One of the knot-weed species.

At one time my state had a picturesque waterway called Gunpowder Falls which is the location of several state parks. Today some parts of this watercourse are completely lined with Japanese Knotweed Polygonum cuspidatum. Ferns, trillium, trout lily etc, in the infected area have disappeared. Invasives such as this are like ebola to the ecology.
Carefully cut the stem at ground level and treat with herbicide and do it today. Remove entire stem including seed and burn them.

here in the St Louis Missouri area we've been invaded by a foreign honeysuckle bush. it is EVERYWHERE. it just shows up in my yard out of nowhere. I'm guessing birds are helping it to spread since I've not seen any type of airborne seeds. invasive species are hard on local plants. this bush has proven to be as hard to get rid of as nut sedge.