Thanks. A second one did the same today. I took them all out of their pots to dry out a bit, in case that's it. If the tuber is still sound I assume I can start over. I would then not bury the tuber under the "soil", like these were.

I've never known what kid of potting soil is best for these.

The interesting thing about this site is that there are so many different ways of dealing with begonias. I think environment has to determine how to grow. I do comment that begonia tubers produce roots and must do so all over the tuber except for the growing center. You hear of one writer who cuts his tubers in thirds, then plants them. It seems a bit much to me as people plainly say a cut tuber will not produce roots on the cut surface.

I can see that none will make it. I unpotted the first one to do it, and couldn't even find what I'd call a tuber. Just a soft mass that felt like it could be a mass of roots or...or....well, like no tuber I ever saw. Not rotted, though...smelled nothing like rot.

I took the other 2 out of their pots to dry thoroughly in the hopes of saving them. It's not that they are precious, mostly to learn from it. I had bought them for their unusual burgundy/brown leaves as much for anything else.

It's the first time I've ever killed tuberous begonias!

Very interesting, at least when it happens to somebody else . . . I suspect these plants were started from seeds or cuttings as the tuber doesn't form on the young plants until days shorten in the fall.

Cuttings, yes, that’s what they must be: though how they can
make such vigorous plants is beyond me.

The cuttings I tried to make from the fallen-off stems
rotted. Must have been compromised. Unpotting and leaving the other 2 begonias’
rootballs to dry out seems to have stopped the decline. One had already dropped
half its stems, the others look ok. The soil-less mix is now bone-dry. I put them back in their pots today and
gave them a dribble of water. I should take any remaining flower buds off.

I have often bought ready made up hanging baskets with begonias.
In the fall the stems drop off normally and I bring them inside, then I can
tell there is a very small tuber there. I store in the basement and a few weeks
ago brought them up and outside; now I can see tiny begonia sprouts. I don’t
expect much from those, it’s more of an experiment than anything else.