MILWAUKEE — About half of Milwaukee County now belongs to a plant hardiness zone that’s completely new to Wisconsin.
The latest USDA map shows zone 6a has claimed a stretch of Lake Michigan coastline from Kenosha to Sheboygan and some areas of Door County. It’s the first time since 1960 that a new zone has occupied state.
Meanwhile, the distribution of Wisconsin’s lowest zone has shrunk considerably: zone 3b, which used to cover large swaths of northern Wisconsin, now occupies just two small patches in Douglas, Washburn and Sawyer counties. As for the rest of the state, Wisconsin has moved up about half a zone in most places.
The USDA looks at data from weather stations around the country to create its plant hardiness map based on the average annual extreme minimum winter temperature.
But plant hardiness isn’t the only factor for gardeners to consider. Gardening expert Melinda Myers told WTMJ: “We have to have the right plant for our other growing conditions — like the soil, the wind, the frost and the moisture.”
Myers said climate change is a factor that affects plant hardiness zones, but also impacts plants’ overall tolerance of extremes.
“Some of the concern moving forward is floods, droughts and extreme heat,” she said.
Myers advised gardeners to proceed with caution and take the new zones as an opportunity to experiment with new plants. Things like sweet cherry, hearty figs, crape myrtle and Japanese maples might thrive better in zone 6a.
It’s also an opportunity for more perennial variety: “A lot of the things we grow as annuals are annuals because of our cold winter.”
Myers said gardeners should consider journaling and paying attention to microclimates — conditions like wind, sun and proximity to urban heat islands — to keep track of how plants fair under various factors.
