Mistflower (Conoclinium coelestinum)

Conoclinium coelestinum, commonly called mistflower, is a late summer to fall-blooming herbaceous perennial that is native to the Eastern United States. It looks like annual ageratum and in that regard is sometimes commonly called hardy ageratum. But it is perennial and can spread aggressively by rhizomes.
 
Easily grown in average, medium to wet, well-drained soils in full sun to part shade. Prefers moist, fertile, humusy soils which do not dry out. Mistflower is a vigorous plant that spreads aggressively by rhizomes and self-seeding. In some areas of the U.S., the species is considered to be a spreading weed. Propagate by clump division in early spring. Plants will grow in wet areas. Taller plants may be cut back in summer to prevent flopping.
 
Zone: 5 to 10
Height: 1.50 to 3.00 feet
Spread: 1.50 to 3.00 feet
Bloom Time: July to October
Sun: Full sun to part shade
Water: Medium
Maintenance: Medium