Answer: Hydrangeas tend to wilt in the hot afternoon sunshine. It's their way of conserving moisture. They generally recover overnight and look fine the next morning. The only way to stop this activity is to move your plant to an area that gets morning sunshine and afternoon shade, or at least filtered afternoon sun. You can wait until this fall to move your plant, or you can move it now if you're careful.
Soak the soil the day before the move and then dig the new hole before digging your plant out of its current home. Make the move as quickly as possible, exposing the roots to hot sunshine and dry air can really shock it. Water it well after moving.
Best wishes with your hydrangea!
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