Grayc, welcome to ATP!
This is not an absolute answer, as I have never gardened anywhere but Ohio... But, I can tell you that my mophead hydrangeas don't tolerate heat very well. They wilt and whine when sun hits them, and cry for water. If they're adequately hydrated they perk up after sunset. They don't tolerate hot afternoon sun well.
I'd try them in the ground, as I think the roots would be cooler and the plant happier. They don't tolerate hot afternoon sun well, even in Ohio.
Karen
