Poplars? Shallow, Sprouting, Interconnected root system in backyard Brooklyn, NY

Hi -
Just purchased a property in Brooklyn NY and trying to rehabilitate the backyard. I found several clustered smallish stumps and roots that run all through the backyard, as well as joined to my neighbors lawn on either side. The thicker the stump the deeper the root, the harder for me to pull up. Looking at my neighbors lawn right now, there is a new shoot every 6-12 inches. The shoots grow fairly quickly, 1" diameter green, with purplish new leaves that green. Leaves are pointed at the end. Any suggestions would be helpful to compare. I've looked up poplars and willows. Willow shoots don't match the shoots I see in the back yard, and so far I have found any poplar shoots that match either, but I'm thinking that still might be what it is.

 

Fallopia japonica, Japanese knotweed.

 

Yes it is! Thank you so much!