When the plants get heat stress, they display symptoms like that. Lack of humidity, too much sunlight, lack of water, too much wind can affect them as they lose moisture through the leaves faster than they can absorb more water. The leaves begin to brown out from the edges inwards. The plant shuts down some leaves by yellowing them so they fall and it does not loose so much water.
Make sure that you are watering enough. A new hydrangea needs enough water to moisten the soil down to a depth of 8". Usually in Spring, they require here about 1 gallon of water per plant, more or less as that depends on soil type or so. As temperatures go up to or stay above 85F, you need to increase the water to around 1.5 gallons per plant. As temperatures typically stay above 95F, you may need to increase to 2 gallons per plant. Tweak as needed for your conditions which may require even much, much more water than those numbers.
Water only the soil early in the morning, starting with the root ball and extend outwards in all directions. Never water the leaves to minimize the chances of fungal infections like powdery mildew or leaf spots. If you use a sprinkler, try to water just before sunrise.
To tell if you watered enough, insert a finger to a depth of 8" and see if the soil feels moist. If not, the amount of water was not enough.
To tell if you need to water, insert a finger into the soil to a depth of 4" and water if the soil feels dry or almost dry. Hydrangea roots normally develop in the top 4" of the soil and that is why we check that way.
You can do the above finger method to a depth of 4" daily, early in the mornings, for 2-3 weeks to control the sprinkler or drip irrigation. Test the soil daily, at about the same time. If the soil feels dry, water the plant and make a note in a wall calendar or an electronic calendar indicating that you watered say 1.5 gallons on June 2. And so forth every time that you water. After 2-3 weeks, review the notes and calculate how often you were watering. Say you look at the notes and determine that on average you were watering 1.5 gallons every three days. Then set the sprinkler to water the plants near sunrise every three days and make the station run long enough to give the plants 1.5 gallons of water. Tweak as needed for your conditions which may require even much, much more water than those numbers. If temperatures change up or down 10-15 degrees and stay there, consider using this set up to see if you need to tweak things.
In the Spring, newly purchased hydrangeas usually need about 1 gallon of water per watering. Once temperatures are usually at or above 85F, you need to ratchet up the water to around 1.5 gallons per plant. Once temperatures are usually at or above 95F, give them 2 gallons of water per plant. Tweak as needed for your conditions which may require even much, much more water than those numbers. And use the finger method if you are unsure. For temperatures above 100F, I would recommend watering manually if the finger method says more water is still needed.
Maintain 2-4" of mulch to minimize soil moisture loss and protect the roots from winds and extreme temperatures in summer/winter.
Once lower temps arrive in the Fall, reverse the process. Cease hand watering when temps drop below 100F. Bring down watering levels to 1.5 gallons when temps are usually below 95F. Bring water levels down to 1 gallon when temps are regularly 85F or less. Water only once a week, depending on local rains, when the plant goes dormant and the leaves brown out in the Fall. Stop watering if the soil freezes or if the plant is getting enough water from local rainfalls during winter. Resume Spring watering levels when you see leaf out.
Make sure that the plant does not get too much sun. During year one, the leaves will be more sensitive to sunlight because the plant was grown in a green house. But mopheads should always get morning sun only, dappled sun or full but bright shade. No sun after 11am or so. They should not get any direct sun in the afternoon or in the evening... although in places with a weak summer sun, they may withstand sun until later times if they also get more water...
If the plants get afternoon sun now, consider transplanting them elsewhere. If they are getting a little sun before 11am but are still sensitive to wilting episodes and you are watering enough then temporarily provide them with anything that will give them additional shade in year 1. I have used outdoor chairs, umbrellas and all kinds of contraptions that I created from scratch. In theory, hydrangeas will perk up after a wilting episode on their own. But they need the soil to be moist enough to do that. If the soil is not moist enough, they appear wilted in the morning. That immediately tells me that the plant needs water so I would give it a watering then and there. If you see an extreme wilting episode (say it was hot but also it was very windy), give them water immediately. I sometimes make by drip irrigation go 'on' the night before a very windy day.
If the plant is getting too much sun, you will also notice that the leaves in direct contact with the sun will turn all yellowish, including the leaf veins. But the leaves underneath will remain dark green. This happened to me when the tree that shaded one hydrangea lost limbs after a bad summer storm.
Does that help? Luis