Garden Formulas

What Are Garden Zones in Celsius?

You often see the word Zone on plant labels followed by a number. What do these mean? North America  is divided into garden “Zones” according to the climate. The lower the number the colder the winters are in your area and the hardier a plant needs to be to survive.



With Zones on the left and minimum temperatures in Celsius on the right the above graph gives you a good idea of what Garden Zone you live in based on the minimum temperature your area experiences during the year. If you want to get real precise, the lowside of Zones are referred to as “a” while the topsides are “b”. So if you experience -44 C temperatures you live in 2a; however if it only drops to -41 you are in Zone 2b. A minimum of -39 and you are now Zone 3a! Frost free days are also factored in.

It seems like such a little difference of degrees but it makes a big difference to trees, shrubs and perennials. You can live in Zone 3 and get by for a few mild years with plants suited for Zone 4 and then whammo! A record breaking cold snap hits and you lose them all.

Perennials that die down to the ground every fall and are slow to emerge always have a better chance of survival no matter what the zone. I live in Zone 3 but if I love something and it’s labelled Zone 4 or 5 but it dies back to the ground in the fall, I’ll try it anyway. To make it more interesting, a lot of the plant information is just plain wrong. We are happily growing many perennials experts thought would only survive in Zone 5 or even 6.

Microclimates can also be created by planting in pockets sheltered from prevailing winds by trees or buildings. A square bale of straw or hay set over the plant after the ground freezes in the fall (waiting for freeze-up prevents mice from setting up a winter home under the bale and possibly damaging the roots) can nudge you up a Zone and lessen the worry of losing your snow cover to a mid-winter chinook.

Bottom line, if you love it and can afford to lose it, give it a try!


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