Most of us are familiar with annual ageratum which is a useful low-growing bedding plant with fluffy blue flower heads. There is, however, a perennial plant that is similar but it grows tall and blooms late summer into fall. It is Eupatorium coelestinum, and it is actually a relative of Joe-Pye weed.
This North American wildflower grows 2-3 feet tall with fluffy 2-4 inch clusters of lilac-blue flower heads in zones 5-9. It needs to be divided every three to four years, as it spread to form large clumps. It can be divided in the spring when one can see how much is coming up since it also liberally self-sows. One needs to be careful where one puts this charmer.
It likes sun to part shade and grows vigorously anywhere that I have put it in my garden. It is invaluable for a flower arranger, as its soft blue flowers make useful filler and combine with most other colors, softening and enlarging a few other flowers into a bouquet. It is another one of those plants that I wouldn't be without as the blooms are so serviceable, as well as soft and pretty in the garden and in a vase.
In times of drought I have seen it wilt, but otherwise, it is trouble free and a plant that certainly pays its freight.