A Climate on the Move

by Jo-Ann Canning

The Canadian plant hardiness zones were first updated in 2000, and British Columbia – affected by both mountains and a warmer ocean – had more changes than the rest of the country combined. Traditionally, each hardiness zone represented an average spread of 10ºF (5.6ºC), based on average annual minimum temperature. When the Canadian map was updated again in July 2025, each 10ºF hardiness zone was split into a 5º F (2.8º C) spread, designated as a) and b). Looking back, the insight of Resources Canada initiating this designation because of our northern latitude and varied geography has proven a real help to gardeners and farmers dealing with a warming climate. 

The reasoning of a) and b) comes from a climatology truism, “latitude mimics altitude.” In other words, the farther north the smaller the zone at any given altitude. An example is Vancouver Island’s mountain tops (1,400 – 1,600 meters / 4,600 – 5,250 feet), where there is alpine tundra even at the north end (50.52º N latitude). North of 60º N, tundra is at sea level. At the south end and along the eastern shore of our island we enjoy the farthest north Mediterranean climate on the globe, yet it extends only 1.5-5 km / 1-3 miles inland and ends at 50º N., at Campbell River / south end of Quadra Island.

Weather patterns are less predictable at a climate zone’s edges and are made even more so by the compass direction an area faces. This is an evolutionary reason that geophytes (bulbs) are found here: they evolved to withstand unpredictable seasonal shifts at the edges of a growing zone. This means that if your town is located near the edge of a climate’s growing zone your neighbourhood may be in “a” while the rest of the town is in “b.” For the island, that “higher up the mountain” and “away from the sun” may be just across the street where it is 3 metres / ten feet higher and a little shadier. 

Another thing for gardeners to remember is the edges of a growing zone also shift more quickly with localized weather. In a late spring or early autumn, the area in question may slip down or rise up into the adjacent half or even full zone where adjacent areas are not affected. Five degrees can make the difference between freezing and not freezing for as much as two weeks in spring and autumn, or create heat days (above 30ºC / 86ºF before 1100h) in summer when pollen becomes sterile and food crops will not set fruit. Those old terms “mini-climate” and “micro-climate” are ways of acknowledging these climatology facts. 

If you are a vegetable grower or an ornamental enthusiast who loves half-hardy cultivars, it also pays to understand two other two elements of hardiness (in addition to altitude and compass direction): the annual sun window and seasonal wind. I once lived on hill where one garden split a climate zone: The back 30%, in the shade of tall trees and the house, had soil that stayed frozen for a full two months each winter. Next to the house, away from the shade, facing the sun and protected by the building, I grew a lush herb garden and half-hardy flowers in ground that never froze. The sun window told me they’d do well there; even my half-hardy rosemary and tarragon flourished, and the crocus bloomed a full four weeks earlier than at the back of the yard.

Wind as well as its seasonal direction affect an area, from fields to urban yards. This is why we see tall trees along edges of farm fields being used as wind breaks. Wind can shift your yard a whole zone at a critical growing time. I once had a large urban garden where I grew fruit trees, berries, and year-round veggies. The first winter I watched the wind shift to a different direction. It froze all my chard and Asian greens. The next year I did a test: I saved all my spring tree clippings and made a foot-high brush fence in autumn on half of windward side of the garden. The other half of that side I planted hardy kale to protect the chard with Asian greens in the centre. The twig-fenced side saw no loss. The kale’s ground level wind-break saved most of the chard, but only about 25% of the less-hardy Asian greens. In year 3, I covered those. That year I had no loss due to winter winds or frozen soil in any of my vegetables.

To help understand the specific pattern for your town, follow this link to the Weather Spark website, which gives an in-depth discussion. From the main site, scroll down to British Columbia. On that page, add your town, then click the X in the upper right of the boxes for other towns. Now you can scroll down to the map, and read the discussion of your seasonal weather. https://weatherspark.com/

Here’s the link to the Natural Resources Canada site with an explanation of the science and methods behind the map updates. https://natural-resources.canada.ca/stories/simply-science/zone-canada-s-plant-hardiness-zone-maps-website-get-update

Here is the link for the plant hardiness site. It has a QR code for your phone. You can also print the map from the page. https://planthardiness.gc.ca/?m=22&lang=en&prov=BritishColumbia&val=D.