I know a lot of FL and CA folks grow begonias in the ground but any I grow in the ground have to be dug up in the fall before winter sets in (although some have come back from the roots the following spring if I missed some).

I've had good luck with a couple growing in the compost heap (I thought they were dead and tossed them and a couple of months later they have come back from the dead and formed a nice plant). From that observation, a porous mix seems ideal for growing them in the ground.

Hi, I"m running an horticultural enterprise in Germany, we plant Begonias in summer beds, especially a nwe orange Typ of Begonia elatior ("Peardrop") and a lght yellow Sort ("Buffy") are very resistable. Before planting we dig up the soil and add industrial compost with a long lasting fertilizer (Osmocote). We plant them also in very hot spots here, but I don"t know how they stand temps more than 35 C

There are many on the FL forum with incredible Begonia collections. You may want to check that out also, FL gardening is unlike any other part of the country. Good luck!