Yes, many plants including hydrangeas wilt during the hottest time of the day to conserve moisture. This also happens if a mole or vole has damaged roots. Try using a mulch of wood chips over the roots to keep them cool and water in the morning on hot days so the roots can stay cool. Perhaps that is why they are called morning glory - not so glorious in the afternoon and closed in the evening. Keep growing, Marianne Binetti