How to - Gardening Basics

Hardiness zones demystified

Katharine Fletcher

What do those zone numbers mean... and how do they apply to my garden?

For gardeners, “being in the zone” matters—a plant hardiness zone, that is. An interactive online map of Canadian zones allows gardeners to zoom in and discover their zone. But what do those seventeen numbers starting at zero, through 1a, 1b and culminating at 8a mean?

The first map and the “new” map
Canada’s original plant hardiness zone map was created in 1967. However, as we gardeners know, times—and climatic conditions—are changing. Scientists from Natural Resources Canada’s Canadian Forest Service, Agriculture and Agri-Food Canada, and Natural Resources Canada’s National Atlas of Canada collaborated to create the new Plant Hardiness Zones Map. Published in 2000, it used data captured between 1960 and 1990.

The new data incorporated information on elevation, so exposure effects are factored into the hardiness equation. The hardiness zone map is useful for home gardeners through to nursery owners and large-scale orchardists to refer to these zones prior to investing in plants.

What is the relevance of the numbers?
The nine major plant hardiness zone numbers describe Canadian growing conditions, where zero (0) describes the harshest environment and 8 represents the mildest. A simplistic way to remember the numbers is to think of zero being harshest, and 8 being very mild. When we look at the map of Canada divided into its zones, this approach works. All of Canada’s north including the Arctic region, falls under zones 0a or 0b, the colour is blue—icy blue.

Usefully, warmer zones go from green to yellow to red, where fuschia (zone 8a) represents the mildest growing conditions, found on British Columbia’s West Coast. 

The general rule of thumb is that when you’re buying plants, you buy plants that are tagged for your zone (or lower) because you know the plant will survive in your zone’s specific weather conditions.

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