During one rather warm day last year I was asked to treat an area behind a few houses that had Japanese knotweed in the 'garden' area. The whole area was full of mature canes. Last year I stem injected as many as the eye could see. This year in the third photograph you can see there isn't any Japanese knotweed growing anywhere!
(Generally as good as the chemicals are there will be a little regrowth over the coming years so monitoring the sites are essential, otherwise all the hard work will be undone in another few years!)