Hi Megan, if you haven't done so already, try posting this in the Name That Plant! forum. It gets a lot more traffic, and they really know their stuff there.

I've known this genus for years....and just a few years ago, I couldn't remember it to save my life...spent a day trying to find the genus via google, so I do know the feeling. And other than here in the SE, most bidens have tiny petals...so they all look far less like the bidens I know. It's hard to id therefore.

Thank you Flora! I am trying to listen what my backyard environment is trying to say...this year lots of volunteer natives are popping up in areas that I tried to plant modern hybrid plants and failed..

The flowers on the second plant really look like ragweed flowers to me. 8 feet tall could be Giant Ragweed, Ambrosia trifida. The leaves I can see in the photo don't look quite right, but a larger image should make it clearer. Or it could be a different species from the two common ones I'm familiar with. USDA lists a bunch of species and I don't have time to go through them now. On a side note, while looking for this I went to the Dan Tenaglia's old Missouri Plants website and discovered that the Missouri Botanical Garden has taken it over and is updating it. That site was the first wildflower ID site I found on the internet back when all this was pretty new and was a real inspiration to me about what you could do along those lines, as well as being a very useful ID tool. It still is, for that matter. I'm really glad it's being continued. WW