I wonder if this is Euphorbia esula, number 6 on a list of prioritized invasive plants in natural areas in Canada: http://www.cba-abc.ca/bulletin/vol_38_4.pdf
Well, the link I should have supplied is this one: Euphorbia esula I was trying to remember which Botany Photo of the Day posting had the link to the prioritized invasives - it was the one on the plant that I think is likely to be the one to ID here.