Common Ironweed (Veronia Fasciculata) - an herbaceous perennial with dense clusters of magenta to purple flowers in the late-summer to early-fall.
 
Growth: Can grow 2 – 4 feet tall.
Light: Prefers full sun to partial shade.
Soil: Thrives in moist soils, prairies, meadows, rain gardens, & wildflower gardens.
Hardiness: Hardy in USDA zones 4-9. 
Maintenance: Self-seeds.
Benefits: Attracts pollinators, including native bees and is the host plant for the American painted lady butterfly and the parthenice tiger moth.