Most people are as drawn to blue flowers as...uh, butterflies are to ageratum!   This member of the aster family is easy to grow, and doesn't even wilt when transplanted.  It quickly forms a clump three feet tall, and beginning in late August and all thru September the show begins, with masses of blue flowers that the butterflies love. 

I combine my ageratum with Helianthus (false sunflower) for a golden contrast that really steals the show in late summer until frost, when most other things are heat stressed and out of bloom.  Sound too good to be true?  Well, there is a catch.  As you may have guessed from my clues, the plant is considerably aggressive.
No concerns about this in the first year.  The second year, a nice patch is established, but be prepared for the third year, as the plants overstep their boundaries with underground runners.   It would help to use steel edging, or some kind of underground barrier around the planting area, to keep it in one place.  How aggressive it can be may depend on location, soil, and moisture conditions.   Ageratum thrives in full sun, and prefers a moist soil. 

If you know someone that has a clump of this beautiful perennial, all you will need is a small piece to get you on your way.  If what you're looking for is a fast-growing plant that entices butterflies,  and masses of blue color the entire fall season, I'm sure you will not be disappointed.