It may be one of the Solidagos (goldenrod.) Not much confidence in that suggestion, but wanted to bump this back to pg. 1. In Canada? Do you know what zone you are in?

Well, I ended up taking it to the extension office and they said it was kochia. It's an annual, so I'm not sure if I should bother to pull them up to prevent seeding. The extension office said they already dropped many seeds, so maybe I'll just let them die over the winter.

.Thank you all. Yep... the most A1 were found right under two October Glory Maples. I have had these trees for 9 years (they were probably two years old or so back then), yet I had never had these seedlings under the trees before. The trees had not started flowering until last year. This year, the trees were pretty with all reddish butterfly look flowers in early spring.

thanks, internet pics look like a dead ringer for it.