Eupatorium coelestinum (Conoclinium coelestinum) (blue mistflower)

Blue mistflower, also known as blue boneset and wild ageratum, will grow 1.5–3' tall and wide. The blue flowers appear beginning in July and last through October. This plant can spread aggressively by rhizome. It does best in full sun to part shade in moist soils but can handle dry conditions in part shade.

Blue mistflower is native to eastern and central North America. It prefers moist soils and can be found in wood edges, sandy woodlands and clearings, wet meadows, and streambanks. Attracts bees and butterflies. Host to various moth species. 

BONAP Native Range Map (bright green indicates county native, dark green indicates state native)  

Photo by Kellen McCluskey