- What Are Hardiness Zones and What Are the Best Plants for Milwaukee?
- Hardiness of Plants
- How to Use the Hardiness Zone Map
- Zone 5 Plants
- Get Planting
- What Is the Planting Hardiness Zone in Wisconsin?
- USDA Hardiness Zones
- Wisconsin’s Climate
- Factors Affecting USDA Hardiness Zones
- Typical Garden Plants in Wisconsin
- Summer Heat Designations
- Where In Wisconsin Do Hardiness Zone Shifts Reflect A Changing Climate?
- August 2010
- What may grow well in tomorrow’s gardens?
What Are Hardiness Zones and What Are the Best Plants for Milwaukee?
Gardening in the U.S. has risen in popularity in recent years, with over one-third of American households now growing food at home or in a community garden.
If you’re one of these green thumbs, it’s more important than ever to know in which hardiness zone you live so. That way you can raise plants that thrive in your climate. For Milwaukee residents, that means zone 5, which means you can grow plants that can survive winter temperatures of -20 degrees Fahrenheit and higher.
This quick guide explains the definitions of zones and plant hardiness, and give suggestions for zone 5 plants that do well in the Milwaukee area.
Hardiness of Plants
When a plant is considered hardy, it means it can withstand living in colder temperatures.
This is important for gardeners to know when deciding upon the best flowers, plants, and grass to choose for their landscape design. And to do that, you need to know in which hardiness zone you live.
How to Use the Hardiness Zone Map
To know which plants are suitable for your geographical area, you will need to look up your hardiness zone. A hardiness zone is a section of the U.S. that is defined by its average annual winter temperature.
The United States Department of Agriculture (USDA) Plant Hardiness Zone Map allows you to find which zone you live in. Each zone is also technically divided into two sections (a and b). While Milwaukee is located in zone 5, most of the city lies in zone 5b.
The USDA’s map divides the nation into thirteen main zones and has been updated through the years as new weather data indicated climate change. The most recent version of the map takes into consideration changes in elevation and closeness to bodies of water when determining where each zone begins and ends.
Prior to the map, farmers had to rely on their own common knowledge and publications (such as the Farmer’s Almanac) to know what crops were most suitable for their climates.
By the 1970s, however, the USDA Plant Hardiness Zone Map emerged as the favorite and most accurate map. That’s why it is used to this day as the planting standard for gardeners and landscapers.
Zone 5 Plants
As a Milwaukee-area resident, you probably know that our weather is defined by its cold, snowy winters and warm, humid summers. What you may not know, however, is that our zone experiences a growing season that usually begins around May 15 (the last average frost date) and ends by mid-October.
Zone 5 is not the coldest zone in the U.S., but it does mean you need to choose plants carefully. Fortunately, there is no shortage of colorful perennials and bulbs that thrive in zone 5 that are perfect for Milwaukee-area gardens and lawns.
Here is just a partial list of flowers that do well in zone 5:
- Garden phlox
- Bee balm
- Purple coneflower
In addition, these plants for great for ornamental touches:
- Zebra grass
- Jacob’s ladder
- Coral bells
Several varieties of grass also do well in zone 5. These include bluegrass, fescue, buffalo grass (which is native to this area), and ryegrass.
There’s still time this season to polish your home’s landscaping. The plants listed above are suitable for zone 5, which is a great place to start.
And, if you’d like help with your garden design or ongoing maintenance, don’t hesitate to contact us today. We offer professional landscaping design and grounds maintenance solutions throughout Milwaukee and Waukesha.
What Is the Planting Hardiness Zone in Wisconsin?
Located in the north-central United States, Wisconsin endures a continental climate with warm, humid summers but pronounced winters with cold temperatures and snow. Although elevation can affect temperatures, as does proximity to a large lake, generally the northern counties of Wisconsin have colder winters and cooler summers than the counties farther south and east toward Milwaukee. The conifer and hardwood forests and occasional prairie openings afford good soil and a variety of plants to enjoy in a garden setting.
