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.

I'm in Zone 9b in the SF Bay Area, and I have a few different zonal Pelargoniums here - some in the ground, and some in pots. We do get some frost every winter, and pretty much all of the Pelargoniums get heavily damaged. Except for exceptionally cold winters, though, most of them survive. The ones on the south part of my Moon Garden largely survive, but the ones on the north side uniformly die. I don't know what the difference is. Ditto some in pots live, but some in pots die...generally the larger the pot, the better the chance of survival, but I haven't a clue why. (Certainly overhead cover or shelter (or not) does not factor into any of these locations.)

The dahlias, though, have uniformly died if left out in pots.

You know, I've only ever had the very tall dahlias and I always plant them in ground. At worst, they usually survive 2 or 3 years, not as long as gladiolas or some of the others but still, they do hang on for a few years. I wonder if being in ground has anything to do with it?

I guess this would be best discussed on the perennials forum where you started the new topic.

"The dahlias, though, have uniformly died if left out in pots"

This past winter took a hit on my dahlia's. I knew I should have moved them into the garage, but I already too many other plants in there.