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
This may fit in with the description of stone flies but I could not find photos like it. It appears to have tightly closed wings over its back, which then turn sideways against one another behind the abdomen. It was resting face up in the shade on a wooden fence.
Carapace about 4" long. Got a little carried away with this one too.