USDA Hardiness Zones
The U.S. Department of Agriculture’s plant hardiness zones divide the state of Wisconsin into areas with similar average minimum winter temperatures. Overall, the state comprises zones 3a to 5b, meaning expected winter lows range anywhere from minus 40 to minus 10 degrees F. The coldest zone (3), dominates northwestern and north-central Wisconsin, while zone 4 is about 50 percent of the state’s center and in a narrow band along Lake Superior. The warmest zone 5 extends in a wide band closest to Lake Michigan and southward into Illinois.
Sunset Magazine considers Wisconsin climate and places the state into three different zones when discussing garden plants. Hayward and Radisson are in climate zone 45, where the growing season is quite short. The majority of the state, including towns like Eau Claire, La Crosse, Madison, Wausau and Green Bay, are in zone 43, which considers the hot, humid summer air from the Gulf of Mexico and the frigid Arctic air that plunges south in winter. Climate zone 41 runs from Sturgeon Bay to Janesville and Milwaukee; it has a milder winter than in zone 43 but tends to receive much more hot, dry air from the southwest in summer.
Factors Affecting USDA Hardiness Zones
The USDA hardiness zones only take into consideration actual winter low temperatures. They do not account for duration of the winter cold, wind, soil moisture or presence of snow cover. Microclimates occur where topography or man-made or natural features create a smaller “pocket” of warmer winters. For example, the south side of a building or grove of wind-blocking trees is much milder in winter than the northwest side or a wind-swept prairie. Lakes can extend the growing season into autumn because of their radiant heat from the water while tending to keep the spring cooler than usual because the water is slow to warm in spring.
Typical Garden Plants in Wisconsin
Overall, many of the same garden plant materials are grown across Wisconsin. The greatest variety of plants occurs in the southeastern counties because of the slightly milder winters. Pine, spruce, oak, maple and crab apple are common throughout the state, and shrubs/perennials like red-twig dogwood, spirea, lilac, peony, iris, coneflower, peegee hydrangeas and tulips grace gardens all over. The closer you are to Milwaukee and Janesville, the more opportunity there exists to grow plants that are more commonly seen in Chicago to the south, such as flowering dogwoods and Boston ivy.
Summer Heat Designations
Since the 1990s, more attention has been placed on the extent of summer heat in the discussion of plant performance. The American Horticultural Society published its heat-zone map and classifies Wisconsin in four different zones of average annual days above 86 degrees F each summer. Zones 2 and 3 occur in north-central Wisconsin and on the Door Peninsula, where no more than seven to 14 days above 86 degrees are expected. Nearly 90 percent of the rest of the state is in heat zone 4, expecting about 15 to 30 days above 86 degrees. In extreme southern counties along the Illinois border and Mississippi River, as many as 45 days above 86 can occur annually. The closer you are to a Great Lake–within five miles–the fewer hot days are expected.
Where In Wisconsin Do Hardiness Zone Shifts Reflect A Changing Climate?
For centuries, gardeners have depended on tradition and long-honed folk knowledge to inform their spring planting decisions. Although many American gardeners may still prefer to sow their seeds guided by the advice of their forebears today they have another source of information.
In most locations around the United States, gardeners now have at their disposal more than a century’s worth of detailed local climatological data. These records provide a wealth of information about climatic trends, and they hint at the types of plants that may, or may not, survive and thrive in a vegetable patch in Beloit or a flower bed in Bayfield.
But what gardener would choose to sift through mountains of historical weather records when they could be outside getting their fingernails dirty?
That work has already been done, though, thanks to generations of mapmakers who have used ever more sophisticated methods to distill heaps of climate data. Their creations establish a geographic system called hardiness zones which since 1960 have provided an evidence-based resource for planting decisions by defining regions with similar extreme low winter temperatures. These hardiness zones can help gardeners and farmers gauge whether a plant can reasonably be expected to survive the winter in a given location.
In Wisconsin, where winters can usher in brutal cold snaps, that guidance can be key to cultivating a successful perennial garden. But in a world with a changing climate, just how accurate are these maps, and how do their makers continue to ensure these tools are as useful as possible?
