Hydrangea leaves wilt easily during the summer months. Foliage loses leaf moisture faster than the roots can absorb soil moisture. Examples of stressor events that can trigger wilting: too much direct sun, windy conditions, insufficient water, temperatures that are typically above 85F, root damage/injury, etc. Since your temperatures have been above 85F almost daily, the heat wave is having a punishing effect on the plants, especially if you have them in such a small pot. I would put them into larger containers using well draining potting soil for ericaceous plants or plant them outside in the ground. But because of the heat wave with temperatures spiking to 100F, consider instead putting them in full but very bright shade for a few days, then in 1-2 hrs of very early morning sun for several more days and then putting them in morning sun until 11am or earlier. Acclimating them in a larger pot will help until you can transplant them/move the pot into a site that now in the summer receives full sun. But transplant/move them after temperatures recede in the fall or once the plant goes dormant in late fall/winter. Aim to keep the soil moist but remember that with temperatures above 85F, there may be wilting. However, they will perk up at night when it becomes cooler and will look recovered in the morning. Can you post pictures for further comments?
