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What the hardiness zone number really means

By Melanie Mathieson
Gardening Guru

Gardeners often hear the word “zone” frequently during conversations with other gardeners or when reviewing plant tag descriptions.
“What zone are we in?” or “What zone is that plant good for?” often are asked by fellow gardeners. But what exactly are the answers to these questions-and what do they really mean?
The first Canadian hardiness zone map was developed by Agriculture Canada in 1967. A newly updated and computer-generated map has become available this year.
Although this new map is more detailed and has used weather data for 30 years to determine the zones, Rainy River District still is inaccurately classed.
Our zone is indicated as a Zone 3a, but I strongly disagree with this classification as I believe we are closer to a Zone 4a.
One reason behind this is the fact there is not a weather station in Fort Frances but across the river in International Falls. Since Agriculture Canada does not use U.S weather data to determine zones along the border, however, we are lumped in with Kenora and Dryden for weather data.
We all know just by watching the weather channel how different the weather can be in Kenora, Dryden, and Winnipeg from here in Rainy River District.
The other reasoning behind a Zone 4a classification is the different forest type in this district. We are in a Great Lakes-St. Lawrence forest transitional zone and this type of forest does not survive north of a Zone 4a in the rest of the province.
The forest around Kenora and Dryden is a boreal forest and the boreal forest is a colder climate forest. With this reasoning, I have tried many perennials with a Zone 4 classification with great success, and I encourage you to do the same.
What exactly does a zone classification mean? Zone classifications were developed for the horticulturist as a guideline to the winter survival of trees and shrubs in the different growing regions of Canada.
Today, nurseries and gardeners use the zone classification system as a guideline for planting not only trees and shrubs but also perennials in their regions.
Although not a precise method, it is a good guideline for determining which perennials will be winter hardy in our area.
Now remember this is a guideline only. There is always a chance a perennial with a higher zone rating could outperform a perennial with a lower one based on your individual growing conditions, or you may not be able to grow Zone 4 classed perennials at all.
I certainly encourage you to use the zone classification system when planning your garden, as well as when purchasing trees and shrubs for your landscape.
I will stand by my theory for zone classification and I personally use Zone 4 as my upper limit for perennial winter hardiness. Anything classed a Zone 4 or lower to a Zone 1 should be winter hardy in our region.
But I also to encourage you to try some interesting plants, like bamboo and pampas grass from higher zone ratings, with the expectation that these will be interesting annuals in our area.
I will try plants such as these from seed as opposed to purchasing them as expensive potted plants. That way, if they do not perform well, I haven’t invested a great deal of money.
I also will try trees and shrubs from a higher zone rating, but only if I can get them in the smaller, more economical pots.
Just keep these rules of thumb in mind if you travel out of our area and see an interesting plant you want to add to your own yard or garden.
Now that you more clearly understand what the zone ratings mean and how to use them when selecting plantings for your own garden, you can make wise plant decisions.
Always look for a zone rating on the plant tag or label on each pot. If you cannot locate a zone rating on the tag (as with pre-packaged plants in large chain stores), you are best to assume the zone rating is for a zone higher than our local zone.
Often these bargain pre-packaged plants, such as roses, shrubs, and vines, are grown and packaged in Toronto and distributed throughout Canada.
Unless you are very familiar with the species of plant and automatically know it is hardy for this area, I suggest you leave it in the store so you are not disappointed when it dies in the winter.
I encourage you to have fun and experiment, and keep the zone hardiness in mind. Using it as a guideline will help you make wise plant selections-and will ensure you remain pleased with your garden form year to year.
Good growing this season!