Did our hardiness zone just get colder?

For gardeners in the Thunder Bay area, yes, our hardiness zone is colder and distributed in new ways.  But it's not the same in all areas of Canada.

First, what is a hardiness zone?
Scientists use hardiness zones as a way of describing and mapping the overall growing conditions in different parts of the country. In Canada, we have nine Plant Hardiness Zones (PHZ). They range from 0 (the harshest) to 8 (the mildest). 

Several factors are used to determine PHZ, including minimum and maximum temperatures, snow cover, rainfall, wind patterns 
and the average number of frost-free days in a given area. The data are plotted on the Plant Hardiness Map of Canada.

Read & Reap

Plant Hardiness Map of Canada, 2000

In 2000, the map was updated, using more recent climate data from across Canada.  Plant survival data for indicator trees and indicator shrubs (the northern and southern extremes where selected species can survive and reproduce successfully), the effect of elevation, and climate-change models were also used.

What emerges is a new distribution of zones.  According to the Canadian Forest Service, "the hardiness index has declined or remained stable in eastern Canada and has increased in western areas."

Have things changed in Thunder Bay?
The Thunder Bay district - my part of Northwestern Ontario - was considered Zone 3-2b on the old 1967 map. 

In our district now, our hardiness is distributed across Zones 3a-2a. There are 3a, 2b, and 2a pockets on the map, indicating that there is greater variation overall within this area of the province. We are part of the eastern cooling trend, rather than the warming western trend.

These changes are consistent, I think, with what northern gardeners have observed in recent years.

Detail of Hardiness Zones in Thunder Bay area

You can view the interactive map at Environment Canada to gain insight on your zone.

How do zone changes affect the way we garden?
Here are a few thoughts ...

• More than ever, it's important to investigate the known hardiness of perennials, trees and shrubs before you buy.
  
• Try to buy cultivars that have been grown in northern temperatures. Divisions from a friend's garden may be more hardy than an imported plant.
  
• Explore the micro-climate options in your own garden. Do you have sheltered places where you can plant cultivars that may be tender?
• Start a plant nursery. While they take time to mature, you can start many perennials from seed. The plants you grow yourself in a corner of your garden will be tough, ready to take on the things our zone can throw at them.
  

What plant hardiness zone are you in?
Have you experienced changes in your garden?
Leave a comment and help us gather our own climate data.

---