A brief history of hardiness zones
Over the past 60 years, the U.S. Department of Agriculture has developed three standard hardiness zone maps, with the most recent version released in 2012. It is far more sophisticated than the previous versions released by the USDA in 1990 and 1960.
However, the 1960 map itself was a major undertaking in its own right. This work was the culmination of a long effort among horticulturists to make use of temperature data that the federal government had collected for decades via its network of weather stations dotted across the American landscape. The map split the continental U.S. into zones according to a multi-year average of extreme cold temperatures recorded at local weather stations, falling within 10-degree increments. Each of the 10 zones were subsequently divided into half-zones in an attempt to provide more detailed guidance.
Swaths of Wisconsin fell into three zones in the 1960 map, with extreme winter temperatures ranging from -40°F in northwestern portions of the state to -10°F in the south and along Lake Michigan.
U.S. Department of Agriculture Wisconsin’s hardiness zones in the 1960 USDA Hardiness Zone map range from Zone 3 (-40° F) in the state’s northwest to Zone 5 (-10° F) in the south and along Lake Michigan.
In 1990, the USDA released an updated map based on more recent climate data and defined by five-degree half-zones. That map broadly resembled the 1960 version, with some notable variations.
For instance, the coldest zone in Wisconsin, Zone 3, expanded over much of the northern third of the state, while Zone 5, Wisconsin’s warmest, shrank considerably. According to this 1990 map, much of the state was actually more likely to experience colder winter extremes than the previous map had forecast.
U.S. Department of Agriculture Wisconsin’s hardiness zones in the 1990 USDA Hardiness Zone map shifted south and east from the 1960 map as the cartographers used a relatively brief and cold climate period to make the map.
However, the methods used by the USDA for the first two maps limited their usefulness. While each provided gardeners and farmers with an evidence-based resource using mapping techniques available at the time, they are blunt instruments by 21st century standards.
For starters, the maps were hand-drawn, and the boundaries between zones lacked spatial detail. Areas with fewer weather stations, such as the mountainous western U.S., and where records were incomplete, were particularly difficult to map in much detail. The 1990 map also drew from a relatively short time frame — just 14 years — and that period happened to include several years with colder than average winters.
These limitations — along with pressure from competing hardiness zone maps put out in the 2000s by organizations like the American Horticultural Society — led the USDA to seek an improved methodology for its mapmaking.
New technologies lead to better maps
The USDA’s solution to improving its hardiness zone mapping approaches came by way of the PRISM Climate Group at Oregon State University headed by geospatial climatologist Christopher Daly. In the early 1990s, Daly developed computer modeling techniques that incorporated more than just weather data to map climate trends in precise spatial detail.
Crucially, Daly recognized the important role of physical geography — known as physiography in scientific circles — in constraining localized climate conditions. His models therefore incorporated variables like latitude, elevation, prevailing winds and proximity to bodies of water, all of which influence weather, including extreme winter temperatures.
The modeling tool Daly developed came to be known as PRISM, and in 2007 the USDA asked him to develop a new hardiness zone map based on its techniques.
Those techniques incorporate sophisticated quality controls, according to Daly.
“We use a number of different screening steps,” he said. “The first one is that we’re pretty big sticklers for data completeness.”
Generally, if a weather station is missing records for more than two days in a month, none of its data are incorporated into the PRISM model, Daly said, though that standard was relaxed slightly for the hardiness zone mapping work.
Additionally, using the physiographic parameters Daly developed, PRISM is able to forecast what a given location’s temperatures would likely be.
“That gives PRISM the power to forecast even in areas where there aren’t weather stations,” Daly said.
With strict quality control measures in place, and using a 30-year climate record, the PRISM group began developing a hardiness zone map with much finer spatial detail than the 1960 or 1990 iterations. The new map was complete by 2009, Daly said, though it took another 3 years for the USDA to release it because the agency needed to build digital infrastructure capable of distributing it and handling expected levels web traffic. The 2012 map was the first to not only incorporate complex computer modeling, but also to live online.
The delay turned out to be prudent as there were tens of millions of accesses within the first few weeks after it launched.
U.S. Department of Agriculture The USDA’s 2012 Hardiness Zone map is the agency’s latest guidance for cold-hardy planting for farmers and gardeners. Computer modeling provided a new level of spatial resolution in the map.
