Can you add a photograph of this tree which is growing japanese knotweed?

Not as such, no. BUT I have seen plants growing into other plants and looking as if they are conjoined. For example we have terrible problems with Couch grass roots growing through bulbs, in one side and out the other.

Mistletoe does this as well.

Sorry Bathgate but mistletoe is parasitic and grown on the likes of oak trees...

I realize that Mg. My point was that it can give the appearance of being grafted in. I was commenting on Owd's response.

Old ivy up a tree grows entwined into the bark and seems to fuse as one

Bark grows over things, think of forgotten tree ties

We have a large hawthorn with a young ash growing out of a crevice where dead leaves had drifted and rotted - don't give much for its chances...

Our ash tree has had a tiny elder bush in a big knot hole thats filled with devris, never gets more than a few inches , over the years it has few more stems, must have been there 20 years...natures wonderful

I have seen weeds of various sorts growing into and through the outer bark of old trees, but not actually grafted in.

No Japanese knot weed wont graft onto another plant.