In 2023, the United States Department of Agriculture (USDA) released the latest Plant Hardiness Zone Map, which helps gardeners across the country make informed decisions about which perennials to plant.
Because perennials last year after year, it’s important to plant varieties that won’t freeze and die in the colder winter months — and the Plant Hardiness Zone Map indicates which varieties are able to withstand the winter temperatures of each area.
The plant hardiness zones are based on the “average annual extreme minimum winter temperature,” according to the USDA. In simpler terms, the zones are created by taking an average of the coldest winter temperature each year from the past thirty years. In the previous map from 2012, the USDA used data from 1976-2005. The current version uses data from 1991-2020.
Based on the average minimum temperatures of each area, the map is divided into thirteen different zones (with 1 being the coldest and 13 being the warmest), each with two subzones. Zones are broken down into intervals of 10 degrees, and subzones are split into 5-degree intervals.
Compared to the 2012 version of the map, in 2023 around half of the country (including many counties in Wisconsin) has shifted half of a zone (or one subzone) warmer. For example, in 2012 most of Brown County was categorized as zone 5b, with an average lowest winter temperature falling between -15 and -20 degrees — in 2023, Brown County is primarily in zone 5a, with an average lowest winter temperature between -10 and -15 degrees.
What does this shift mean? Should you start investing in new varieties of plants better-suited for warmer climates?
Don’t scrap all of your garden plans for next spring just yet! The hardiness zones don’t have much influence over annuals, and most varieties of perennials are suited for a wide range of zones.
Additionally, even if warmer-weather perennials can now survive the lowest temperatures of Wisconsin winters, there are still many other winter elements that would cause them to fare poorly here.
Many areas across Wisconsin have been seeing a decrease in snowfall each year, which acts as an insulator against the cold air.
“In the last few years we haven’t had much snowfall, so the cold temperatures we do get are able to penetrate the soil further and potentially cause damage to roots, which are the least hardy part of a plant,” says Mark Konlock, the Garden’s Director of Horticulture. “Even if winter temperatures aren’t as cold as they have been historically, the cold may go deeper into the ground because of less snow cover.”
The warmer winters don’t just impact plant life, though. Garden pests and fungi that were once unable to withstand the cold Wisconsin winters have been able to overwinter in recent years, making it easier for them to overtake your garden in the warmer months.
One example of this can be seen in squash bugs, which feed on vine crops and release toxins into their leaves. Historically, these bugs aren’t able to survive winter in Wisconsin, which keeps their populations at bay. However, recent Wisconsin winters haven’t been cold enough to kill them off, so they’re becoming more common throughout the state.
Similarly, Southern blight is being detected more frequently in Wisconsin. This fungus, which is commonly found in the South, affects over 500 different plant species – including vegetables, ornamentals, florals, and even small trees, in some cases. While this fungus is best-suited for tropical and sub-tropical climates, it’s recently been able to overwinter in Wisconsin due to the warmer winter temperatures.
Additionally, while the plant hardiness zones aren’t inherently tied to the growing season, frosts, and summer temperatures, they do indicate that we may see greater variation or warmer averages in these other elements, as well.
When it comes to gardening, a few degrees can have a big impact. The updated plant hardiness zones don’t mean that you should dig up all of your perennials and completely redesign your garden, but you might want to start paying closer attention to your plants. By staying informed and keeping an eye out for pests and disease, you’ll be well-prepared to keep your garden thriving for years to come.
Sources
- https://planthardiness.ars.usda.gov
- https://fonddulac.extension.wisc.edu/usda-unveils-new-plant-hardiness-zone-map
- https://apps.npr.org/plant-hardiness-garden-map/?name=Green+Bay&state=WI&lng=-87.986568&lat=44.521542
- https://stateclimatologist.web.illinois.edu/2023/11/16/new-usda-plant-hardiness-zones-map
- https://hort.extension.wisc.edu/files/2023/12/New-USDA-Cold-Hardiness-Zone-Map-copy100.pdf
- https://planthardiness.ars.usda.gov/pages/how-to-use-the-maps
- https://www.pbs.org/newshour/science/climate-change-is-shifting-plant-growth-zones-heres-what-to-know-for-your-garden-this-year
- https://pddc.wisc.edu/2024/03/27/march-2024-warming-wisconsin-weather/#:~:text=Southern%20blight&text=The%20fungus%20can%20also%20kill,and%20subsequent%20wholesale%20plant%20destruction.
