I'd agree with snakeroot. I have Hammock Snakeroot, Ageratina jucunda ( http://butterflies.heuristron.net/plants/AgeratinaJucunda.html
) down here in South Florida, and it looks similar, but it does not grow as far north as you are. White Snakeroot, Ageratina altissima does though, and it looks like yours: http://plants.usda.gov/java/profile?symbol=AGALA
It's a great plant. In fact, I collected some Hammock Snakeroot seeds
just a couple of days ago so I could have it in my yard. Butterflies absolutely love it.