Verdifish, that's a bit extreme don't you think? I absolutely get where you're coming from though!
Demonfarmer - illegal or not, just don't. End of.
Even if your hens thank you for the knotweed, your parents certainly won't. Having knotweed could cause major structural problems with your house and basically make it unsellable.
What's wrong with the perpetual spinach that I proposed the last time you asked a similar question?