I’m north of Kingston in average soil. Our hydrangea shrubs are a few years old and this year blooms are wilting from the bottom and eventually the whole bloom becomes brown and wilts. The wilt is not dry to the touch. It has been watered through the heat. It does not get direct sunlight. Leaves are fine and plant appears to be otherwise healthy. No evidence of insects.
Hello north of Kingston!
You didn’t specify the type of hydrangea shrub you have, but the most common is the Annabelle mophead hydrangea (Hydrangea arborescens), which has huge snowball-like flowers. They are pretty tough, low-maintenance plants, but are susceptible to some diseases and insects.
Here is a good reference to the diseases, by hydrangea type: https://www.ag.auburn.edu/hort/landscape/kerrysmith.html
However, since you say that only the blooms are being affected and that the rest of the plant is doing well, I do not suspect wilt, which is usually evidence of root rot problems caused by fungi or by overwatering.
It is important to ensure that you are correctly watering deeply a couple of times a week, instead of frequent light ‘sprinkles’. Check the soil moisture by moving away any mulch and inserting your finger as far as you can into the soil near the roots. The soil should be moist, not dry nor soaking wet.
You have found no evidence of insects and those most likely to damage the hydrangea blooms are Japanese beetles and rose chafers, both of which are pretty easy to see.
It is possible then, that the blooms are responding to the extreme heat. They are not terribly happy when the temperature gets into the 30’s and we have had far more of those extreme heat days than usual this summer. This year, we are seeing many plants going through their growth, bloom, and dormancy cycle much faster than usual.
Please monitor your plants over the next 10 days or so and let us know if there is any more deterioration of the plant itself. And do attach a photo if you can.