Welcome to NGA, BMurph70. Wilting of the hydrangea foliage can be triggered either by heat stressor events or by root disturbances. Examples of heat stressor events are -individually or in any possible combination- temperatures much above 85°F; windy conditions, lack of water or lack of enough water; direct sunlight in an amount that the plant is not yet acclimated to; etc. Root disturbances can be caused by recently planting or transplanting the shrub; performing gardening tasks that disturb/injure the shallow, tiny, fibrous hydrangea roots that are normally found only about 4 inches deep or so; damage from underground pests like voles, grubs, etc; root rot due to over-watering, which maintains the soil wet/soggy for long periods of time.
You can rule out over-watering if you know that the downspout will not dump large quantities of water by the hydrangea's drip line; if you frequently test the soil before watering; and if you only water as soon as the soil is dry or almost dry at a depth of 4 inches, the typical depth of new hydrangea roots. You can also insert two fingers into the soil to a depth of 4-8" and extract some soil in between the two fingers. Then apply some pressure and observe if the soil forms water droplets as you apply pressure to the soil. If you see water droplets form, the soil may be too soggy/wet because it recently rained, your soil drains slowly or you are watering too much.
An unusual symptom in the picture is that wilting of the leaves is only affecting part of the plant. Normally all leaves wilt about the same as the environmental conditions affect all the leaves in the same way... unless one side is not watered properly, gets more sunlight or unless the roots on that side have sustained an injury or have root rot. You can rule out root rot by watering -one time only- after the sun goes down. When temperatures recede below 85°F, the leaves will then slowly perk up on their own and look very nice in the morning. However, if the plant has root rot/injury, wilting will occur permanently 24/7 in the affected area. So water tonight and observe if the leaves that are wilted in the picture will perk up in the morning or not.
Most likely though, the issue is because it was planted during hot enough environmental conditions and it suffers from heat stress as it is not acclimated to these summer heat waves at the plant nursery. A scan of temperatures for Plainfield since June indicates that temperatures above 85°F were rare at first (twice per week) but then they shot up to hit 99°F multiple times since. Maintain 3-4" of mulch at all times of the year. In the summer, check the soil moisture daily (6-8am) and water as soon as a finger inserted to a depth of 4" feels dry or almost dry. Water the soil -not the leaves- deeply early in the mornings (6-8am) using the finger method and use enough water to get the soil moist/damp at a depth of about 8".
Once temperatures typically reach or exceed 85°F (sporadically in May but more often in June for Plainfield), consider increasing the amount of water from 'spring watering levels' to 'summer watering levels'. Then once temperatures recede in the fall and typically stay below 85°F, revert back to 'spring watering levels'. Make note of these dates in a wall calendar or an electronic calendar for your future reference. Once the shrub goes dormant and the foliage turns brown, water only once every week or once every two weeks based depending on local rains and the finger method but... stop once your soil freezes. Once your soil thaws in spring 2023 and you observe new growth, begin watering, mulching and protecting from late frosts until your average date of last frost; for Plainfield, that is the 3rd-4th week of May. Begin fertilizing afterwards. The last application of a slow release fertilizer should be done about three months before your average date of first frost; the average date of first frost for Plainfield is around the 3rd-4th week of October so stop fertilizing at the end of July.
You can fertilize using a cup or so of organic compost, composted manure, cottonseed meal. Or you can use a general purpose, slow release fertilizer with a NPK Ratio of around 10-10-10 per label directions.
It is normal for hydrangea paniculatas like yours to show their feet during the summer months by yellowing some foliage near the bottom/center area of the shrub. As long as you maintain the soil as evenly moist as you can with no inconsistent watering, there should not be too much leaf loss.
Lastly, make sure that water from the downspout/gutter does not collect often and in large quantity under the drip line of the hydrangea. I could not clearly tell in which direction the water was going to drain so I thought best to mention it.
Does that help you, BMurph70? Luis
