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.