Natural Resources Canada recently updated its plant hardiness zones map, providing gardeners with valuable information for planning their gardens. The previous map was published in 2014, and significant changes have been observed since then, with approximately 80% of the land in Canada showing an increase in hardiness zones—typically by half to a full zone. The plant hardiness zone map categorizes regions from 0 to 9, with each zone further divided into a and b. This new map is based on data collected between 1991 and 2020 and uses seven criteria, including temperature averages and frost-free periods, averaged over 30 years to provide reliable assessments for various regions.
In southern Ontario, there’s been a noticeable increase in zones, with the Greater Toronto Area (GTA) moving from 6A to 7A, reflecting a warming trend. John Pedler, a research scientist at the Great Lakes Forestry Centre, noted that the “heat island effect” may contribute to these changes. As urban populations grow and urbanization increases, temperatures in metropolitan areas can rise due to concrete and infrastructure. The Windsor region also experienced a significant increase, moving from 7A to 7B, marking the first appearance of a 7B zone in Ontario. This shift opens up new gardening possibilities; for instance, gardeners in Zone 7 could grow canna lilies and dahlias as perennials, and even attempt to cultivate more sensitive fruits like peaches and figs.
However, the updated map comes with some caveats, as pointed out by Dan McKenney, another research scientist at the Great Lakes Forestry Centre. While the new data provides a general guideline based on 30-year averages, it does not account for yearly variability. An exceptionally cold winter could still damage plants better suited to warmer climates. To address this uncertainty, researchers at Natural Resources Canada have developed species models that allow individuals to assess the hardiness of specific plant species based on their local climate conditions, which can be particularly useful for commercial growers who must weigh their risk tolerance.
The most substantial zone changes occurred primarily in Western Canada, particularly in British Columbia and Alberta, where some areas saw increases of up to two full zones. Victoria experienced the most significant change, moving from 7B to 9A, the highest hardiness zone in Canada. Other notable increases include Vancouver, which jumped from 8A to 9A, and Calgary, which moved from 3A to 4A. The map also highlights changes in central Canada, with Winnipeg increasing from 2B to 3B and Toronto evolving from 6A to 7A.
Interestingly, some areas, particularly the eastern portion of Newfoundland, saw a decline of half a zone. This phenomenon is believed to be linked to climate change, resulting in more iceberg activity in the North Atlantic, which could be cooling the region despite the general trend of warming. Pedler emphasized that while increased hardiness zones present exciting opportunities for gardening, climate change is fundamentally altering natural ecosystems in ways that pose significant risks, like longer and more intense fire seasons observed in Northern Canada.
It’s important to note that Canada’s plant hardiness zones do not align directly with the United States Department of Agriculture (USDA) map, which primarily considers extreme temperatures in its assessments. Snow cover plays a significant role in plant survival, and the Canadian approach incorporates various climatic factors that impact hardiness. For gardeners and farmers seeking to adapt to these zone changes, Natural Resources Canada provides interactive maps and historical data on its website, assisting in informed decision-making as climate conditions continue to evolve. Though the map offers new possibilities for gardening, it underscores a pressing concern regarding climate change’s impact on ecosystems and agriculture across Canada.
