Ask Kitty
Hello my dear gardeners,
I have had several letters asking about the numbers on a plant or seed package that detail hardiness zone, followed by numbers. This Ask Kitty article is all about those numbers.
Plant hardiness zones tell you where different types of trees, shrubs and flowers will survive. The first hardiness zone map in North America was created in 1960 by the United States Department of Agriculture and indicated 12 different zones based on winter temperatures.
Agriculture Canada created a map in 1967 just for Canada using a wider number of variables. Seven to be exact — including winter temperature, frost free time, rainfall in summer, highest temperature, how long the snow stayed, January rainfall and highest wind speeds. This was a much more accurate zone indicator. Interestingly on the U.S. map our area is a zone 5 or 4, which is much colder temperatures than we actually experience.
The newest map for Canada came out in 2000 and divides the country into 9 major zones with a few subzones indicated by the letters “a” or “b”.
The harshest zones are 0 and the mildest are 8. Toronto for example is a zone 6. The closer you are to Lake Ontario it may be zone 6b. The zone may even become a zone 7 in a wind protected area, or a 5a or 5b in an open, windy area.
It’s best to figure out your own zone for your garden area. If your garden is in a sheltered spot you can use plants for a slightly warmer zone 7, or even an 8. If it’s in a windy corner you might be a zone 5. Your garden is actually its own micro zone.
While all this zone information seems a bit unnecessary, it works for both too-warm and too-cold areas. A plant for zone 2 may find our zone 6 winters too warm to survive because it needs the deep cold to hibernate and rest.
So when in doubt, stick to trees, shrubs and plants that fit your zone. For a plant to be locally hardy here it should be at least a zone 6 or down to zone 3.
It’s important to note that while zones are important, the soil and water needs of a plant still need to fit. So a plant that is zone 6 hardy, but needs loam and lots of moisture, won’t grow well or winter over in clay that gets little rain.
Hopefully I didn’t zone you out (sorry, I couldn’t resist!)
Laughing in my zone,
Kitty
