A shift of growing zones in Earth’s changing climate
The weather, it’s a-changin’. And it’s due to climate change — which is not news to almost anyone.
Our globe’s long-term warming trend is reshaping how we garden, like it or not. While we do not like the overall trend, and support measures to curb this change, we also accept that there is a certain level of warming which is at this point locked in.
We are more likely to experience hot summer drought and extremely cold winter conditions that can kill plant species which would need time to adapt.
So what does a dominant warming trend with the occasional cold snap mean for gardeners? Kevin Kavanagh has spent a lot of time thinking about this. After a career as a conservation biologist with both World Wildlife Fund Canada and Nature Conservancy of Canada, Kavanagh established his small nursery South Coast Gardens — near the shore of Lake Erie, in St. Williams, Ont. — in 2005.
“While the trend is warming, along with that we also get more extreme weather,” Kavanagh said. “We don’t escape the wild swings that have given us some of the coldest weather. But there are still some new opportunities for plant species.” Anew growing zone for Toronto: The opportunities he refers to are those plants which we haven’t historically considered hardy, based on the old U.S. Department of Agriculture growing zone maps. “There are published reports (showing) they are looking at how the zones are shifting,” Kavanagh explains. “In the past 30 years, zones have shifted northwards about 150-200 kilometres. The new Canadian system is trying to match up with the USDA system, but for now you could try going a half-zone higher than what you’ve tried in the past.”
That means Toronto — which has historically been a USDA zone 6a — is now a 6b.
Eastern Flowering Dogwood and Redbud are two examples of plants which, historically, were not widely planted in our part of the province — but are increasingly widespread. New magnolia varieties, such as Daybreak and the Yellow Butterflies variety, are also proving hardy. Sweet gum ( Liquidambar styraciflua) is a beautiful and common forest tree in the southeastern U.S. that a lot of people have not yet tried but can thrive in Toronto’s “new climate.” Helping these marginal species adapt: Growing a tree or shrub outside of its traditional growing zone is never as easy as dropping it in a hole and forgetting about it.
Start by selecting a local seed source. As Kavanagh puts it: “Shipping plants here from the Deep South is probably not going to work for us.” Sweet gum from seed sources as far north as Indiana have proven to do well in southern Ontario, as the genotypes are more locally adapted than the populations growing further south.
Protect from wind and sun. In late autumn, place a screen of white fabric around plants without wrapping too tightly — a cheap set of white sheets will do. The white fabric deflects the winter sun, which can burn plants, while protecting against wind.
Create a microsite by planting simple barriers, such as evergreens to the west and north of your home. Fencing also works to create a protected area on the east side for planting tender varieties.
Mulch and water evergreens deeply in the fall. The freeze-thaw cycle can be particularly damaging with warm spells followed by cold snaps, something which is increasingly common. White plastic tree guards can prevent bark split or sunscald during early spring warm spells.
Experiment and take risks. Trying new things is half the fun of gardening. Nobody knows exactly what the new climate holds, and as Kavanagh puts it “plants can always surprise us”. Backyard Urban Farming for Beginners — Feb. 12, 19 and 26. Toronto Botanical Garden will host this threeweek intensive course about growing food, by Backyard Urban Farm Company. Week 1 covers garden planning, companion planting and crop rotation; week 2 covers raised beds and containers; week 3 covers growing from seed and planting. Students will create their own garden plans. For more info: torontobotanicalgarden.ca.