I have tried round up and digging them up. I started off with three large plants. I thought I had it gone by the start of fall last year. The snow melted two months ago, and now I have at least 7 of them!
I asked at Home Depot and Lowes. Both places didn’t know what it was, or how to kill it. They suggested posion ivy killer. They even suggested dumping bleach on it.
I have since found online a way to hopefully kill it, which I am going to try.
Mix Herbicide with Diesel fuel or vegetabel oil. The oil ensures that the herbicide will cover the plant thoroughly and be absorbed into the plant.
Use 15% Remedy (Herbicide) and 85% diselse fuel or vegetable oil. Make sure to sake the mixture until it is thoroughly mixed. Spray the center of each whorl and spray for at least 2 seconds.
The article I read did not say how long before you see results.
...Oh, and I have nick-named mine, the Devil Plant from Hell