Prairie Ironweed (aka Smooth Ironweed) is a perennial native to central US and prefers sunny wetland habitats, although it can tolerate average soils. The purple compound flowers are unusually brilliant, on top of tall stems.  Works well as a tall, background plant and in rain gardens.
 
Direct sow on the soil surface in fall. If started indoors or in the spring, it needs two months of cold/moist stratification. Can self-seed.