Old names for this relative of Joe Pye Weed are Blue Eupatorium, Mistflower, and Hardy Ageratum. The new Latin is Conoclinium coelestinum.
It is a hardy perennial wildflower that blooms all over Middle Tennessee, but one must look to mail order to bring it to southern New Hampshire. It is a notorious leaper and goes everywhere in good soil, but when it comes into bloom in late summer, I do not care where it spreads.
In Spring it stays underground late and one must remember that when temptation comes telling us to plant something new in that empty spot.
When I first saw it in bloom in Nashville I thought it the most beautiful late wildflower I had ever seen.
I have not changed my mind.
I lost 30% of my dooryard garden this year to woodchucks. My phlox and asters were bitten to the quick. My marigolds and Blackeyed Susans were chewed to stubs.
The only consolation is that there are plants that the woodchucks will not touch, and the Mistflower is one of them.