Nothing like the promised “giant” 5-10 cm tubers, I was nevertheless surprised to get maybe 3 times the yield of what I planted of chinese arrowhead tubers – Sagittaria trifolia subsp. leucopeta (syn. S. trifolia var. edulis)…a much bigger yield than when I tried North American wapato (Sagittaria latifolia).
211018: I finally got round to trying some. I didn’t peel them and didn’t trim away the edible shoots and started steaming them (as I usually cook potatoes).  Then halfway through I remembered a post by Alison Tindale (see https://backyardlarder.co.uk/2017/11/ducks-eat-duck-potatoes )  where she mentions that they were slightly bitter after boiling,  I therefore boiled them (to reduce bitterness for the second half).  The verdict: one of the tastiest tubers I’ve ever eaten…the texture is like floury potato, but the taste not unlike chestnuts and yes a slight bitterness of the good sort, adding to the overall taste experience…and to cap it, the shoots taste like artichoke hearts!!
I think I will just steam them the next time!
I hope I will manage to overwinter them as I really need to grow more next year! I’m trying to overwinter in the cellar (about 3C and dark), on a window sill in a cool room and in my pond about 10 cm deep to protect  from the worst frost…maybe also covered with spruce branches!