link IDd as an invasive Lythrum salicaria (purple loosestrife). .. in the perennial forum ken

That's a pretty good guess and would certainly provide a similar effect.

Context would help with id, but the gray foliaged plant looks like stock (Matthiola incana). I recognize the other one also, but cannot seem to pull the name out of the brain at the moment.

It is not knotweed......and it does look like a tree seedling (root shoot), what type I am not sure. But you will get faster and accurate ID's if you use the forum - Name That Plant - that is intended for plant identification. This is not the right place :-)