Bev, zone 8 is what the sources I've looked at agree upon. I've got mine in pots on the deck again this year, where I'm getting more sun than last year due to some tree removals. Still partial shade, but they're getting enough sun now to grow well and are blooming now.
Barb, I wonder if the roots got too hot in the containers? They are in the Lily family, and Lilies prefer cool root space, so it's a possibility. The leaves don't seem to mind sun and heat. Since you're in zone 8, I bet you'll have good chances of overwintering them in the ground- let us know if you try it and how they do
