Purple Loosestrife - Invasive Plant
They are herbaceous annuals or perennials. Typically they have square stems, narrow stalkless leaves, and spikes of star-shaped flowers in shades of purple, pink and white. They are especially associated with boggy areas, river banks, and ponds, though in cultivation they often tolerate drier conditions.
They quickly spread by seed and take over the area.
From Huge Album of Wisconsin Wildflowers , Weeds, Invasive