This isn't a begonia site, but I would imagine someone will answer your question guest.

If there are any frosts after the shoots have developed they will just melt to a mush. If you have somewhere to protect from frost you can pot them up when they show signs of shooting, and put them in flowering position when you think the risk of frost in your area has passed. I forgot my large ancient begonia corm, last year, till very late. I stuck it in the middle of some new corms that were budding up to flower!. In the end it bloomed just as well as the rest! They seem very tolerant but they hate frost.