Calgary’s Garden Zone
Contributed by Jolene Ottosen for the Chaparral Green Thumbs
If you’re shopping for plants or seeds, you may have noticed a zone range listed on the tag or packaging. This range indicates the plant’s hardiness and the climates where it will thrive.
Since the 1960s, Canada has been divided into eight hardiness zones, determined by seven climatic factors: minimum winter temperatures, length of the frost-free period, summer rainfall, maximum temperatures, snow cover, January rainfall, and maximum wind speed. The harshest climates in the far north are classified as Zone 0, while the mildest climates in Vancouver and Victoria fall under Zone 8. Each zone is further divided into sub-zones “a” and “b”. Microclimates caused by factors like elevation changes or wind exposure can create variations within these zones.
This system is similar to the one used in the United States, though the American system considers only annual extreme minimum temperatures. While the U.S. has updated its calculations for zones, Canada has not followed suit.
In 2014, Canada’s hardiness zones were updated using data from 1981–2010 collected at 104 test stations across the country. These updates added a ninth zone to the system and revealed significant shifts. Zones in western Canada increased by one to three levels, while eastern zones generally increased by one. The area covered by Zones 3 and 4 expanded by about 200%, and most regions now enjoy an average of eleven additional frost-free days compared to 50 years ago. In Calgary, we average 114 to 117 frost-free days annually.
Traditionally, Calgary was classified as Zone 3a or 3b, depending on the source. Today, most of the city is considered Zone 4a. Southeast Calgary is especially hospitable to plants compared to higher elevations in the west and northwest, including areas along the Bow River Valley.
What does this mean for Calgary gardens? Hardiness zones are a helpful guideline when selecting plants. Living in Zone 4 means that Zone 3 perennials should thrive, most Zone 4 plants will also do well, and some Zone 5 plants might survive under favourable conditions. Plants classified as Zone 6 or higher would likely be annuals unless moved indoors for winter. Lower-zoned plants may also grow here but could struggle due to Calgary’s unpredictable winters, with Chinook thaws followed by -30°C cold snaps.
As the climate changes, so do Calgary’s gardens. Plants once thought impossible to grow here, like certain Japanese maple varieties, are now viable. Gardeners are also succeeding with pears, large apples, grapes, and hydrangeas. So, the next time you plan your garden, consider experimenting with something new—you may be pleasantly surprised by the results!
For more information or details on upcoming events, email us at [email protected].
Click here to the Chaparral Community News home page for the latest Chaparral community updates.
