Not sure but I would water when dry (not bone dry) for starters and work from there. You can begin to probably figure out what is needs from there.
Michele

contrygirl,
I have a Rieger begonia. It is doing well in mostly shade. Maybe it gets some very late afternoon direct sun but that is it. I water when the surface of soil is dry. The flowers will turn brown, I think it is normal. They will turn brown and drop and new ones will come out. Now mind you, my plant is down here in Florida and you are in Maine. As for temperatures, mine lived through a low of about 40 but I don't know if it could take a freeze. And the Florida direct sun was a big no-no! It was slowly cooking and that is when I moved it to almost full shade. It grew so fast after I moved it that I had to cut it way back and it is growing like crazy again. I don't think I've ever fertilized it and I have it almost a year. Good luck with yours! I would say in Maine anything up to part sun/part shade in summer is fine.

I have acquired 6 beautiful Rieger Begonias and wish to know if they can survive outside in Jacksonville Fl. Of course they would be placed in full shade. I don't want to sacrifice them to my ignorance. Thanks in advance for any advice.