Even though it appears this is just one of your posts, I don't see the others.
That said, your weed is one of the nasties from the Ranunculaceae group. Need an image of the full plant -- roots too -- to know precisely which one.
In any event, you have quite a project to deal with. And the longer you delay, the longer it will take to win.
Yes, for some reason, I posted two other pics in the gallery 'side' of the weeds forum. Not sure how I did that.
See final image of an individual plant here
Here's some info for management of Ranunculaceae weeds
Plant Conservation Alliance: http://www.nps.gov/plants/alien/fact/rafi1.htm
Here is a link that might be useful: some info here
We have these weeds here. I sit down with a small digging tool and pull them out, root and all. Each year I get fewer of them. Were it not for the fact that they grow in adjoining yards, I'd have had full victory years ago.
Poisoning them is a poor option. Just sit down, get comfortable, and wage war. You will win. If you do nothing, you will regret it. This is an invasion, yet it can be stopped, but only with a bit of effort.