I am putting it into my report as Ambrosia artemisiifolia, even though I haven't seen ragweed with such a reddish stalk before. Still curious to hear other people's thoughts.

Do the leaves have the typical whitish/silver undersides?

@missingtheobvious No the leaf color is relatively uniform. You can see the backs of some of the leaves at the top of the plant and they are green on the underside.

The leaves aren't at all like the Ambrosia artemisiifolia I'm familiar with: not only the reddish color, but also much too finely cut. I believe I've read that most if not all in those genuses/genera are known for leaves with undersides which are whitish or "silver."

If you don't have a similar source for Canada, this at least shows southern Canada: you can look under Asteraceae, then click on Ambrosia and Artemisia to see which types live where, then search for photos on the Internet. [Near the top of the linked page is a link to the "Map Color Key".]
http://bonap.net/NAPA/Family/Traditional/County

Thank you for that website. What an awesome tool. You are right that it's not Ambrosia artemisiifolia. I am thinking Artemisia biennis now.

Glad to be of help!

Tamarix?
http://en.wikipedia.org/wiki/Tamarix

I think it's a Tamarix.

Not Tamarix.