Perennials for window boxes
A good rule of thumb for growing perennials in containers, including window boxes is that for them to survive the winter in the container, you need to grow plants that are at least 2 USDA Hardiness Zones colder than where you live. It could be difficult to find such perennials for Wisconsin. However, another option might be to remove the window boxes each fall, place them on the ground and mulch them in heavily. In that case, you should be able to grow almost anything that grows in your garden.
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