Redrawing the Map: The 2023 USDA Hardiness Zones
In November, the USDA released an updated Plant Hardiness Zone Map (PHZM) for the first time in over ten years. While other hardiness maps have come and gone, the USDA map, first created in 1960, has remained the most authoritative version, and it’s widely considered the standard we use to determine which plants are likely to succeed in any given location in the United States and its territories. In addition to the 80 million gardeners and growers who make up the largest user base of the PHZM, it also serves as a data point in scientific models and as a reference used by the USDA in setting crop insurance standards. Like the 2012 map, the latest version divides the country into 13 zones, with each zone further divided into “a” and “b” half zones. However, the 2023 update has shifted about half the country into a warmer zone. In addition, the latest map is more user-friendly and interactive, and it includes a “Tips for Growers” section, with information on how to use the map, as well as videos and articles intended for growers and home gardeners.
The biggest takeaway from the newest map is the expansion of warmer zones and the shrinking of colder ones. The map offers a stark visualization of the warming of our region and our country. When you compare the 2012 and 2023 Illinois maps side by side, you will see that Zone 6a advanced about 60 miles north, from Springfield in 2012 to Peoria in 2023, and expanded in the Chicagoland region. Zone 7a has also continued its northern march, and, for the first time ever, a sliver of southern Illinois is now designated as Zone 7b. Similarly, Wisconsin now has four cold hardiness zones due to the addition of Zone 6a along the western shore of Lake Michigan and parts of Door County, and Zone 3 is rapidly vanishing from the state.
The plant hardiness zone map is no doubt an invaluable tool for growers, and it’s become much more than that over the years. Referencing the hardiness zone of the plants we grow has become instinctive, almost second nature. When we’re introduced to a new plant, one of the first questions we ask is “What zone is it?”. But those of us in the green industry are also well aware of the inherent limitations of the hardiness map and understand that it has the potential to be deceiving to our customers and the casual home gardener. This is due, in large part, to the temperature data used by the USDA to designate the zones: the 2023 map is based on averages of the lowest annual temperatures at specific locations over the 30-year time period between 1991 and 2020. So, the maps don’t take into account the lowest actual temperatures, variability from year to year of extreme winter temperatures, or lowest average temperatures prior to 1991.
There are more than a few problems with this. One is that we’ll continue to experience cold snaps in our Midwest winters where temperatures drop below the average range for our zone. When this inevitably happens, severe temperatures, combined with wind chills, will push the limits of marginally hardy plants, regardless of whether they’re considered suitable for the zone they were planted in. Another weakness of the maps is they don’t account for microclimates - those tricky areas within a landscape that are warmer or colder than their designated zone. But perhaps the biggest limitation of cold hardiness maps when it comes to predicting plant survival is the fact that cold hardiness is extremely complex, depending on factors that go well beyond the average low temperature in any given season. Dormancy in plants is triggered by a gradual decrease in temperatures in combination with the shorter days of fall. When cold hardy plants are allowed to properly harden off and achieve full dormancy, they can withstand extremely cold temperatures. But it’s become clear in recent years that our climate will no longer allow this type of textbook fall scenario. We’re not only seeing warmer temperatures later and later into fall, but also more instances of erratic temperature fluctuations. If warm temperatures have delayed plants from reaching dormancy, stems and buds are far more likely to be damaged or killed from a sudden freeze or cold snap. Along those same lines, prolonged warm temperatures in mid-winter can trick plants into de-acclimating, making them more susceptible to damage from freezing temperatures in late spring. This is especially true for non-native plants that don’t rely on photoperiod to break dormancy. On top of all this, our increasingly hotter and drier summers will continue to impact the hardiness of woody plants, as drought-stressed plants are more susceptible to winter injury.
Having said all this, no metric is perfect, and the hardiness zone maps are still an indispensable tool when it comes to guiding our decisions as we plant and plan our landscapes and gardens. But our industry will need to be focused on educating and supporting our customers more than ever going forward. There are few things more frustrating to any gardener, regardless of experience, than losing a plant, and we should do all we can to grow and recommend plants that are most likely to succeed in their landscape. After all, successful gardeners are the key to long-term growth in our industry.
