There's a flowering plant with a hard black seed that once it gets going, it's almost impossible to get rid of. I wish I knew what it was, because OP could accidentally on purpose toss a few of those seeds around MIL/SIL's yard and it'll be off to the races.
I first saw it as a kid (45+ years ago). It was a single plant at the corner of 26th & McBryde. We kids liked the seeds and would pick them and carry them all over the place. I think the homeowners all over North & East Richmond are to this very day still trying to eradicate it.