I've never actually seen Japanese knotweed, only on TV. But I heard it can push through concrete! Also, even rhododendrons are causing problems here in the wild, their roots poison all the soil around them and people are digging them out frantically to preserve the native wild flowers and plants. Like Oxford ragwort (terribly toxic to cattle and horses) all these non-native plants misbehave when they arrive, they seem to be real bullies. I wonder if it's because they haven't brought any natural enemies with them!