by Christine Freeburn, Master Gardener
Having just come back from places where tropicals are growing wildly and flowers are blooming, I wanted to discuss growing zones and the importance of knowing your zone for overwintering plant success of perennials, trees and shrubs.
Plant hardiness zones can be a confusing subject. In Canada, we generally use the USDA (United States Department of Agriculture) system because it is the one we often see on labels of the potted plants we purchase at greenhouses and big box stores.
The USDA system was designed in 1960, updated in 2012 and again in 2023. These hardiness zones are determined as “geographic areas that have an average annual minimum temperature of a certain degree.” No other factors are taken into consideration. There are 13 zones with zone 1 being the coldest and zone 13, the warmest.
Natural Resources Canada has their own system which uses factors that include the number of frost free days, amount of rainfall, snowfall, and wind as well as mean temperatures in the coldest and warmest month. There are 9 zones. Interestingly, Ontario has 7 of these zones.
To complicate things further, there are also ‘a’ and ‘b’ zones. Zone 3a is colder than Zone 3b.
When you look at the government of Canada site, you will see zone dates from 1961-1990 and then an update of 1981-2010. In most cases, zones have moved by at least half a zone. In 2024, Natural Resources Canada was to release an updated version. At press time, I did not find an update.
Plants are described as being hardy to a zone #. If a plant is listed as zone 6, it will survive in Zone 6 or higher. If you live in Zone 4, and try to grow and overwinter a Zone 6 perennial, you will probably not be successful. You are better off to look at lower numbered zones from what your area suggests, to be sure a plant is going to survive and be robust.
Reading plant labels and buying plants that are zone appropriate will help you to have more overwintering success. You will notice on most labels the zone is the USDA system. The label may say “perennial” but look at the zone. A plant can be a perennial…..but only in Florida (Zone 8 or higher).
Other factors for overwintering success include snow cover, wind, and humidity. Snow cover is a great insulator, wind can cause windburn, humidity will help plants stay hydrated.
With climate change happening, our Peterborough area has changed from 5a to 5b which means our average annual minimal temperature has moved from -26.1C to -23.3C.
Below are some towns and cities and their current zones:
- Bancroft – Zone 4a ( interesting to note, this zone did not change in the update)
- Haliburton – Zone 4b
- Port Hope – Zone 6a
- Pickering – Zone 6a
- Hamilton – Zone 6b
- Thunder Bay – Zone 3b
- Lakefield, Lindsay, Norwood, and Ennismore are all the same as Peterborough’s – Zone 5b
Things to Remember
- Canadian and American systems are not the same. For instance, Zone 5 under the USDA system should be considered Zone 6 in Canada
- You can stretch the zones, if you have a micro climate or spot protected from wind, good snow cover and a milder winter
So depending on your gardening confidence, you can stick to Zone 3 or 4 perennials for zonal success or be adventurous and try that beautiful Zone 6 perennial. Just remember to read your labels and check your zones.
Resources
Plant hardiness zone maps are not created equal
The Differences Between US and Canadian Plant Hardiness Zones
The Peterborough & Area Master Gardeners 