Possibly, difficult to tell at this size. Yank it out and burn to be on the safe side.

Thank MG, will do.

It certainly looks like it might be. do they definitely have it in their garden? There are strict rules regarding its disposal. burning is by far the best option for you.

I'd also be tempted to spray the leaves with glyphosate type weedkiller and leave it for a few days before removing it. the problem when you take the top growth off it promotes more side shoot growth.

I thought you had to notify the council if you knew Japanese knotweed was growing somewhere. I would encourage the people next door to get it sorted before it turns into a massive problem for you all.

As Cammomile advises, speak to your neighbours and get the council involved. I would leave the one by your fence to show how it is spreading before dealing with it later. Lots of advice on the net.

I am not so sure that it is JK. The young leaves are generally all pale green in colour. I would have a peep over the fence and see what the plant looks like on the other side. It looks more like Red Dock to me, but then..............

Thanks for all your advise. The neighbour in question doesn't actually live in the property they are doing up, it was an old hall originally built in 16something or the other.
I have had a look on the neighbours side it looks like a fiburuss bamboo. We have sprayed it with the glyphosate and now keeping an I on it.
Thanks again.

it certainly isn't a bamboo so I suspect it is JK.

It could cost them a fortune to get rid of it, so the sooner the better.