Conoclinium coelestinum, formerly Eupatoriium coelestinum, doesn't exactly roll off one's tongue, so let's just call it what it is - Hardy Ageratum or Blue Mist Flower.  I have looked for this hardy Ageratum for years because to me, fall just wouldn't be complete without this beautiful blue/lavender native wildflower.  Hardy Ageratum is native to low woodlands, moist meadows and even ditches, but it is just as at home in the garden.  It is long-blooming, makes wonderful cut flowers, is loved by butterflies, and is beneficial to birds and insects.  And one more thing:  it is deer resistant.  Now if that doesn't sway you, nothing will.  Hardy Ageratum prefers full sun, but it will easily tolerate shade, and is drought tolerant.  It grows from two to three feet in Zones 5-9 and is a spreader, so give it room.  This is not the annual Ageratum that is seen so often in nurseries every spring. (Photo by Kingdon, from Wiki Commons, and in the public domain.)