Keep cutting it down with a strimmer, removing all leaves, never let it form leaves again. This will weaken the plant over time as it will be getting no light from leaves. Spray with roundup, as that is systemic and gets to the roots and kills it there. It is a long term (2 year) battle but can be done.

The most effective use of glyphosate on these is to drip it neat down the hollow canes, so cut first, then drip...

Thanks killer price and bamboo

Thanks hope I have done this cant get hang of how to send thanks

My son did what Bamboo advises after trying everything else and it did work to some extent eventually. I think just spraying the leaves with roundup would only give it a temporary setback - its a very robust and determined beast. Cutting first and applying roundup afterwords is no use as its absorbed by the leaves and doesn't affect the soil.

I disagree - you need to cut off all the leaves. If Roundup needs leaves to soak in then use a different treatment herbicide that is taken up via the roots. If you keep the leaves on all you are doing is feeding the plant.

Round up (glyphosate) won't kill japanese knotweed if its sprayed on the leaves and stems, that's for sure. Do as Kildermorie says, but leaving enough of the central stem or cane so that you can drip the treatment inside, neat. You will need to do it for a considerable period of time (years), and it may only be a partial solution - the roots go down 9 feet, and that's a lot of root to kill with anything other than a nuclear blast.

Otherwise, glyphosate is usually sprayed or watered onto soft green growth - just not for knotweed.