Posted: 16/04/2015 at 23:40
I successfully overwintered begonias from 2013 last year (2013-14) by allowing all the soil on them to dry out and then knocking it off carefully and keeping them in boxes and trays on thick newspaper, but not covered up (To avoid mould).
Around Feb /March 2014 I covered them in trays of slightly damp general purpose compost and left them for a few weeks just keeping the compost slightly moist. Eventually new shoots appeared (Around April). That's when I potted them into troughs, pots and hanging baskets.
I did the drying out process again this year by putting them on a rack to air dry. The reviving stage has been less successful because I think I accidentally used a mixture of new compost and spent compost. Nothing happened. I thought I'd lost them but on uncovering them all discovered that most of them are just about showing new growth at last. I've recovered them with new compost and abject apologies for mistreating them and am now hoping I've done enough to bring them back. In the meantime they staying in the greenhouse until I'm sure there's no more risk of frost.