When Fred First identified this plant as Virginia knotweed, I feared that it was related to the invasive exotic, Japanese knotweed. While both are in the same family (polygonaceae), Virginia knotweed is in the genus persicaria while Japanese knotweed is in the genus fallopia. Whew!! The plant is also known as Jumpseed, because the seeds stick on the stem into the fall and jump when you try to collect them. The leaves have a characteristic dark spot near the midrib and the flower stalk is very long with tiny white flowers arranged along it:
I have a lot of this plant, also, but I don’t believe that it is anywhere near the problem as the wingstem is. I observed a number of seedling trees growing amongst the jumpseed, probably because this plant doesn’t get very tall – I didn’t see any taller than about 12″. Thus, it does not shade out small seedlings and prevent them from growing as the wingstem does.