My only guess would be Eupatorium. That's some nasty, evil goutweed behind it.

Agree with Donna.in.Sask on both counts.....

I second your comment on goutweed, it is a persistant little bugger. I seem to have it contained and hopefully the sedums will eventually take over where I have cleared some of it away. That would make sense on the Eupatorium then, I did throw quite a few randomly collected seeds into that area last autumn. I will keep an eye on it and see what becomes of it by mid-to-late summer.