Hydrangeas do not fare well using rocks as mulch. As summer arrives, the rocks absorb a lot of heat and release it at night, keeping the shrubs warm far too long. In Spring, they are ok with rocks for a while. I suggest other mulches like hardwood mulch, pine needles, etc. It does not have to be acidic mulch necessarily as they can tolerate some alkalinity.

The wilted look shows that they are loosing internal moisture thru the leaves faster than they can absorb water thru the roots. If you have not already done so, I also suggest upping waterings a notch due to the arrival of the summer months. I already had to do this back in May since Tx is so much hotter.

The wilted look happens when the leaves loose moisture way too fast. That is common when it is hot or when it is windy. It is common in the summer, especially the first few summers when planted outside. But in a few years, they should fix them overnight (check the in the morning and water then if they appear still wilted at 6am-ish)... provided that the soil is kept uniformly and sufficiently moist. No periods of wet soil, dry soil, wet soil, etc. Aim to get water down to a depth of 8" each time you water. Start with about 1 gallon of water and tweak as needed. In my weather and soil, I am up to 1 to 1.5 gallons a day. Just an example. Your soil, temps, etc are different but it gives you an idea. When newly planted, water the soil -not the leaves- from the root ball outwards (i.e., from the crown where the stems originate from).

Thanks Luis!

I'll try upping the watering. It looks different than other wilting I've seen due to heat, but I hope your right! It's just so odd that it was only happening on the right side of the plants.

I don't think I can switch from rocks to mulch at this point, but hopefully most of the rocks will be covered as these shrubs grow.

Regarding soil, Pittsburgh has a lot of sand and clay. There is some loam / top soil, including leftover landscaping soil from previous plants. I added top soil around the root ball when planting too. We are by the river / low altitude, so the clay/soil usually stays a bit moist.

Yeah, the rocks "cook" the roots and stems under the soil in the Summer. I sometimes wish the shrubs could handle it as your pics and those of others were very nice looking... ones one would like to copy! ;0)

Do you have landscape cloth AKA weedblock fabric under the rock mulch? If so that is one more problem that they have to contend with. I would plan on over time converting the rocks back to organic mulch such as shredded wood or bark or compost or pine straw if you want to grow hydrangeas there. Hydrangeas are plants of woodland edges and so the rock mulch really doesn’t benefit them. Rock mulch is also a lot more difficult to weed.

I do not have weed block under the Tiny Tuff Stuff. I have some material in some spots, but peeled away from the plants.