Canadian Gardeners: Beware of US Hardiness Zones!

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Sorry, American gardeners! I know that this blog is mostly read by Americans, but today I have a special message to share with my fellow Canadians. So the information in this blog doesn’t really concern you. However, if you want to understand a problem that causes much frustration to gardeners north of your border (or east if you’re from Alaska), read on!

20150412AEnglishMost Canadian gardeners know their hardiness zone and know that, when it comes to choosing a hardy plant (perennials, trees, shrubs, etc.), it’s best to choose one adapted to their zone or any colder zone. For example, if you live in zone 6, you should choose plants from zone 6 or even colder zones: 1, 2, 3, 4, or 5. If you live in zone 3, you’d choose plants from zones 1, 2 and 3. It’s not really that complicated.

(To find your Canadian hardiness zone, go here).

20150412BWhat is less known is that the Agriculture Canada hardiness zone system does not quite match the one used in the United States, that of the USDA (United States Department of Agriculture). Among others, the American system only considers the minimum temperature in a given region, not the duration of the minimum temperature. But when temperatures drop to -15˚C (5˚F) for one night, it doesn’t affect a plant nearly as much as when the temperature stays at -15˚C (5˚F) for a week. The Canadian hardiness zone map reflects this detail and also other factors (snow cover, etc.), giving a more realistic picture of local conditions.

The result of these differences is that there is about a one zone difference between the two systems. For example, a plant labelled with the USDA hardiness zone 5 is actually a zone 6 in the Canadian system. Here is a chart that compares the two systems:

Comparing the USDA and Agriculture Canada Hardiness Zones

USDA Zone
0
1
2
3
4
5
6
7
Canadian Zone
1
2
3
4
5
6
7
8

So if you read an American gardening magazine or book or visit a gardening website as Dave’s Garden, just mentally add a number to any zone that is indicated. I must admit my own books include both zones, but I believe that I’m pretty much the only author who does that.

When Nurseries Lie to Us

Crossed fingersIf it were only a question of mentally correcting the zones while reading, the situation wouldn’t be so bad, but unfortunately, many Canadian nurseries use, without ever stating it, the American hardiness zone system. I can’t say whether they do this on purpose or if they simply don’t know they’re making a mistake, but you have to admit it is definitely more advantageous for them to exaggerate the hardiness of a plant a little. The result is that many plants labels seen in Canadian garden centres are misleading: they exaggerate the cold resistance of the plant they identify. And that causes much confusion among Canadian gardeners who rely on the plant’s hardiness zone to help them purchase plants hardy enough for their gardens.

This is not a minor problem: each year, due to misleading information, millions of plants are planted in areas where they are not hardy and therefore either die over the winter or are severely damaged by the cold. Indeed, some plants in Canada are almost always sold with the wrong zone. Here are a few examples:

Hardy Orange (Poncirus trifoliatus)
Zone usually indicated: 6
Actual Canadian hardiness zone: 7

Butterfly Bush (Buddelia davidii)
Zone usually indicated: 5b
Actual Canadian hardiness zone: 6b

Rose of Sharon (Hibiscus syriacus)
Zone usually indicated: 5b
Actual Canadian hardiness zone: 6b

Kousa Dogwood (Cornus Kousa)
Zone usually indicated: 5b
Actual Canadian hardiness zone: 6b

So be careful when buying supposedly hardy plants in Canada: some nurserymen have a marked tendency to exaggerate hardiness zones… in their favor! And you’re the one who loses out!

Caveat emptor!

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2 thoughts on “Canadian Gardeners: Beware of US Hardiness Zones!

  1. Somebody changing the playing field again?!! Oh, no! Just like reporting stats of urban crime–just change how they are measured and the crime decreases. Changing the playing field. I’ll just go out and do some weeding. 🙂

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