Gardening zones important for picking plants

IN THE GARDEN

A risk of frost, you say? We’ve heard that word already and will hear it more frequently this month. But, so far so good around my place.

Out in the country or only a little way north, tender annuals and windshields might well have been iced up already. In the heart of a town or city, it’s unlikely much is harmed this early in fall.

That’s due to the heat retained by roads and buildings, and the heat from all those furnaces that kicked in last weekend has to go somewhere. This is why the hardiness of a particular plant can vary depending on where it’s growing, especially perennials; many of our annuals are actually perennials in their native habitat, with greater or lesser degrees of hardiness.

Based initially on field trials and on observations, plants are given a hardiness rating: zone 5, for instance. A lower zone number means it’s hardier, while a higher number less so. Zone numbers are also given to all locations in the country, with the letter ‘a’ or ‘b’ added to further subdivide them.

Where I live in Waterloo is rated zone 5b, half a zone milder than Grand Valley to the north, which is 5a. Further south, Windsor is a balmy zone 7a.

Plant tags don’t reference a or b, simply a zone number, and this is enough of a guide when buying plants. For this area, always choose plants rated as suitable for zone 5 or any lower number. Should you have zone envy, and most gardeners do, a zone 6 plant might make it through winter with good snow cover or plenty of mulch — providing you’re prepared to take the risk.

It’s worth noting that the U.S. system is a little different. Their zone ratings are based on the minimum low temperature of a given region, whereas here we take into consideration factors like duration of cold and snow cover. The difference is roughly one zone number. This means a plant listed as hardy to zone 5 under the U.S. system is actually a zone 6 in the Canadian system, meaning it would be less hardy.

Keep this in mind if you’re reading plant information in a U.S. gardening magazine or on a U.S. website.

This one zone disparity might not always make a difference. Depending on the severity and depth of snow, a zone 6 plant can often make it through winter in this area but if there’s a January thaw followed by a sudden drop in temperature, it’s a gamble.

As for those gardens in the heart of a big city, they can be one big microclimate. Toronto is rated as a much milder zone 7, despite being in our cooler zone 5 region. So if you’re hearing from a friend who gardens in downtown Toronto that they’re growing plants you only wish you could grow, that’s the reason.

In any garden there are microclimates where a less hardy plant will have a better chance of surviving. These are the sheltered, south-facing places, usually close to the house — under the exhaust from the clothes dryer is not a good place. You might enjoy sitting in a sauna then rolling in snow, but plants won’t like the rapid temperature changes. Now, if you were to keep the dryer running all winter you could probably grow tomatoes there — very expensive tomatoes smelling of fabric softener.

A final note on zones: although they are clearly marked on zone maps, there isn’t an immediate change from one to another. They’re there as a guide. I’ve never heard of anyone mowing their lawn in spring while their poor neighbour across the street in a colder zone is still shovelling snow.

Are you zoned out now? For zones of Ontario towns and cities see https://tinyurl.com/zonesontario.

To chat with local gardeners, share tips, pics, or discuss hardiness zones, see Grand Gardeners on Facebook at www.facebook.com/groups/Grandgardeners.