Not only was the new map more precise than ever, but it more closely resembled the 1960 version than the one released in 1990. In Wisconsin, Zone 3 reverted back to a relatively small portion of the northwestern portion of the state, centered around Hayward, while Zone 5 expanded well beyond the Lake Michigan shore to include most of southern and eastern Wisconsin.
U.S. Department of Agriculture Wisconsin’s hardiness zones in the 2012 USDA Hardiness Zone map shifted back to the north and west from those in the 1990 map as cartographers used a later and longer climate period to make the maps.
Reflecting PRISM’s incorporation of variables outside of weather data, the 2012 map also for the first time displayed the impact of physical geography on plant hardiness in Wisconsin. In particular, the boundary between zones 4 and 5 closely follows the rolling topography in parts of the Driftless area in southwestern Wisconsin. Hilltops and higher ground are part of the warmer Zone 5, while river valleys are generally classified as Zone 4.
“Cold air pools in valley bottoms and river valleys, even shallow ones,” Daly said. “There is some of that in Wisconsin. Cold pools at night, especially during an arctic outbreak, and if the wind dies down, cold, dense air settles into river valleys and depressions. This can change your ability to grow certain perennials quite dramatically.”
However, there are still questions about the localized usefulness of this most recent map. Lisa Johnson, a horticulture educator with University of Wisconsin-Extension Dane County, said topography remains a confounding variable for some Wisconsin gardeners, as do differences between rural and urban areas, the latter of which often experience a heat island effect.
“I take it with a grain of salt,” Johnson said of the USDA’s 2012 map, especially since extreme winter temperatures are not the only important parameters for gardeners and farmers to consider.
The timing of spring and autumn frosts can also dramatically impact the ability to grow certain plants, though frosts are usually more of a concern for annual flowers and crops that require a certain period of frost-free weather to reach maturity. Hardiness zones and first and last frost dates may generally follow similar trends — i.e., more northerly locales will generally have shorter growing periods and colder winter extremes — but they aren’t totally correlated, Daly explained.
“One of the misuses of the map is that people don’t really understand what the plant hardiness statistic is,” he said. “It’s often used to describe when last frost in the spring might be, but it doesn’t have to do with that. You could have a plant hardiness zone that’s pretty warm but the last freeze is late.”
How does climate change fit in?
One major phenomenon that will likely alter Wisconsinites’ ability to grow certain perennials in the decades to come is climate change. Models show that by the 2080s, many locations in Wisconsin are projected to have climates similar to those currently experienced in locales far to the south and east, like Kansas City and Philadelphia.
But while some regions in the USDA’s 2012 map shifted to warmer hardiness zones from previous iterations, Daly cautioned against consulting it as a proxy for understanding climate change. That’s because hardiness zone maps provide only a tiny (albeit important) snapshot into local conditions: extreme low temperatures.
Plant hardiness is “a really volatile statistic in time,” Daly explained. “It’s only one day per year over 30 years. Even if the average is slowly creeping up on us, as a single, volatile statistic it’s just not a very good metric for looking at climate change.”
His view reflects the USDA’s own guidance against interpreting the maps as evidence for climate change.
Moreover, the three USDA maps were produced using different methodologies, so a comparison of their zone boundaries is as much or more a comparison of the mapmaking process as it is the underlying data, Daly said.
The USDA’s 1960 Hardiness Zone map was the agency’s first attempt at providing cold-hardy planting guidance to farmers and gardeners. U.S. Department of Agriculture The USDA’s 1990 Hardiness Zone map was the agency’s second attempt at providing cold-hardy planting guidance to farmers and gardeners. U.S. Department of Agriculture The USDA’s 2012 Hardiness Zone map is the agency’s latest guidance for cold-hardy planting for farmers and gardeners. Computer modeling provided a new level of spatial resolution in the map. U.S. Department of Agriculture Wisconsin’s hardiness zones in the 1960 USDA Hardiness Zone map range from Zone 3 (-40° F) in the state’s northwest to Zone 5 (-10° F) in the south and along Lake Michigan. U.S. Department of Agriculture Wisconsin’s hardiness zones in the 1990 USDA Hardiness Zone map shifted south and east from the 1960 map as the cartographers used a relatively brief and cold climate period to make the map. U.S. Department of Agriculture Wisconsin’s hardiness zones in the 2012 USDA Hardiness Zone map shifted back to the north and west from those in the 1990 map as cartographers used a later and longer climate period to make the maps. U.S. Department of Agriculture
Another federal agency weighs in
Around the time the USDA was releasing its 2012 map, the National Oceanic and Atmospheric Administration developed another set of hardiness zone maps for the explicit purpose of tracking a changing climate.
