Winnipeg's sweet spot is still 3b

Winnipeg Free Press

a day ago

In late 2023, when the U.S. Department of Agriculture (USDA) released a new version of its plant hardiness zones map for the first time in a decade, the map showed an increase of approximately 1.39 degrees Celsius warmer than the 2012 map across the 48 states.
Plant hardiness zones are divided into two halves, denoted by the letters a and b. For example, zone 3 is divided into zone 3a and 3b, with the 'a' being slightly colder than the 'b.' With the 2023 map, about half the country shifted to the next warmer half zone and the rest remained in the same half zone.
Gardeners in the half of the U.S. which shifted into a hardiness zone half a zone warmer were ecstatic about all the new plants they could grow.
Natural Resources Canada photo
Canada's updated plant hardiness zones map, released in July, uses climate data for the 1991-2020 period and shows shifts in zones in many parts of the country.
At the time, social media in this country lit up with questions from gardeners who wanted to know when a new plant hardiness zones map for Canada would be created. After all, the last update occurred in 2014 using climate data from the 1981 to 2010 period. In November 2023, when I first reached out to Dan McKenney and John Pedlar — the Canadian Forest Service researchers and plant hardiness experts who are the collaborative minds behind the hardiness zone map updates over the past couple of decades — they were working on a refresh.
An updated plant hardiness zones map for Canada was released in July by Natural Resources Canada. The new map uses climate data for the 1991-2020 period. It shows shifts in zones in various parts of the country, with many areas changing by half a zone or more (for example, Yellowknife, Northwest Territories, formerly zone 0b, is now zone 1b, a full zone increase). Zone values have increased by as much as two full zones in parts of British Columbia and there have been zone increases in southern Ontario.
In Winnipeg, the hardiness zone remains at zone 3b.
Wait, what?
If, as a gardener, you were half-
expecting — even hoping — that the new hardiness zone map for Canada would show an increase by half a zone or more for Brandon, Winnipeg, Steinbach and Winkler, as occurred in the 2014 update, you may be feeling disappointed.
McKenney and Pedlar work at the Great Lakes Forestry Centre in Sault Ste. Marie, Ont. Experts in developing climate maps, they have built models that cover all of North America. They work closely with Environment Canada and Agriculture Canada as well as with the National Oceanic and Atmospheric Administration in the U.S.
'Plant hardiness zones are developed on 30-year averages,' McKenney said in a recent interview. 'The 30-year period was first developed by scientists at Agriculture Canada. Their original intent was to look at zones that are fairly stable based on longer-term conditions. They knew, as we know, every year is different and maybe there is an argument to be made that the hardiness zones could be developed on shorter time periods.'
Natural Resources Canada photo
Compared to the last update in 2014, the new plant hardiness zones map for Manitoba stays the course for Winnipeg, Steinbach and Winkler, which remain zone 3b, and Brandon, 3a.
For now, though, McKenney and Pedlar base their modelling on 30-year averages.
In Canada, there are 10 different hardiness zones, ranging from 0 to 9.
'Plant hardiness zones for every location in Canada are calculated by a formula which consists of seven variables,' said Pedlar.
These are average minimum temperature of the coldest month, average maximum temperature of the hottest month, growing season length, total rainfall from June to November, winter factor (which combines January rainfall and minimum temperature), snow depth and maximum wind gust.
It's important to note, said McKenney, that the U.S. system is based solely on extreme minimum temperature conditions.
When the original map was developed by scientists at Agriculture Canada in the 1960s, the statistical formula of seven climate variables was based on 108 locations across the country, explained McKenney.
'They assessed 174 plants (cultivars and species), did a bunch of statistical analysis and came up with this formula — but remember, it's a model. There are places like Winnipeg where there are extremes in temperature or chinook winds in Calgary or the high wind speeds in Newfoundland that can really have a big effect. There may be winters where there is very little snow cover. Those kinds of conditions are not represented in the model. It's not all about temperature — it's about combinations of temperature, precipitation, et cetera. What's complicated is that these things are changing across the country in different ways for every location.'
Natural Resources Canada photo
John Pedlar, a research scientist and biologist, has worked closely with Dan McKenney to develop climate maps.
But let's get back to understanding the new map, which has Winnipeg's hardiness zone stuck at 3b.
'That hardiness zone is in comparison to the last update in 2014,' said Pedlar. 'In our first update that covered the 1961 to 1990 period — it's one we often go back to when comparing the changes over time — Winnipeg was zone 2b at that point. The shift to 3b happened with the 2014 update and you've stayed there for this update.
'There are index values for Winnipeg which range between zero and 100 and these get classified into the more familiar zone designations like zone 3a, zone 3b, et cetera. Technically the index value for Winnipeg is 36. Zone 3b ranges from 35 to 39 in terms of the actual index values that get classified as zone 3b. So I was hoping to be able to tell you that you're right on the verge of flipping into zone 4a, but 36 isn't even at the high end of the index range for 3b.'
Plant lust and zone envy are real. But so is recency bias, a cognitive bias that favours recent experiences over historic ones. 'Many people think about the last four or five years,' said Pedlar, 'but this is a 30-year average so that is really going to override a warm winter here and there.'
Pedlar and McKenney receive many requests for more frequent updates to the plant hardiness zone maps for Canada.
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'We could use shorter time periods — say, 10-year periods which would give a more recent picture — but we are a bit reticent to do that,' said McKenney, 'because we've really been faithful to the original work. This model was developed using long-term averages and it's not really clear that it's meant to be used with shorter-term time periods.
'So far we haven't really seen drastic shifts in most places across Canada, even over the 10-year update periods. There have been consistent positive increases, which is, I think, a real interesting piece of evidence about the reality of climate change, especially given that those seven variables aren't all … equally connected to a changing climate. We expect temperature to increase, but it is less clear how things like snow depth and precipitation are going to shift with climate change.
Natural Resources Canada photo
Dan McKenney is a research scientist and plant hardiness expert at the Great Lakes Forestry Centre in Sault Ste. Marie, Ont.
'But we're certainly thinking about more frequent updates. It might happen.'
Canada's plant hardiness site (https://planthardiness.gc.ca/) is an excellent resource, where you'll find climate maps, hardiness indices and lists of plants that grow in different areas.
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Colleen ZachariasGardening columnist
Colleen Zacharias writes about many aspects of gardening including trends, plant recommendations, and how-to information that is uniquely relevant to Prairie gardeners. She has written a column for the Free Press since 2010 and pens the monthly newsletter Winnipeg Gardener. Read more about Colleen.
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