Wisconsin Natural Resources magazine

Wisconsin Natural Resources magazine

Photo of gardeners © Robert Queen

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

August 2010

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.