Laidback Gardener Tip of the Day

Canadian Gardeners: Beware of US Hardiness Zones!

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

Canadian Zone

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!

8 comments on “Canadian Gardeners: Beware of US Hardiness Zones!

  1. I just bought Hedera helix ‘Baltica (usda 4) which I suppose will have to grow indoors since we live close to Calgary. I also bought Parthenocissus tricuspidata ‘Veitchii supposedly usda zone 3. Last year I got sent aguntas instead of kolomikta in a kiwi set.

  2. You can see what we grow here on Salt Spring Island on YouTube Bananajssi

  3. Banana Joe Clemente

    Plants can’t read zone maps . This region of Canada ?? where we live is zone 9a as are the other Southern Gulf Islands ???Proof is in the pudding with all of the long term exotic vegetation and palm trees that grow here . ?????

  4. Guess why I’m here on this post???
    Because I’m Canadian, and I just bought new Muscari bulbs (at a major Edmonton area greenhouse & garden centre, — one that should know better!) that, granted were on rock-bottom end of season clearout, but I’d never heard of the particular variety before… they’re gorgeous, so I snapped up a couple of bags.
    Well, wouldn’t you know it, I look it up online and it’s… zone 5.
    Yeah, if I go by the “new” hardiness zone changes done by some official Canadian Gov’t Body last year to account for climate change, I’m zone 4 — but the fact that I’m not IN Edmonton, I’m in one of the suburban towns surrounding Edmonton, I’m not sure if it was just the city that was changed or surrounding areas, too.
    ONE website I read said it was rated for zone 4, but the rest said 5… but if they used the USA system , that means it’s zone 6 and probably seriously pushing it too far. GRR. Good thing I didn’t pay much for them or I”d have been REALLY choked!
    So I decided to plant 1 bag of 10 outside, and the other one inside. For outdoors, I put a few at the base of a Little Lime Hydrangea shrub, and the rest of the bag in my south-facing garden, fairly close to the house, and I threw a bucket of finished compost on top, then mulched that with a few inches of corn leaves. I’m crossing my fingers!
    The ones that are staying indoors — a necessity weather-wise since we’ve gotten about 6 inches of snow in the last 2 days, (whatever isn’t in the ground by Halloween isn’t going in the ground around here)– I’m thinking that I’ll plant them in a fairly flat planter, then put them in my spare fridge for the winter so they get their period of cold then bring them out in March-ish & put them in my grow room under lights
    For the record, the Muscari I got is “Paradoxum”.
    Thanks for this post! I’m so happy to find a fellow Canadian gardener who understands the challenges of gardening in Canada!

    • Actually, it’s not even a Mascara, but a Bellevallia, a bigger and less hardy plant. I’ve tried it in zone 4 and it did “all right”, with a bit of bloom, but didn’t last long, even though I mulched it abundantly, whereas a good muscari can live practically in my climate forever without mulch. I take that as a sign that it just found my climate a bit too tough

      It really has to be considered as a zone 6 plant for Canada.



  5. Pingback: How to Read a Seed Packet – Laidback Gardener

  6. 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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