Milder winters changing USDA plant hardiness zones in Minnesota

Winters in Minnesota are not what they used to be. The vicious winters of the 1970s are few and far between as we move through the 2020s. And yes, climate change is the primary reason.

Data from the National Oceanic and Atmospheric Administration and the nonprofit group Climate Central show winter on average has warmed more than 5 degrees across Minnesota since 1970.

The updated 2023 U.S. Department of Agriculture hardiness zone map shows changes in Minnesota and the Midwest. The map is based on the average annual extreme minimum temperature at any given location.

Here’s the detailed description from the USDA:

The USDA Plant Hardiness Zone Map is the standard by which gardeners and growers can determine which perennial plants are most likely to thrive at a location. The map is based on the average annual extreme minimum winter temperature, displayed as 10-degree F zones and 5-degree F half zones.

You can see on the map at the top of this post that Minnesota ranges from zones 3a (minus 35 to minus 40 degrees) in the far north, to zone 5a (minus 15 to minus 20 degrees) in the Twin Cities and most of southern Minnesota.

Those numbers represent the average annual extreme minimum temperature. Another way of saying that is it’s the average lowest temperature of the year at a given location.

Changes in 2023

The 2023 update to the map includes some notable changes for Minnesota and the Midwest. Here are a few bullet points that caught my eye as highlighted by the Midwest Regional Climate Center:

Changes to note when comparing the USDA Plant Hardiness Zones from 2012 to 2023 across the Midwest:

• More of the Chicago area & Lower Michigan are now in Zone 6.

• Twin Cities moved from 4b to 5.

• *Most* places south of I-80 are now zone 6 (save southern IA & northern IL)

Taking a closer look at the Twin Cities area you can see our designation as zone 5a is similar to places like Mankato and most of southern Minnesota:

It’s logical to say that part of the change is due to the continued trend of climate change and milder winters overall. The lowest extreme temperatures in the Twin Cities area have warmed around 12 degrees on average since 1970, according to data from NOAA and Climate Central.

But part of the change is also likely attributable to the Twin Cities expanding urban heat island effect. The effect of more asphalt and concrete reradiating stored heat from the daytime produces milder overnight minimum temperatures in the urban heat island zone.

So the overall trend of climate change in Minnesota continues to be magnified up in multiple data sets. New types of plants and animals can survive the Minnesota winters of today compared with the 1970s. Our lake ice season is 1 to 2 weeks shorter most years and ice is thinner and less stable.

These are just some of the evolving changes as Minnesota’s winters continue to trend milder due to climate change.