NOAA’s maps are based on two 30-year periods called climate normals. One covers the period between 1971 and 2000 and the other spans 1981 to 2010. The 2012 USDA map also uses the 1981 to 2010 data, and a comparison of the two shows fairly similar zones, though the PRISM-produced map is more finely detailed and shows cold hardiness zones stretching farther south in the upper Midwest, including Wisconsin, than does NOAA’s map of the same period.
NOAA’s Hardiness Zone map using climate data from 1971-2000. National Oceanic and Atmospheric Administration NOAA’s Hardiness Zone map using climate data from 1981-2010 shows hardiness zones moving northward as compared to the map using data from 1971-2000. National Oceanic and Atmospheric Administration This map highlights the U.S. regions within warmer hardiness zones in NOAA’s 1981-2010 map versus its 1971-2000 map. National Oceanic and Atmospheric Administration
At the same time, a comparison of the two NOAA maps reveals that hardiness zone shifts seen in Wisconsin could be due to the changing climate. These shifts affect a narrow band from the state’s southwest corner to the Michigan border near Marinette, as well as a large region in the northern third of the state. Hardiness shifts would be expected under climate change modeling, which project a larger warming effect on nighttime minimum temperatures than on daytime highs, and hardiness zones are based on the most extreme of those minimums.
If climate projections bear out over the coming decades, hardiness zones could continue their northward march.
Oregon State researcher Christopher Daly said he is hopeful the USDA will continue to update its maps going forward, adding that he would be happy to provide PRISM’s services again. He has had informal talks with the agency about an update once the latest climate normals are in after 2020.
“I would like to see the map updated every 5 years or so,” he said.
Over the long-term, Daly hopes the USDA’s maps could provide an increasingly detailed snapshot into local growing conditions and how they are changing due to both natural variations and human-caused climate change. Daly viewed it as a responsibility of agency to continue the updates, noting that these maps will always be in high demand by gardeners, farmers and the horticulture industry.
“Once you start leaving a map not created there’s a vacuum and it’ll be filled by less reliable maps. We want these to have the highest integrity possible,” he said.
The Farmers’ Almanac promised a “wet, wild, very cold” winter for the upper Midwest. Instead, snowdrops appeared at Olbrich Gardens before the holidays. Daffodils emerged in late February. Where are we? Atlanta? Not quite. Try Chicago. According to the recently released USDA 2012 Hardiness Zone Map, Madison is no longer in Zone 4. We are now in Zone 5, along with Chicago. Gardeners and growers follow the Hardiness Zone Map to determine which plants will thrive where they live. The map is based on the average annual minimum winter temperature, divided into 10-degree Fahrenheit zones.
The map, which hadn’t been updated since 1990, clearly shows a changing climate. In a news release, USDA takes pains to point out that the new map is based on temperature data from a longer time period (1976-2005). The 1990 map was based on temperature data from only a 10-year period (1976-1986). And the shifts in each area are small: just five degrees, or one-half zone.
Bottom line: Don’t start planting figs and bougainvillea just yet. But do think about making slight adjustments in your gardening habits, and be prepared for a few surprises.
Samantha Peckham, a horticulturist at Olbrich Gardens who manages the wildflower and herb gardens, says the variances they’re seeing are slight, but noticeable.
“We are sticking with what has always done well here, with a few experiments here and there, like leaving some annuals in that we think might make it through,” says Peckham. Petunias and marigolds might have survived one of the top 10 warmest winters ever in Madison.
