Apparently the time to harvest these things is when the leaves turn brown and begin to drop - so about now!
I up-ended the pot and tipped out the plants (I say plants, plural, because I originally planted 3 small tubers). This is what I saw:
The plants had produced a mass of underground rhizomes, like Mint does, and the edible tubers are the tips of these rhizomes.
Well, what can I say? I picked off all the little tubers and this is what I got - a total of 160 grams - probably about enough for one 2-person serving. It's not an impressive yield for a whole year's growth, is it?
On the plus side, I'll just say that when eaten, these little tubers are pleasant enough, though in my opinion not a great delicacy. In terms of taste and texture I think they are quite similar to Water Chestnuts and because of this we'll probably use them in a Chinese dish. Due to their strange shape they need careful cleaning, though thankfully they do not need to be peeled!
I grew these for their novelty value, but I don't think I'll bother with growing them again, because I think they have a poor VSR (Value for Space Ratio).