Can I compost...
Japanese knotweed?
No.
In the UK, since the 1981 Wildlife and Countryside Act, it's illegal to allow Japanese knotweed to spread so it is best not to add it to a compost heap, even when cut down, in case it regrows and spreads - whether in the compost heap itself or when you spread mulch around the garden.
If you feel you really must compost it though, do it separately from the main pile so you can know that it's well and truly dead before using the resultant soil.
In category Garden waste