But what’s good for annuals is bad for the native perennials, woody shrubs and evergreens that have adapted to our cooler climate. Peckham reminds gardeners that freeze-and-thaw cycles are hard on plants, stressing their root systems, as warm spells trick them into expending stored energy too soon.
“The best winters are ones with a lot of snow, like the previous two years,” says Peckham. “Plants are insulated by the snow, not damaged by sun or wind, and they are safely dormant.”
Burpee Seed Co. acknowledged the cold’s detrimental effect on perennials in a news release about the zone shifts (the entire gardening world, it seems, has been abuzz about this). But chairman and CEO George Ball claimed it was a boon for vegetable gardeners. “For seed customers, global warming results in slightly warmer and longer summers, which means – since annuals literally feed on heat and light – higher yields of vegetables and longer displays of flowers. In other words: good news! Certainly nothing to worry about,” Ball said, according to the release.
Laura Jull, associate professor of horticulture at UW-Madison, takes a more cautious approach. “The zone map isn’t a predictor of the future,” she warns. “It doesn’t affect when you plant your veggie seeds outside. That has to do with the last frost-freeze date.”
Jull advises gardeners to be conservative when it comes to planting. Check the cold hardiness zones on plant tags, but don’t take them as gospel. Out-of-state growers and mail-order companies will sometimes overestimate cold hardiness zones. Buy local, she advises, since local nurseries “have to be accurate.”
Just as plants have a certain critical minimum temperature, so do insects. UW entomologist Phil Pellitteri, who grew up in Madison, is seeing species here that weren’t present 35 years ago. Some of these have a detrimental effect on the garden, such as the bean-leaf beetle that chews down the leaves of green beans, and the euonymus caterpillar, which decimates the burning bush shrub. Until recently, the caterpillar couldn’t survive our winters, but Pellitteri says he’s watched it migrate to Madison, and now to the Dells, over the years.
“I feel like I’m in central Illinois, instead of Wisconsin,” says Pellitteri. He points out, though, that not all insect migrants are undesirable.
“Mantids – they’re good guys in the garden -are not native to the Midwest, but I’ve seen them flying overhead and at the Arboretum,” he says. “Giant swallowtail butterflies are doing great.”
Warmer temperatures are almost always the reason a new insect shows up further north. But the picture gets a bit more complicated when you’re talking about unusual numbers of a pest that’s been here a long time, such as Japanese beetles, a species that can survive cold pretty well. Regardless of why the pests are here, gardeners have choices in how to deal with them, says Olbrich’s Peckham.
“In our new rose garden, we’ve incorporated woody perennials and trees into the beds, instead of just planting roses en masse. This minimizes the need for chemicals to fight powdery mildew and Japanese beetles. If you intersperse with other plants, the beetles can’t just hop from plant to plant.”
Staff at Olbrich have been working to educate themselves, and others, on sustainable gardening practices. The botanical garden is using fewer chemicals, minimizing lawn, turning to local experts on composting, and offering a variety of classes on native plants, bird-friendly gardens and low-impact gardening.
“We see not only the new USDA map, but all the other research that’s out there,” says Peckham. “We understand that humans are impacting the planet, and we are trying to model, to the public, that you can do things in a sustainable manner and still have a beautiful garden.”
The notion of a longer growing season with warmer nights and warmer winters combined with wetter springs sounds very favorable for Wisconsin gardeners, but the projected weather extremes are less rosy.
© Robert Queen
What may grow well in tomorrow’s gardens?
Climatologists share a vision of how Wisconsin gardens might change as the climate warms up.
Michael Notaro, David Lorenz, and Daniel Vimont
While global warming seems intangible and remote to many, slow signs of climate change are increasingly clear across the state. For example, our gardening experiences are likely to be transformed as the century progresses.
Studies of weather station records in Wisconsin from 1950-2006 by UW-Madison researchers Shawn Serbin and Christopher Kucharik show the average temperature has warmed 1.1° F (2.5° F in northern Wisconsin) with the greater warming in winter (2.5° F) and spring (1.7° F). Annual precipitation has also increased in the past 60 years by about 3.1 inches per year, especially in autumn.
