are expressed as numbers from 0 to 9, sometimes accompanied by a letter (a or b) for greater precision, with higher numbers corresponding to warmer climates. Plants suited to your hardiness zone or a lower (colder) zone can generally be grown, but not those suited to a higher (warmer) zone. In the United States, the USDA system is based solely on the average annual minimum temperature.
These zones are an essential tool for gardeners, agronomists, and horticulturists, as they help them choose plants that are suitable for their local climate, thereby increasing their chances of success in the garden or in cultivation.
What Has Changed
The results of the most recent update to the hardiness zones show a widespread increase in hardiness zones across Canada. In many regions, there has been an increase of ½ to 2 full zones, which represents a significant change for gardeners, farmers, and horticulturists. This trend confirms that plants that were once marginal in certain regions are now more likely to survive the winter there.
The biggest changes have been observed in British Columbia and northwestern Canada—regions that are highly sensitive to climate warming. When comparing weather data from 1961–1990 with data from 1991–2020, we find that almost all of the zones that have changed have become warmer.
This warming is mainly due to three climate variables included in the Canadian hardiness calculation:
- the minimum temperature of the coldest month, which influences the direct winter survival of plants;
- the frost-free period, which prolongs the growing season;
- the maximum temperature of the hottest month, which contributes to better ripening of heat-loving plants.
These factors combined explain why some plants that struggled to survive 30 years ago are now thriving in several regions of Canada.
Consult the hardiness zone map by municipality to find out which zone you are in. Have there been any changes in your area? Let us know!
Practical Implications for the Garden
With the widespread increase in hardiness zones, many gardeners and horticulturists are discovering new growing possibilities. Plants that were once reserved for warmer regions are now becoming viable further north, as the growing season is longer. For example, the cultivation of vines, hardy fig trees, and even some more delicate fruit trees is gaining ground in regions such as southern Quebec, Ontario, and certain valleys in British Columbia. This is good news for those who like to experiment and diversify their gardens.
But it is important to keep in mind that the climate remains unstable. Despite the warming trend, the risks of late spring frosts, summer heat waves, and periods of intense cold in winter are still very much present. That is why many producers and amateur gardeners prefer to proceed with caution. The door is opening to new crops, but it is best to enter one foot at a time, so you don’t get caught out by the cold!