By Cheryl Harrison, Master Gardener
Gardeners are used to seeing the “zone” as part of the information provided on the label of most perennial plants, trees and shrubs purchased from a nursery. Some labels will list a zone with the corresponding temperature in brackets afterwards. This seems like an easy way to determine if your chosen plant will successfully grow in your garden–you will need to know the average lowest winter temperature, in your region, then figure out your zone. You then just buy the plant labelled with your zone for success! Easy, right? Yes and no!
In Canada, the original 1960’s plant hardiness zones were based on information gathered at 640 weather stations across the country along with the survival results of selected trees and shrubs. In 2012, revised maps were released using elevation, more locations and more environmental details resulting in a hardiness index which corresponds to a zone . At present, there are 13 zones ranging from 1-13. You may also see an “a” or “b” beside the number. This indicates a slight variation within that region. See Plant Hardiness Zone by Municipality to find your zone.
In the USA, the system is different. Again, there are 13 zones ranging from 1-13 and “a” or “b” indicates variation within the region. See US Plant Hardiness Zones. However, the US system is only based on minimum temperature and no other factors. You may read about a plant grown in the US and designated US zone 5. A US zone 5 labelled plant may require warmer winter conditions than it’s corresponding zone 5 labelled Canadian plant. Bottom line, US zones may not be the same as Canada’s.
Canadian data is based on previously collected information but we know that the weather does not always follow past patterns — it can, and will, fluctuate (e.g. freeze/thaw cycles). There may be microclimates (see Microclimate in Wikipedia) in your own gardening space. Good gardening practices (e.g. soil preparation) will contribute to the successful growth of your plant. Plant survival and growth depends on your gardening expertise and knowledge. The zone is just one indicator of whether, or not, the plant that you are contemplating will grow well in your garden. Oh, and Peterborough is a Canadian plant hardiness zone 5 b.
#1 The plant hardiness zone site now can be used to explore where the specific plant that you are thinking about purchasing will grow through the “Species-Specific Models and Maps”. It does this by creating a “climate profile” of the plant and then mapping where it will grow in Canada. These are preliminary maps but click here to try it out.
#2 Grow native plants! Native plants are specifically well suited to an area especially if you can purchase locally grown plants or find locally sourced seeds.
#3 Consult local nursery staff and your knowledgeable fellow gardeners. They will know what will successfully grow in your area.
#4 Experiment! If there is a plant that you really must have, but the zone is off a little…..try it. You will need to choose a protected location for planting and probably baby it along but sometimes it may be worth it. Talking to other knowledgeable local gardeners may be especially helpful….someone may have successfully grown that special plant in their garden and may share their growing tips with you!