Yes, hydrangeas will wilt in the sun or heat because they have large leaves and going limp helps to conserve moisture. Your hydrangeas should perk up after sunset and can continue this pattern of wilting on hot days all summer. To look and grow their best hydrangeas prefer some shade during the hottest part of the day but if you are willing to put up with hot day wilts your hydrangeas can thrive for years in the hot spot. A mulch over the soil may help keep them cool. Keep growing, and stay cool, Marianne Binetti