I just can't believe that this plant is still widely available to the general public, it ticks all the boxes as a pernicious weed!
I've seen no end of situations where its been planted by unsuspecting armature gardeners and after a short time, it can takeover the whole garden, plus neighboring gardens.
I think its at its worst when it gets into trees, where it can smother the foliage,killing the tree and acting like a sail, resulting in the tree getting blown down in high winds!.
I wish you luck in removing as much root as you can, then treating (individually) any regrowth that may appear.