Thanks for writing. It's not uncommon for some plants, such as hydrangeas to wilt in hot sun. You may find that they wilt even when the soil is moist --- what happens is that their roots can't absorb moisture fast enough. If the plants recover in the afternoon, it's probably not harming them and they should be fine. Otherwise, you might move them to a spot where they get a little more protection from the sun.
---Justin, Senior Garden Editor, BHG.com