Ah ha! Here I thought it was maybe not enough water. I'll pull them in under the eaves next summer, keep them cooler next to one of my patio doors, too.
Patty gave me these, and this huge, beautiful one last spring that almost croaked during the summer but is coming back now. I'm sure I over-watered it, treating it the same as my big cane begonias that seem to like any amount of water.