Roundup in it's strongest format seems the best way.
2 ways to sort it
1st.General spraying if it's in a thick clump. Usual rules apply when
spraying.
2nd. cut off each stem about 1-2" from the ground and inject with Roundup.
repeat as often as necessary.
JW's root system can bury 7' down and spread 21' in diameter. So unless
you're barmy don't yank it out of the ground.
Both methods will probably not kill it off in the first go. Be prepared to
attack it for three years or so.
Regards




I'd just like to point out that roundup contains glyphosate. Glyphosate is a
translocated herbicide (sorry, I'm getting technical here) that is only taken
in through the leaves of the plant. Injecting stems with the stuff is likely
to be very expensive, especially as it's likely to be a complete waste of time.
Though spraying is effective on large clumps, the best way to kill off
Japanese Knotweed is to paint the leaves with a glyphosate based herbicide
using an appropriate brush. Needless to say that rubber gloves or something
similar are advisable.

If you follow the instructions on the packet you'll see that you can pull up
weeds after a few days (I think they state 3 days). You could either cut down
the knotweed after this period of time, or better still dig it up as far as is
reasonably possible. Retreat any new growth that appears as soon as possible
and you should get rid of the stuff. The same applies to bindweed.

Dave.