If you didn't over water then I would think the next issue would be how many hours of sun they get a day. I have some of mine in hot Atlanta in full sun for several hours in the morning and one big pot on the front porch that gets indirect light. Despite all the rain we've gotten this year so far, they are amazingly doing quite well.

Front porch - took it down from getting hot afternoon sun but still gets lots of bright light.

This one gets hot sun from early morning until around 2 to 3 PM. The New Guinea Impatiens are crowding it out though.

This is the pink series (the one above is more orange than pink orange as shown). This is in the same sunny spot as the one above just in a different pot.

And finally two non-stop types in a hanging basket in an Okame cherry tree. These aren't doing as well as the boliviensis type begonias though.

hc is the king of begonias! What is your secret? I can grow canes just fine. I still have mallets that I am nursing and am optimistic about.

But rexes and rhizomatous start dying from the minute I buy them.

I'm in TN. Not far from you. Release the magic please!

Thanks for the comments Kyle.

Mallets grow very well in summer but suck in winter.

Rexes and Rhizomatous are fairly easy - do not keep them soaking wet but don't let them get bone dry either. So I check most everyone of them every day just to see if any are wilting and water when I see this for sure. I also like to run a mist/cone shower over them as well to keep them somewhat humid. They need light to excel but no full sun during the hot part of the day either. As the summer winds down I usually like to move them to a sunnier location but again only a few hours of direct sun. Some like callosa are in full day sun and seem to be loving it. If your begonias are rotting then maybe it is time to repot with a faster draining mix (again I use Miracle-Gro potting mix so I don't have to worry about any secret formulas - I've gone through 8 full size bags so far this year and bought 3 more ready for use). Top off any pots that are low as the fertilizer included helps the plants get a boost.

Here is a sample of what I have growing pretty well (two weeks ago)

valida next to U358

Sylvan Triumph

I think the trailer is Bob-o-link and the one in front is ludwigii grown by our club's youngest member from seed

a few begonias

Virginia Jens (big leaf) and aconitifolia (about six feet tall now)
platanifolia (very large leaf on a thick cane like stem)
my own seed grown maculata (maybe a hybrid but it looks like its mother plant)
Snow-Capped, Cancan, Island Magic, and Judy Cook in among rose of Sharon seedlings

Speculata with some torn leaves from falling debris
U445 is really cool - tall shrub or thick-stem (?) with palmate leaves.

Thanks for the advice, hc. I had two gorgeous mallets last summer. I am still trying to recover them but at least I haven't lost them, yet.

Your begonias are spectacular!