With the arrival of spring, gardening fever gains intensity. Any amateur gardener who hopes to achieve success with their flower beds, vegetable garden and fruit trees benefits from knowing the horticultural zone in which they are located to ensure they plant varieties adapted to their region. But this data could change in the coming months, as Natural Resources Canada works on a new map of plant hardiness zones to better reflect the reality of global warming.
In the 2000s, the island of Montreal was located in zone 5b. In 2014, thanks to a revision of the plant hardiness zones, it moved to zone 6a. Rimouski, for its part, jumped from zone 4a to zone 5a. Elsewhere, this indicator has not changed, such as in Gaspé, which remained in zone 4a.
This map allows gardeners to know which plants will best withstand the rigors of winter or will benefit from a sufficient growing period in their region. To determine these zones, Natural Resources Canada uses several criteria evaluated over a period of 30 years. The ministry takes into account in particular the monthly average of the daily minimum temperature of the coldest month, the average annual number of frost-free days (above 0 °C), the amount of rain from June to November, the Maximum thickness of snow cover and maximum gusts.
The first plant hardiness map in Canada was published in 1967 before being updated in 2000 and again in 2014. “Generally speaking, plant hardiness ratings have increased in most regions of Canada since the original map from 1967,” says John Pedlar, a biologist at the Great Lakes Forestry Center in Sault Ste. Marie, Ontario. “The biggest changes have been in western and northern Canada, up to three zones in some places. In Quebec, the changes were less significant. »
Global warming is not unrelated to these changes in areas which will continue to evolve. According to John Pedlar, however, the differences observed between the different regions of Canada remain somewhat of a mystery. “It is unclear why western Canada is experiencing more rapid climate change than the east, but studies suggest it is due to the intensification of the El Niño phenomenon, which tends to cause greater warming in the West, and the weakening of the polar vortex, which brings cold air to eastern Canada,” explains the expert.
The time has now come to revise this map, and Natural Resources Canada has taken on the task, promising a new map by next winter in anticipation of the 2025 gardening season.
Risk management
Climate change, however, shows worrying warming. “Nature is out of control,” admits Bertrand Dumont, horticulturist, podcaster and author of numerous books on horticulture.
Over the decades, climate warming has made it possible to plant varieties previously considered non-hardy. “25 years ago, for example, boxwood was sold in garden centers as an annual plant. Today, we see them everywhere,” says Mr. Dumont.
The plant hardiness map should not, however, be considered as a guarantee of plant survival, because if, overall, the climate is warming, the weather can play tricks on the gardener, who is not at home. shelter from a late frost, a prolonged period of heat in mid-winter or a light snow cover. “I noticed that, every time we had early springs, we had disgusting Mays,” observes the expert.
Even an experienced horticulturist like Bertrand Dumont takes precautions. “I haven’t changed my sowing schedule. I kept the dates exactly the same because I have no idea where I’m going. This is where climate change is going to be complicated because we have no idea what awaits us. »
This is why he got into the habit of relying on soil temperature, and not on the calendar date or air temperature, to determine the planting time, even if it meant waiting a few more days. “If I don’t have 14 to 18 degrees in the soil, I don’t plant my tomatoes. Because otherwise, they will suffer thermal shock and they will stop growing. And it will take them ten days to leave. The most important thing in a plant is the roots,” recalls Bertrand Dumont.
The hardiness zone map remains essentially a “guide” which you should be a little wary of. For a region zoned 5, it is generally recommended to choose plants adapted to zone 4. “This is advice that I give, but which is not followed,” points out Mr. Dumont.
The agricultural world
Sometimes the desire to try new plants takes precedence over wisdom and caution, admits Louis Lévesque, president of the Society of Horticulture and Ecology (SHE) of Saint-Félicien, in the Saguenay region– Lac-Saint-Jean. In this regard, global warming and changes in hardiness zones are opening new horizons. “Enthusiasts like me always want to discover new plants, but we try to be reasonable. »
This climate change is already having effects on the agricultural world. Louis Lévesque, who is also an agronomist and former agricultural producer, points out that the cultivation of grain corn, formerly reserved for southern Quebec, is starting to make headway in the fields of the Saguenay–Lac-Saint-Jean region. In contrast, reduced snow cover is likely to impact the cultivation of alfalfa, a perennial forage crop grown in the region that, he says, produces high-quality, protein-rich hay. “But when the snow cover is insufficient, it cannot survive the winter. The fact of no longer having snow cover will mean that we will no longer be able to cultivate it. These are challenges for farmers. »
The agricultural community fears the drop in precipitation at strategic times during the summer and frosts during the flowering period which destroy hopes of a harvest. Farmers will have to adapt.
Global warming has visible impacts in other spheres. Wildlife is gradually migrating north, as evidenced by the presence of wild turkeys and opossums in Quebec.
It also attracts unwanted insects from the south, such as the emerald ash borer or the Asian long-horned beetle. “We didn’t have the emerald ash borer, but it has now reached La Tuque. It’s not far from Lac-Saint-Jean,” underlines Louis Lévesque.
Even if the climate gets out of control, the new map of plant hardiness zones should not present major changes in Quebec, believes John Pedlar. “However, we can expect more frequent updates of this map in the future, especially as climate change is expected to accelerate over the coming decades,” he says.