What Determines Plant Hardiness Zones?

What’s In A Growing Zone?

Hardiness zones are an attempt to match plants to the geographical areas in which they will survive.

In the last half-century, North America has adopted two systems for determining growing conditions; the USDA growing zones and the Canadian Plant Hardiness Index. While both methods use “Zones,” they differ in how they are determined. Knowing their difference can help you choose the best plants for your garden and provide insight into pushing what’s possible in your garden.

USDA Growing Zones and Extreme Minumum Temperatures

In 1960, the United States Department of Agriculture (USDA) created a series of plant hardiness growing zones based on the 30-year average annual extreme minimum temperature.

The USDA plant hardiness scale divides growing condition into increments of 10 degrees Fahrenheit (5.5 degrees Celcius). The lower the USDA growing zone, the more extreme the minimum temperature can be. An area with a USDA growing zone 3 has a maximum low of 10 degrees Fahrenheit colder than USDA zone 4. The USDA growing zones are sometimes further divided into “a” and “b” – a difference of 5 degrees Fahrenheit (2.8C).

The only thing that matters when determining the USDA growing zone of a region is its extreme temperature lows.

Canadian Plant Hardiness Index

In 1967, Agriculture Canada used plant survival data to create a Canadian Plant Hardiness Index. Unlike the USDA’s single criteria, Canada’s plant hardiness index is calculated using seven separate factors:

1. Average low of the coldest month (negative effect)
2. Average number of frost-free days above 0 C (positive effect)
3. Amount of rainfall from June to November (positive effect)
4. Average high of the warmest month (positive effect)
5. Precipitation in January (positive effect)
6. Average maximum snow depth (positive effect)
7. Maximum wind speed experienced in the last 30 years (negative effect)

These seven climatic conditions are used to create a numerical value that is translated into zones. A detailed description and formula used to determine the Canadian Plant Hardiness Index are found on the Natural Resouces Canada website. Unlike the USDA scale, there is no standard temperature difference between growing zones.

The Canadian Plant Hardiness Index is much more nuanced than the USDA, and the two systems do not necessarily line up. For example, Edmonton is a USDA zone 4a, and Calgary is a USDA zone 4b, but both cities have a Canadian Plant Hardiness Index of Zone 4a; while Edmonton can get a few degrees colder, its growing season is almost a month longer.

Since the USDA and Canadian plant hardiness index use “zones,” it might not be apparent which system a plant tag is displaying. While the USDA system is more common it is useful to look for the country of origin.

Zones Not Stones

In both cases, the USDA growing zones and Canadian Plant Hardiness Index are imperfect attempts at mapping growing conditions. Both systems are useful tools in selecting plants suitable for your location but should be taken with a grain of salt. In many cases, knowledge of where specific plants can grow is incomplete. While growing USDA zone 8 plants in a USDA zone 4 climates will end poorly, it’s sometimes worth gambling on plants that are a growing zone out of reach – especially if you have a protected location.

The real power of growing zones, especially the Canadian Plant Hardiness index, is that they provide us with a blueprint on how to hack our local growing conditions in favourable ways. Zones aren’t fated. Each of the seven factors in determining the Canadian Plant Hardiness Index of an area (or backyard) can be manipulated through good design and planning.