Its a sucker from a tree in the genus Populus, not a weed as such. Either there was a tree which has been cut down and has sent suckers up from the roots or you will find a parent tree in the vicinity somewhere. In the lawn it can be mown. In flower beds there's nothing you can do other than pulling unless you can find the source and kill it. Which you might not want to do if its a mature tree.

Wow! You nailed it. I didn't realize it was a tree. The neighbor has what looks like a (white?) poplar about 40 feet away, on the other side of a fence behind two other trees. From a distance the leaves look very similar. That's got to be it. Mowing them down leaves a cluster of 'stumps' behind, so it would be great to stop them from sprouting. Trying to find and cut the roots would be one option. If I could treat them with a weak herbicide just enough to stop the suckers without damaging the parent tree that would be nice. Not sure if that's possible. Triclopyr is recommended for use against poplar, but it sounds risky. I don't want to kill the tree, just the suckers.