I'm afraid I can't make out any features of the flower other than it's yellow. Too fuzzy. But compare with Bidens pinnata and see what you think.

Thank you, floral_uk! I think you are right. The first picture that I saw of Bidens pinnata flower was too daisy or coreposis-like, and I haven't seen such distinct petals, but two other photos that I found do match - especially the mature flower head which has spiky hair like projections from the center. If you look just about the yellow blossom in my picture you can see one of the older blossoms with the spiky hairs (I realy did try to get it in focus but my phone would not cooperate - had to be either distant & in better focus or closer up and not.)

ETA: apparently those are seeds forming. I'm yanking the plant out tomorrow!