There are other more subtle changes across Wisconsin landscapes. Nina Leopold Bradley compared first bloom date for 36 spring flower species in Sauk County during the 1970s-90s with observations her father, Aldo Leopold, made during the 1930s-40s. She found shocking evidence that first spring blooms were occurring 2-2 ½ weeks earlier for common plant species such as baptisia, butterfly weed and forest phlox. Given such dramatic responses, how might Wisconsin gardening change later this century as warming is projected to amplify?
Working groups from the Wisconsin Initiative on Climate Change Impacts (WICCI) are making such projections and developing strategies to adapt to those potential changes. The table to the right outlines what we might anticipate at mid-century and the end of the century if greenhouse gas emissions are moderated or continue at high rates. The key changes projected for Wisconsin’s climate this century include:
- Significant warming, particularly during winter and spring, with more warming at nighttime than daytime
- A longer growing season, including an earlier onset of spring
- More hot days and fewer cold nights
- Shorter frost and snow seasons
- Greater annual precipitation, particularly in winter and spring
- Prolonged droughts, with rainfall more concentrated into fewer, more intense downpours
Based on these WICCI climate projections, new maps of the USDA plant hardiness zones for Wisconsin were produced using the anticipated annual average minimum temperatures. Gardeners and seed producers alike use these zone maps to gauge which plant varieties they choose to plant because the cultivars are hardy enough to survive the harsh extremes of winter and summer weather in a given location. The maps on the left show the plant hardiness zones for modern-day (1980-99), mid-21st century, and late-21st century. The maps of possible future conditions are further separated into what might happen if we are able to slow the rate of greenhouse gas emissions or if these emissions continue at their current high rates.
Currently, Wisconsin’s plant hardiness zones range from a low of 3b up north in Washburn County to a high of zone 5b in Milwaukee County. By midcentury, warming will likely shift hardiness zones to growing conditions typically found one or two zones further south, with Washburn County moving to zone 4b and Milwaukee County classified as zone 6a. These higher temperatures would make northern Wisconsin more conducive for perennial plants such as common foxglove, garden phlox, yellow archangel and common periwinkle, which previously flourished further south. Likewise, the transition in southeastern Wisconsin into zone 6a by the middle of this century would allow such warm weather perennial plants as climbing aster, stinking gladwin, swamp sunflower and fringed campion to thrive in southern Wisconsin gardens.
Climate projections are likely to diverge even more by century’s end. Shifts of two hardiness zones are projected for Wisconsin under the more favorable scenarios of lower greenhouse gas emissions vs. shifts of three to four zones if the world is emitting more CO2. If we hit these higher levels, by the year 2100 we would predict the growing conditions in Wisconsin would favor the plant species currently grown from central Illinois as far south as northern Mississippi. Given those conditions, northern Wisconsin gardens might flourish with a dramatically richer variety of perennial flowering plants than is presently possible. The reduced winter chill would favor such plants as chrysanthemums, spring starflowers, Aaron’s beard, and Stokes’ aster along with the perennials that currently do well in southern states like southern maidenhair, Autumn sage, Confederate roses and atamasco lilies.
The notion of a longer growing season with warmer nights and warmer winters combined with wetter springs sounds very favorable for Wisconsin gardeners who would like to grow a wider variety of perennials that are currently restricted by bitter winter temperatures. However, the weather extremes projected for such growing seasons are less rosy. More frequent floods and heavy downpours, prolonged droughts and extreme heat could become the norm. Summer would bring higher evaporation rates and drier soils that would need greater care, mulching and preparation to sustain crops. Rising temperatures would likely encourage the further spread of invasive plant species. Our aim in sharing these projections is to spur efforts to reduce greenhouse gas emissions and to dampen the magnitude as we adapt to changing conditions that climate change can bring to the face of Wisconsin.
Michael Notaro and David Lorenz are associate scientists for the Center for Climate Research (CCR) at UW-Madison’s Nelson Institute. Daniel Vimont is an assistant professor of Atmospheric and Oceanic Sciences at UW-Madison and also affiliated with CCR.