The United States Department of Agriculture (USDA) Plant Hardiness Zone (PHZ) map is the definitive resource used by gardeners and growers to determine which perennial plants can survive winter temperatures in a specific location. This system divides the country into zones based on long-term climate data, providing a standard reference for plant selection. The map guides plant choices for perennial survival by assessing the likelihood of cold weather damage. Knowing what hardiness zones cover Wisconsin is the first step toward a successful landscape in the upper Midwest.
Understanding the Hardiness Zone System
The USDA Plant Hardiness Zone map measures the average annual minimum winter temperature over a 30-year period. This average is the single metric used to define the zones, which are broken into thirteen zones, with each number representing a 10-degree Fahrenheit range.
A lower zone number indicates a colder climate; for example, Zone 3 is colder than Zone 5. Each major zone is further divided into two 5-degree subzones, designated by the letters ‘a’ and ‘b’. The ‘a’ subzone represents the colder half of the temperature range, while the ‘b’ subzone represents the warmer half. The most recent map update, released in 2023, incorporates data from 1991 to 2020, resulting in more precise zone boundaries.
Wisconsin’s Specific Hardiness Zones
Wisconsin spans a wide range of zones, covering Zones 3, 4, 5, and a small part of 6, according to the 2023 USDA map. Zone distribution depends heavily on latitude and proximity to the Great Lakes. The coldest areas, featuring isolated pockets of Zone 3b (-35°F to -30°F), are found in the far northern reaches, specifically in parts of Douglas and Washburn counties.
The majority of the state, including central and northern regions, is dominated by Zone 4, split into subzones 4a and 4b. Zone 4a (-30°F to -25°F) covers a large portion of the northwest. The more moderate Zone 4b (-25°F to -20°F) extends across the central part of Wisconsin, including cities like Eau Claire and Wausau.
The warmest zones are concentrated in the southern tier and along the Lake Michigan shoreline, which benefits from the lake’s moderating effect. Southern cities, including Madison, mostly fall within Zone 5b (-15°F to -10°F). A narrow strip along the southeastern coast, encompassing the Milwaukee-Kenosha corridor, has shifted to Zone 6a (-10°F to -5°F), the warmest zone in the state.
Applying Zone Data to Planting Decisions
Gardeners use their local hardiness zone number to select perennial plants that can reliably survive the winter. A plant’s hardiness rating indicates the coldest zone in which it is expected to survive. For instance, a plant labeled for Zone 4 is likely to fail in Zone 3 but will thrive in Zone 5. The rule is to choose plants rated for your specific zone or any zone with a lower number, as they tolerate colder conditions.
This zone data is relevant for trees, shrubs, and perennial flowers, which must survive multiple winter seasons. When selecting plants, gardeners should compare the zone listed on the plant tag with their location’s zone to ensure a match.
The hardiness zone system is not a factor in choosing annual plants, as these complete their life cycle in a single growing season and are not expected to survive the winter. For long-term landscape planning involving perennial species, using the zone data correctly is fundamental to avoiding costly plant losses due to severe winter cold.
Beyond the Zones: Local Climatic Factors
While the PHZ map offers a solid foundation for plant selection, it does not account for localized conditions that create microclimates. These small-scale variations can significantly influence a plant’s ability to survive. Elevation changes, common in western and northern Wisconsin, can lead to cold air pooling in valleys, making those spots colder than the official zone suggests.
Proximity to large bodies of water, particularly Lake Michigan and Lake Superior, provides a tempering effect, often delaying the first and last frosts. Urban heat islands in cities like Milwaukee and Madison are also warmer than surrounding rural areas because buildings and paved surfaces retain heat. Planting near a south-facing brick wall or in a sheltered courtyard can raise the local temperature by half a zone, offering a survival advantage to marginally hardy plants.
Other environmental factors not reflected by the hardiness zones include snow cover, which acts as an insulating blanket for plant roots, and soil drainage. A plant rated for a specific zone may still perish if its roots sit in waterlogged soil during a freeze. Gardeners must consider these local nuances along with the official zone data to ensure long-term success.