When gardening and planting trees up here in northern Wisconsin, I pay a lot of attention to United States Department of Agriculture (USDA) hardiness zones.
My general region is rated Zone 4, which means it’s too cold for many popular trees and plants to survive the winters. When perusing plants catalogs and websites, I always check to see which zones the plants are rated to handle. If one says “Zones 5-9,” for example, I’m forced to choose a different option.
I know I’m not alone in seeking zone-appropriate plants for my region. You’ve likely done the same. The question is, what do the USDA hardiness zones actually define? And are they the be-all end-all when it comes to determining whether a given plant will grow in a particular region?
Before we dig into our soil for to start gardening, let’s dig into the USDA hardiness zones and learn more.
What are the USDA hardiness zones?
In order to help farmers and gardeners determine which plants are suitable for their region, the USDA has divided the U.S. (and Puerto Rico) into 13 zones based on temperature. The result is a color-coded map with each color representing a different zone.
The zones are further subdivided into “a” and “b” categories. So “Zone 7” technically encompasses “Zone 7a” and “Zone 7b,” with the “a” zone being slightly colder.
Read more: Starting a garden? You need a wheelbarrow.
How are the USDA hardiness zones determined?
The USDA hardiness zones indicate the average minimum winter temperature for a given region, based on data spanning 1976-2005. They measure nothing more and nothing less.
If the coldest annual minimum temperature in your region averages around -17 degrees F, then you live in Zone 5—or, more specifically, Zone 5a, which covers the -20 to -15 degree F range.
Are there shortcomings to the USDA hardiness zones?
Since the USDA hardiness zones only measure one data point—average extreme minimum temperature—there’s actually a lot of variables they overlook. Under the right circumstances, you might be able to grow a Zone 5 plant in Zone 4. Conversely, challenging circumstances can make gardening difficult if you grow plants on the edges of their comfort zones.
A plant might be hardy to Zone 4. But that doesn’t 100 percent guarantee it will survive everywhere in Zone 4.
What are some of the factors the hardiness zones overlook? Well, since the zones are based on average extreme minimum temperature, it’s possible for the minimum temperature in a given year to drop colder than average. This can strain plants on the bubble of tolerability.
Typical snowfall is another consideration. A blanket of snow can protect roots from harsh cold, but only if it’s reliably in place during cold spells. If your region receives cold temperatures, but inconsistent snowfall (with melting in between), you won’t get as much snowfall insulation as in places where snow consistently covers the winter ground.
The list of variables goes on and on.
- What are the typical earliest and latest frost dates for your region?
- Will they interfere with spring blooms or fruit ripening?
- Are summer temperatures hot or mild?
- Does your region afford enough chill hours to grow plants that require a certain amount of winter cold?
And what about your specific microclimate?
- Do you live in an open area exposed to harsh winter winds, or are you sheltered by windbreaks?
- Do you live in a valley (where cold air sinks) or on a hill?
All these factors can affect whether a given plant tolerates the cold or succumbs to freezing weather.
Takeaways from the USDA hardiness zones
Despite their shortcomings, USDA hardiness zones provide solid guidelines for choosing plants based on winter hardiness. And if you’re growing plants rated a zone or two hardier than the zone you occupy (for example, growing Zone 3 and 4 plants in Zone 5), you should be pretty much good to go.
But with a little thought and care, you might be able to stretch the rated hardiness of plants further than you might imagine.
Proper mulching, protection from windbreaks, consistent snow cover, and planting near buildings can all improve the chances of a plant surviving in a hardiness zone on the edge of what it’s rated to handle.
Have fun planting!