Bidens frondosa /Aster Family
These composite flowers usually do not have "petals" (ray flowers). The flowers are not very showy, they could be easy to bypass. You might not see them when they go to seed either, but if you pass by too closely you'll probably take a few seeds home with you! The seeds have 2 sharp barbed prongs that stick into clothing.
Where seen: Oak Ridge Greenways, Great Smoky Mountains NP, Warriors' Path SP