How to figure out your zone

Has a gardening friend or nursery salesperson ever asked you if you “know your zone?” It’s not a pick-up line (well, it’s usually not a pick-up line. I suppose someone might try…) or a way to ask your astrological sign. They’re trying to talk about your growing zone.

“Growing zone” is gardener shorthand for what plants you can grow outside in your geographical area. Once you know what growing zone you live and garden in, you have the keys to unlock information on plant tags, seed catalogues, and garden websites.

 

How to find your growing zone:

If you live in Canada go to Natural Resources Canada’s plant hardiness website and click on the interactive map “1981-2010”. Find where you live. See what colour it is. And then compare that colour to the legend on the side of the map to get your number. That number is your growing zone.

 

If you live in the United States go to the United States Department of Agriculture’s plant hardiness website, click on your state on the map and then figure out the colour and corresponding number. Or, (This is very slick. As a Canadian, I’m jealous.) just enter your ZIP code in search box at the top left hand corner of the page and it will tell you your zone number.

 

The bigger the number the warmer the zone

“I’m zone 4. What does that mean?” It means you are likely (in an average year) going to be able to grow plants that have been rated to grow in zones 1 to 4.

“I live in zone 10, what can I grow?” You can grow plants that have been rated to grow in zones 1 to 10, although you may be challenged with plants that can’t take the heat. This is why many plant tags show two numbers–the minimum and maximum range that a plant will likely grow in. If you see zones 4-8 on a tag and you’re gardening in zone 10, you may be out of luck.

 

What’s the difference between the US and Canadian Growing Zones?

I’d heard that there were some differences between the two maps, but until I starting researching this post I didn’t fully realize how different the maps are. It’s not that the results are so different but it’s the process–the maps were created with completely different methodologies. Let me explain:

 

• The US map is based on the average annual minimum temperature—the lowest average annual temperature that can be expected each year. Pretty straightforward.
• The Canadian version of the map is based on a plant hardiness or suitability index, arrived at by a VERY complicated formula (Y = -67.62 + 1.734X₁ + 0.1868X₂ + 69.77X₃ + 1.256X₄ + 0.006119X₅ + 22.37X₆ – 0.01832X₇) that includes daily minimum temperatures of the coldest month, and daily maximum temperature of the warmest month, amount of rainfall, depth of snow, and maximum wind gust speed in 30 years! Yikes.

 

One would think that these very different approaches would yield wildly different results. And that the Canadian map, with its use of multiple points of data, would be the more accurate one.

 

Well, I looked up my garden on the US map (a version the Canadian government provides, using the US methodology for Canadian locations) and it’s zone 6a. And on the Canadian map it’s zone 6b.

Hmm.

Hello, Natural Resources Canada? We may be overthinking this thing…

 

How accurate are the zones?

Well, they represent averages. Is the weather hotter or colder some years than others? Are some areas of a garden more protected and therefore likely warmer on average than others? Does living closer to a lake or at a higher elevation change “the weather”, even in different areas of the same city? Yes, yes, and yes.

I trust the zones enough that if I, living in zone 6, see a plant tag indicating a plant is zone 8-10 I know that I’m probably only going to enjoy this plant for one season or, if I want it for longer, I’m going to need to bring into the house for the winters. If I see a plant tag that’s says “hardy to zone 7” and I’ve fallen in love with the plant, well, I will probably buy it and do my best to give it a more zone 7-like environment—planting it in my back yard, which is south facing, in a spot that’s sheltered from the wind. If we have a particularly harsh winter I might lose it. But then again, it might live.

 

Do you have any other zone questions? Write them in the comment section below and I’ll try to find you an answer.