Plant hardiness zones provide a standardized way for gardeners to determine which perennial plants will survive the winter in a specific location. This system is based on climate data and helps standardize the language used by nurseries and seed companies. Utilizing this information increases the chance of successful gardening by matching a plant’s cold tolerance to the local climate.
Milwaukee’s Official USDA Hardiness Zone
Milwaukee, Wisconsin, falls into a transition area on the current United States Department of Agriculture (USDA) Plant Hardiness Zone Map, primarily designated as Zone 5b, with warmer pockets of Zone 6a near the lakeshore. This designation is based on the average annual extreme minimum temperature recorded over a 30-year period. The Zone 5b area indicates that the lowest winter temperatures typically range from -15°F to -10°F.
The influence of Lake Michigan often pushes neighborhoods closest to the water into the slightly warmer Zone 6a. This Zone 6a designation means the average annual minimum temperature is between -10°F and -5°F. Understanding these specific temperature ranges is important because a plant labeled hardy for Zone 6 might not survive an extreme winter in a Zone 5b location.
Interpreting Hardiness Zone Temperatures
The USDA Hardiness Zone system divides North America into 13 main zones, each representing a 10-degree Fahrenheit range of the average annual coldest temperature. To offer greater precision, each main zone is split into two 5-degree subzones, labeled ‘a’ and ‘b’. The ‘a’ subzone is the colder half, while the ‘b’ subzone is the warmer half.
The data used to establish these zones utilizes 30 years of temperature records to calculate the average winter minimum. This provides a basis for predicting plant survival through winter. It is important to recognize that this system measures only cold tolerance and does not account for other factors like summer heat, rainfall, or soil conditions.
Applying Zone Data to Planting Schedules
Milwaukee’s Zone 5b/6a designation directly informs the selection of perennial plants, trees, and shrubs, which must tolerate the lowest expected winter temperatures. Gardeners should select plants rated for Zone 5 or colder to ensure reliable, long-term survival. Choosing a plant rated for a warmer zone, such as Zone 7, means it is unlikely to survive a typical Milwaukee winter outdoors.
The presence of microclimates, particularly the warming effect of Lake Michigan, means some areas can successfully cultivate plants rated for Zone 6. Urban heat islands, created by dense pavement and buildings, can also slightly elevate local temperatures, allowing for a broader plant selection in those specific neighborhoods.
For annuals and vegetables, the hardiness zone is supplemented by the average last and first frost dates. The average last spring frost date, typically around early May, guides when to safely transplant seedlings outdoors. The first fall frost date, usually in mid-October, marks the end of the growing season for many crops.