Social media is buzzing today about a new USDA Hardiness Zone map for 2023 that puts much of the Twin Cities and a sizeable section of southwestern Minnesota in hardiness zone 5a. As one gardener noted on my Instagram post about this, this is an identity crisis for many of us who pride ourselves on being able to grow things in the toughest weather. Are we really the same hardiness zone as northern Iowa or the area just west of Madison, Wis.? And, if we are, what does that mean for gardeners as they select plants?
What are Hardiness Zones?
The U.S. Department of Agriculture calculates hardiness zones based on the coldest temperature likely to occur in a region each year. It’s not the coldest temperature ever or the coldest it could be, just the average coldest. USDA Zone 4b, where the Twin Cities was located on the 2012 version of the map, has average lowest lows of -20 to -25 degrees F. Zone 5a, where the new map puts us, has lowest lows of -15 to -20. It doesn’t seem like a huge change, but for gardeners in the North, zone 5 is sometimes seen as the Holy Grail because there are so many more plant options that can withstand zone 5 temperatures compared to zone 4.
The new map was calculated by the PRISM Climate Group at Oregon State University using climate and weather data from 1991-2020. Prior maps used data from 30-year periods before they were issued. The new map is based on GIS (Geographic Information System) data and allows gardeners to look at finer detail than earlier maps. For instance, you can check your zip code directly to find out which zone your yard is in, a particularly helpful tool for those on the border between zones.
What Changes Mean?
According to the USDA and Oregon State website, the changes reflect more accuratee and more recent data. Obviously, technology has advanced tremendously even since 2012, so the data in the set is more specific. Here’s what the report says:
The new PHZM is generally about one quarter-zone warmer than reported in the 2012 PHZM throughout much of the United States, as a result of a more recent averaging period (1976-2005 vs. 1991-2020). However, some of the changes in the zones are the results of additional data sources and improved interpolation methods. These zone shifts can sometimes result in a cooler, rather than warmer, zone. The most substantial changes produced by additional data sources and improved interpolation methods are seen in upland areas of Alaska.Plant Hardiness Zone website
Minnesota Zones Now
According to the new map, the Twin Cities from the Mississippi Rivers to halfway through Hennepin County and from about Farmington up to Ham Lake in Anoka County are zone 5a. If you live in Scott, Carver, northern Washington or southern Dakota counties, you’re still zone 4. The area from near Winona south right on the Mississippi and a big swath of southwestern Minnesota, including towns like Mankato, Slayton and Marshall are zone 5.
Zone 4b has now shifted up past Glenwood in Pope county. Zone 3, which was once almost half of Minnesota is now just the extreme northern quarter of the state, from the Canadian border to about Hubbard County. Grand Rapids, for example, is not zone 4.
Changing What You Plant?
Does this mean you should go wild planting warmer-zone plants? Maybe, maybe not. Plant hardiness and plant survival depend on many things other than low temperatures. Snow cover, early or late frosts, drought, wind, soil moisture and how long a cold snap lasts. You also can have a micro-climate (cold or warm) that is not captured by the map.
If a plant is thriving in your garden now, it will continue to thrive. All our great northern plants such as peonies, serviceberry, grasses, native flowers, shurbs and trees will continue to thrive. If you have a zone 5 plant you are itching to try, go for it. But my mantra is always, don’t plant anything you can’t afford to kill. Or, as Don Engebretson said on Instagram last night, “$20 plants.” What he meant was don’t plant anything too expensive until you are sure your garden is really zone 5.