Interestingly, this plant's leaves show some sign of a check in fecundity in its growth: note the curling and discoloration of the leaves. It can be guessed then, that there may have been intervention with glyphosate last season(2010; further treatment in late summer would be necessary, with additional ones over the ensuing years, to ensure containment and possibly piecemeal eradication.
However, fresh and established growth outside the confines of the site will need a different plan of action.