Centaurea maculosa (Compositae) - I recently saw a potted spotted knapweed for sale for $9.95 (next to the cut goldenrod)...One person's weed... Blooms mid to late summer. (Photographed in WI)

Liatris spicata (Compositae) - This "Dense Blazing Star" is only marginally related to knapweed (they are both members of the daisy family), but they sure look like cousins... Blooms in late summer. (Photographed in WI)