Marsrover, it sure does look like white mistflower, particularly Ageratina havanensis. Like Char said, you are very lucky to have it growing! Oh, and thank you for highlighting my type-o's in both of the quotes.
I've made the corrections.
Here's a link I found with pics of A. havanensis.
http://www.backyardnature.net/...
Oh and just found this article by Frostweed.
http://garden.org/ideas/view/f...
