This plant causes bad skin burns and blindness. It's easy to spot, so warch out for it, and notify the MOE if you see any. The plant in the picture doesn't appear very tall (yet), so don't assume they are all 6M tall.
http://news.nationalpost.com/2010/07/13/giant-weed-that-burns-and-blinds-spreads-across-canada/
http://news.nationalpost.com/2010/07/13/giant-weed-that-burns-and-blinds-spreads-across-canada/