Overwintering begonias

They're in containers.
Can I wait until the first frost? If not, how do I know when to dry them?

 

I'd start drying them off now, I'd not wait for a frost before doing it as you do with plants like Dahlias.

 

Thanks @pete

 

When I say drying them off I mean the pots as a whole rather than digging the tubers up.
Then as they dry out the stems usually detach them selves from the tubers.

 

Thanks @pete . I've never grown them before and there's one or two I'd like to keep.
Million kisses was/ still is pretty stunning but RHS reckons it's usually grown as an annual. I'll give it a go anyway. Some of the lairy ones I'll leave to their fate.

 

I have been know to just leave the pots dry in a garage overwinter once the stems drop off, or you can break it all down and just remove the tubers to store in a similar dry place.

You can get vine weevil in the corms so by taking them out in the autumn you find them before they can do full damage.
Obviously you will need to dig out the corms and plant in fresh compost next year anyway.

 

I don't grow the tuberous ones But I always grow the semperflorens. They are easy to overwinter. I just pot them up, cut them back, and keep them in a frost free shed. I won't be doing it to them for a few weeks yet.

 

I leave mine until the first kiss of frost. Clean them up,cut any soft bits off, dry them and if I remember dust with a bit of sulphur. 9 out of ten make it, a few are 10 years old I think.

 

I haven't grown any for a while, but I used to depot the tubers, check these for any soft bits and discard if found, then pop them on a piece of kitchen roll in the airing cupboard to dry out. After that I'd wrap them individually in fresh kitchen roll, put them in a paper bag, and store this in a cool, dry, dark place, eg understairs cupboard, till Feb.