Why call a plant "Virginia Knotweed" when you could call it "Jumpseed", and watch how the seeds jump when you pull your fingers up the stem? There are so many less than ideal common names for native plants--many a beautiful plant has "weed" in its name--that it's a relief when a good one comes along.
So Jumpseed it is--scientific name Polygonum virginianum. The photo shows a whole understory of jumpseed, ready to live up to its name, over next to the Pettoranello Gardens parking lot.
Thanks to Carolyn Edelmann for providing the name, and an explanation of its meaning.