The leaves remind me of sow-thistle Calin, I checked one I have but it has soft spines at the edges. This morning I spotted others next to the spiny one which resemble these leaves but they don't reach that height, there might be one which does in Ontario. I'm thinking more along the lines of Asteraceae, could easily be wrong!
See how similar the leaves and stem are to Sonchus oleraceus ..
http://luirig.altervista.org/c...
http://luirig.altervista.org/s...
http://www.thewildflowersociet...
