Canada's Growing Zones, Moving North

Every seed packet and nursery tag in Canada assumes you know your hardiness zone. That number comes from a national map Natural Resources Canada redraws every decade or so, as new climate normals come in. Put the editions side by side and they disagree. Gardens have quietly been getting milder. I wanted to show that change as motion rather than as two maps to squint at.

What it shows

The animation runs from the 1961–1990 climate to the 1991–2020 climate. The milder zones, in orange, creep north out of southern Ontario, up the St. Lawrence and across the Maritimes. On the Prairies the cold blues retreat toward the boreal forest. Across most of the populated country the shift is half a zone to a full zone, and in places up to two. For gardeners, that’s the difference between a plant that survives the winter and one that doesn’t.

How I made it

NRCan publishes each edition as a raster of the underlying Plant Hardiness Index, a continuous score built from winter minimums, frost-free days, summer heat, snow cover and wind. That’s better than the coloured zone polygons, because it can be interpolated. I placed the three editions at the centre years of their climate periods and fitted a smooth monotone curve (PCHIP) through each pixel’s three values. That curve gives one frame per year, so the map flows instead of jumping between editions. Everything is projected in Canada Atlas Lambert, the federal standard.

Caveats

There are only three real snapshots. Every frame in between is an interpolation, not a measurement. The editions also differ slightly in method and station network, so a small part of the change comes from the mapping itself rather than the climate. The Arctic is left blank because the index isn’t published there.