Probably planted somewhere nearby, but spreading through sidewalk cracks on its own. Rhizomatous?
A bunch of these were flying from tree to tree near my house, possibly mating. I'm not sure what to ID it as, but the undersides looked like a pale yellow and the head looked almost crested. Too bad I didn't get a photo.
The string coming off its back is a cat hair. This guy somehow came into my car through an open window