When lower leaves turn yellow it could be a sign of too much water but as you have newly planted hydrangeas it may also just be transplant shock. Hot weather and lots of rain could also encourage fungal infections on the foliage. The good news is that hydrangeas are tough shrubs. You can remove the yellow leaves if they are unsightly but just waiting another month or so will show if your hydrangeas are going to adjust to the new location. Try not to water too much - feel the soil down three inches and do not water if the soil is still damp. Hydrangeas will wilt in hot weather even if they do not need water. A wilting hydrangeas does not mean it needs a drink. Hopefully they have been planted where  they have shade from the hot afternoon sun. If not, dig them up and move them to the shade. Keep growing, Marianne Binetti