Plant hardiness zones can answer the question "Will this plant survive in my garden?"
You’re in the garden center and see this plant that is begging to jump on your cart. You know that there’s a little label stuck somewhere in or on the pot outlining the plant’s common name, botanical name (who cares!), and whether it “likes” sun, shade, or something in between. Then you see something along the lines that your new baby is “recommended for zone 6/5/4/3” and you wonder “what is this zone nonsense?”
To help growers of crops and ornamental plants, American Canadian agricultural authorities have divided and colour-coded the country up into climatic zones in which certain plants are assumed to be able to survive. Here's the Canadian plant hardiness map produced by Agriculture and Agri-Food Canada:
|Plant Hardiness Zones of Canada|
Survival factors include the area’s historical (typical) minimum winter temperatures, number of warm days per year, number of sunshine hours per year, soil type, volume and persistence of snow cover, and wind velocity and direction. A plant’s “hardiness” (ability to survive) is then given a zone designation and corresponding number and map location in which experts believe a certain plant should survive in. The higher the number, the higher the minimum winter temperatures (and conversely, the lower the zone, the lower the expected minimum winter temperatures.)
The Toronto area is rated, using the Canadian system, in the 5 zone (American zone 6) so you should feel secure buying and planting perennials, shrubs and trees with a rating of 5 or lower. (Most annuals, herbs and vegetables we grow in the GTA don’t have a zone rating or a rating that is 8 or higher. This means that they will not survive a typical Toronto outside.)
Well, so what?
Remember that these zones were created by plant scientists (i.e. humans) so they’re really best guesses about a plant’s survival in a specific geographic area, all things being equal. Of course, all things not being equal, don’t be shocked that a perennial labeled for zone 4 doesn’t make through a particularly harsh winter or that a “tender” plant somehow does. A deep and persistent layer of snow really “pushes” the zone higher for many perennials and shrubs in our area.
Zone designations are not perfect (because the folks who dream them up are not, just saying) but instead give you a good, quick and easy to understand (hopefully) indication whether that expensive and trendy plant you just need to buy before your neighbor does will likely live and thrive. Zones are more like probabilities not absolutes.
For me, it’s better to think of growing plants in Toronto (or your particular area) within a range of zones that can be taken advantage of (“pushing the zone”) if you want to work extra in creating “less harsh” growing conditions by covering plants up for the winter, growing them along a south facing wall for us in the northern hemisphere or covering them with an extra deep layer of leaves in the fall to act as a protective mulch. Or you can be conservative, not get involved with all this pampering, and stick with plants with 3 or 4 designations. Boring, yes, but less heartache and pain on the credit card.
Toronto gardeners love to push the zone, earning bragging rights, and occasionally pushing back against their Vancouver/Victoria cousins. You can too but just remember that when it comes to a number on a label called the “hardiness zone”, plants will die (and thrive), zones be damned!
By Paul Jung, author of "garden muses: a Toronto gardening blog"