Thanks for writing. It's very common for plants such as hydrangea to wilt during the day if they get direct sun and then recover at night. What happens is that the plants lose more moisture through their leaves during hot weather than their leaves can absorb, no matter how moist the soil is. The best thing you can do to prevent this from happening is to move your plants to a spot where they get shade during the afternoon.
---Justin, Senior Garden Editor, BHG.com