USDA Hardiness Zones are the first step in determining which plants will reliably perform in a given space. The map breaks the US into numbered zones (1-13) based on the mean annual minimum extreme temperatures over the last 30 years, which are divided in increments of 10° Fahrenheit. In Wisconsin, zones range from 3b in the Northwest (-35° F to -30° F) to 5b along Lake Michigan (-15° F to -10° F).
To determine a location’s hardiness zone, simply look at the map below and see what zone applies. Zones can also be searched by zip code at the USDA Agriculture Research Service.
The hardiness zone is not, and should not, be treated as the final factor in plant survival. While a location may be listed as zone 4a, local conditions may result in colder (or warmer) minimum winter temperatures. For example, plants experience different temperature extremes when sited in valleys, on hills, or in urban areas. There are also multiple factors beyond minimum temperatures that determine whether or not a plant will survive local conditions. These include average winter snow cover, heat and humidity, and moisture and light availability.
Even when temperature and moisture levels are ideal for a plant, cold snaps in early fall or late spring can damage vulnerable new plant tissues, leading to dieback and premature wilting. This is especially noticeable on marginally hardy plants like Black Gum or Tuliptree, which are hardy zone 4 when established but will not survive Wisconsin winters as a sapling.
In short, while hardiness zone is a useful tool, it is not the only information needed to select the right plant for the right site. If you struggle to find a plant that will work in your space, or are unsure of where you fall on the hardiness zone map, call us to speak with an experienced horticulturist.