Persicaria lapathifolia ?


SK 3183 29 November 2021: 7 very high res. images.

Location: Mugu, Rara, West Nepal
Altitude: 2913m.
Date: 20 August 2021 
Habit : Wild
Persicaria … ???

I tried all sp. from efn but unable to find a match.

And the stem underwater looks hollow !

Persicaria lapathifolia I hope

I guess Persicaria lapathifolia (L.) Gray is correct ID !

Looks very close to P. lapathifolia. However, P. lapathifolia will usually have large dark black/purple blotch on the adaxial part of the leaves. Therefore, I am skeptical.
Floral details and achenes should also be observed before confirming the species ID.

Is that a mandatory characteristic?
From FoP: Stem simple or branched, often thickened at the nodes, usually with red dots, internodes usually shorter than the leaves
And the stem part underwater looks hollow.
Can we make out from these points?
2 images

Most Polygonum species will have knotty stems, therefore, they are also called knotweeds. Other characters mentioned by you are common to many of its species. Therefore, it is difficult for me to confirm the species ID.

OK … The location being a national Park I could not collect the specimen !