One of the most important things cold-climate gardeners need to know is their plant hardiness zone. Cold hardiness zones are established by the U.S. Department of Agriculture every 20 years or so. The lower your zone number, the colder it is.
The most recent cold hardiness zone map was released in 2012 and most of Minnesota is in USDA Zone 4, which means the extreme low temperature in the zone gets to between -20 F and -30 F. The northern third or so of the state is in zone 3 (-30 to -40). To find out what your hardiness zone is down to the zip code level, check out this handy interactive map from USDA.
Each zone is divided into “a” and “b” zones, each of which has about a 5 degree difference in extreme cold temperatures. So, gardeners in St. Cloud are in USDA Zone 4b (-30), while those in the northern suburbs of the Twin Cities, just 30 miles away are in 4a (-25). A tiny arc of southern Minnesota in Jackson and Martin counties are rated zone 5b.
Why Hardiness Zones Matter?
Hardiness zones are used by plant companies to establish ratings for their plants so gardeners know whether plant particular species will survive in their area. (Oaks in Minnesota, yes! Crepe Myrtles, no!) When they test new plant introductions around the country, they determine how cold it can get before a plant dies. While some gardeners think plant breeders tend to be too conservative — rating a plant as zone 5 (Iowa, southern Wisconsin, Illinois) when it will survive in Minnesota — hardiness zones are a great guide for determining which perennials, shrubs and trees will survive in your area.
If you happen to live on the cusp of one zone or the other, such as gardeners in Brainerd or Pine County, you need to consider the microclimate of your particular property. Wind breaks, extra sun or shade and a variety of other factors can affect the specific climate in your yard.
Plant Tags and Hardiness Ratings
To determine whether a tree, shrub or perennial will survive in your yard, check the plant tag. Plant tags almost always list the hardiness rating, as do seed packets for perennials. If it’s not on the front, turn the tag over to look at the back side under care instructions. You can plant anything with a plant hardiness zone colder than your zone. So, gardeners in USDA Zone 4 can plant all the zone 3 plants they want. If you are planting zone 5 plants, though, don’t be surprised if it struggles.