Last month, The U.S. Department of Agriculture updated their plant hardiness zone map for the first time in over a decade. Unsurprisingly, the new “lowest likely winter temperature” in all 48 states south of the Canadian border is now 1.4C warmer. In an AP news article, Boston University plant ecologist Richard Primack, who was not involved in the map project, said: “Half the U.S. has shifted to a slightly warmer climatic zone than it was 10 years ago.” He called that “a very striking finding.”
Photographed in Toronto, June, 2015
How does this affect Canadians?
We Canadians have our own plant hardiness zone map, of course. In fact, just to one up the Americans, we actually have two. One is based on the same approach as the USDA’s using extreme minimum temperatures and another reflects a wider range of climate variables including the lengths of frost-free periods. You can find both maps here. Unfortunately, they’re sell-by date was 2010.
I like to think we can look at the new USDA map as a kind of snapshot of what’s to come. Just like trends in everything from our stock market to our sneaker choices, what happens in the U.S. pretty much always influences what’s going to happen in Canada so why not their gardening zones as well?
Looking into the not-too-distant future
Interestingly, we wouldn’t be alone in seeing the new USDA map as a glimpse at the future. For instance:
- The USDA Risk Management Agency uses it to set crop insurance standards.
- Scientists use the data in research models when extrapolating the spread of exotic weeds and insects.
- Researchers look for signs of less over-winter kill-off of insects carrying potentially harmful diseases such as ticks and mosquitoes
The U.S. Forest Service is using hardiness zones to predict the range and speed of trees marching north (not literally but you get the idea) as the climate warms. One would have to assume this march won’t stop at the 49th parallel. Perhaps that lonely Loquat Leaf tree, the only one of its kind in Canada, may have company sooner rather than later.
SIDENOTE: Check out The Washington Post’s interesting article, published back in April, with its interactive plant hardiness zone map that projects how zones will change (and subsequently affect what tree species will thrive or die out) right up to the late 21st century (2070-2099).
Ministry of the fence 
Excellent post – very timely! I am a landscape designer in Ottawa (was zone 5b in 2010, but is edging toward zone 6, especially in the heat islands in the city). The city of Ottawa has published a detailed heat zone analysis of different regions of the city, and as expected, the wealthier parts of the city have a cooling buffer of trees and Green Belt and parks, and the less affluent parts have noticeably fewer trees and parks. The public health department uses these municipal heat maps to plan where cooling centres (aka heat wave emergency centres) need to be located to provide large, air-conditioned shelters for those who unfortunately don’t have nearby trees, parks, or central AC where they live. The elderly and economically disadvantaged are at particular risk of heat-related health consequences, but it affects us all in one way or another.
In my work as a residential landscape designer, I encourage families to plant trees and large shrubs if they have the room. There’s usually room for at least one columnar (space-saving) tree even in the smallest lot, especially if you ask your neighbours for permission to plant near the property line. Most people actually want some shaded area, so generally neighbours say, go right ahead!
I believe the Chinese have a saying that goes along the lines of: The right time to plant a tree…was yesterday. Failing that, today!
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Thank you so much for sharing your insights, Martine! I hope everyone gets a chance to read your comments. What a shame that we’re dealing with emergency cooling centres now when trees left standing would’ve dealt with the problem to begin with. A shame, too, that you can so easily see the impacts of climate change along economic realities. You are so right – better late than never to plant a tree.
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