Thank you for your question, Patty.  You can place lightweight, white cloths over your hydrangeas during the heat of the day, which helps some.  At least as importantly, water them thoroughly early in the morning (drip system, preferably), and again once the sun no longer is beating down on them.  Just make sure soil is well-draining, so you don't attract fungi that cause root rot if it sits there.

Good example of "the right plant in the wrong place is the wrong plant"!  Good luck!