I learned a little about this years ago when I moved from southeastern KY to the far western part of the state. I brought with me several of those plants that for years my mother had grown in her garden and had always stored inside for winter. Glads, cannas, dahlias, all of them I brought with me and did the same thing she did until the year my son was born in September. That year I was unable to bring all those bulbs/tubers/etc inside so I just left them where they were in the garden. It was a nice surprise when they popped up again the next year and the next and on and on. Of course some didn't live forever, but they did survive for several years.
SE KY is part of the Appalachians and is considerably cooler than here in the western corner, about 350+ miles and a whole zone of difference. I think it's zoned 6a there and 7a here; so that was quite a lesson for me. I've never tried leaving geraniums outside though, I don't think they'd survive.
