Let the frost kill the tops off and seal the tubers. Store the tubers for winter either in the pot or dig them out and put them in a cool dry spot and check on growth in early spring. Start watering then and bring them to a warm brightly lit area to resume growth. Fertilize as needed.

Other folks who grow tuberous can chime in on winter care for these types of begonias.

Thanks, hcmcdole! So, it's a tuberous then? I've spent the last week looking at begonias until i'm blue in the face and was struggling for an ID.

They were purchased in one of those boxes from costco, so were either tubers or bare root. I just haven't had the heart to go digging around to see what's under the soil because i've neglected these for much of the summer and they are still putting on a show.

I guess they should go back outside and wait for the frost to do its dirty deed!

Yes, tuberous. You can leave them indoors and watch them slowly wind down for this year. Don't be alarmed when this happens though. Just do some searches on winter care for tuberous begonias and you should find plenty of advice.