The success of any garden or farm in a climate with distinct seasons, such as Wisconsin, depends on selecting plants that can survive the winter. Plant hardiness zoning provides a fundamental guide for this selection process. Since Wisconsin spans a large geographical area with varying influences from the Great Lakes, its climate is diverse. Gardeners must identify their specific zone to ensure their trees, shrubs, and perennial flowers can tolerate the coldest temperatures of the season.
Understanding the Hardiness Zone System
The official tool used by growers to determine plant survival is the United States Department of Agriculture (USDA) Plant Hardiness Zone Map. This map divides the country into zones based purely on the average annual extreme minimum winter temperature observed over a 30-year period. The current map uses data collected between 1991 and 2020.
The system uses 13 main zones, with each zone representing a 10-degree Fahrenheit temperature range. Lower zone numbers indicate colder climates, while higher numbers represent warmer climates. Each 10-degree zone is further divided into two subzones, ‘a’ and ‘b,’ which represent 5-degree increments for greater precision. For instance, a plant rated for Zone 5 is expected to withstand average annual minimum temperatures between -20°F and -10°F. This designation allows gardeners to compare a plant’s cold tolerance against the typical low temperatures in their location.
Mapping Wisconsin’s Climate Zones
Wisconsin’s geographical position and proximity to the Great Lakes cause it to encompass a range of cold hardiness zones, covering Zones 3b through 6a. The coldest areas are located in the north, and the warmest are along the lake shores. Northern regions, particularly the Northwoods and areas near Lake Superior, experience the most severe cold, falling into Zone 3b (average minimums of -35°F to -30°F) and 4a. These regions have a shorter growing season.
Moving south and west, the central and southwestern parts of the state are primarily classified as Zone 4b and 5a. Zone 4b covers areas with average minimum temperatures between -25°F and -20°F. Zone 5a is slightly milder, with minimums between -20°F and -15°F, including areas like the Driftless Region and central river valleys.
The warmest zones are found along the Lake Michigan coastline, extending into the southeastern corner of the state. This area benefits from the “lake effect,” where the large body of water moderates winter temperatures, resulting in zones 5b and pockets of 6a. Zone 5b is characterized by average annual minimum temperatures of -15°F to -10°F, and Zone 6a by -10°F to -5°F. Major urban centers like Milwaukee and parts of Madison often fall into the warmer 5b or 6a classifications due to the lake effect and the urban heat island effect.
Selecting Plants Based on Zone
Once a gardener identifies their specific hardiness zone, they can use this information to make informed choices about plant material. Plant tags and seed packets for perennial species typically list a hardiness zone rating, indicating the coldest zone the plant is expected to survive. For a perennial to thrive, its recommended zone must be equal to or a lower number than the local zone. For example, a plant hardy to Zone 4 will reliably survive in Zone 5, but planting it in Zone 3 carries a risk of winter damage or death.
The hardiness zone map is a guide focused solely on winter minimum temperatures and does not account for other factors affecting plant survival. Localized conditions, known as microclimates, can modify the effective zone within a small area. A sheltered area next to a south-facing brick wall might be half a zone warmer than the open yard, while a low-lying area where cold air settles may be colder. Factors like soil drainage, winter snow cover for insulation, and summer heat tolerance are not addressed by the USDA system, yet they significantly impact a plant’s overall health.