This could be a symptom of a moisture issue (over- or under-watering) or a commonplace fungal infection, such as Botrytis. When the plants are irrigated, is the foliage wet by a sprinkler or hose, or is only the soil being moistened?

Heat stress could cause premature flower fading as well. While spider mites might cause damage, they're not a common pest of hydrangea and would probably be seen damaging foliage first. The marginal browning and general yellowed color of these leaves doesn't suggest spider mites, but you can check on the leaf undersides. Look for stippling and webbing; sharply tapping suspect leaves over a white sheet of paper should reveal specks of mites crawling around (or squished into reddish-brown streaks when folded). You can discontinue treatment with the pesticide as this is likely not the issue, nor will its disease-control component cure any infections already set-in. Spraying pesticides during hot weather can also damage plant tissues, so use caution going forward if any other plants need treatments.

Has this plant been fertilized recently, or is it close to lawn which is fertilized annually? While not all hydrangea varieties have equally-dark green foliage, this plant's leaves do seem to be a bit off in color - too yellow. Sometimes, over-watering causes this as roots struggling to get enough oxygen in overly-damp soil don't function well, and as a result can't absorb the nutrients they need. In other cases, the nutrients themselves might be deficient or present in excess, the latter of which also harms roots. If any other nearby plants are not a healthy color, you could consider a soil test as a way to check on nutrient levels and pH. While most often recommended for lawns or vegetable beds, it can be useful for troubleshooting decorative plantings as well. If the symptoms are limited to this plant alone, then a soil test may not be needed since that suggests root problems with this particular shrub.

Yes, you can trim off the browning flower heads. Only water more often if the soil is drying out frequently. Feel for moisture levels around four to five inches deep (not including the mulch) and water well if it's getting pretty dry. Otherwise, soil that is damp to the touch should not need irrigating. Heat-stressed hydrangeas can wilt on very hot days even when they have enough soil moisture; they will recover in the evening and do not need more water. Even if the extra moisture doesn't drown roots, it may speed-up the leaching-out of soil nitrogen, which contributes to leaf yellowing. Make sure the mulch doesn't contact the plant's crown (where stems emerge from the roots) and keep the layer no thicker than three inches.

Miri

The watering has been done via sprinkler system in the very early morning (4 am or so) with an occasional watering near the base of a plant using a hose with a sprayer.

The lawn doesn't get fertilized.  It's a mix of grass and clover that gets cut weekly.  I haven't fertilized the flowers, either. We are in a fairly new subdivision, so we have a lot of clay and the only soil that shows reasonable signs of life (worms vs grubs) is in a raised bed in the back for tomatoes.

There are 3 hydrangeas in a row which all look similar to the photo.  I don't think there is anything on the underside of the leaves.  One picture shows both top and bottom.  There is a fourth brand new one with different colors that I put in to replace the evergreen that died off.  It looks generally better, but has probably only been in for a month or so.  There is slight discoloration on the leaf edges.  (Picture shows all 4.  The original 3 are a wee bit washed out, but they would never be as vibrant as the new one.) Since I planted it myself, I dumped in a fair amount of leafgro from the garden center.  The other three were planted by a landscaper, and I don't know how much he worked into the soil.  Many  of the snapdragons I planted last year came back with a vengeance, and most of the perennials I added seem to be doing OK.  It's the first year for them.  The shrubs the builder put in are looking a little dicey ( close-up photo included), but I think it likely that they picked up the spider mites from the other plant.  There is a larger shrub / tree to the far left, but I can't use more than 3 photos, and I'm not certain what it is.  The flowers have given way to some sort of red berry clusters that stick upwards.  It doesn't seem particularly unhealthy, but some of the leaves do seem a bit on the yellow-green side.

I just redid the mulch in the last two weeks, so the problems predate the mulch.  That said, I will make sure to push it away from the crown.  I will also attempt to get soil testing done this weekend.

Thank you for your help.

Looks like I can add more pictures if I do them in a separate post.

Thank you for the additional photos. We concur there doesn't seem to be any pest issues on the hydrangea foliage.

Depending on the duration of how long the sprinklers are run, and how often they are programmed to run (unless they're on manual), it may be that the beds are being overwatered. As a rough guide, shrubs and perennials might need a soaking once or twice a week, though of course it will depend a bit on soil characteristics and weather. Feeling the moisture levels by hand tends to be the best way to judge when watering is needed; waiting until plants wilt means that root damage is already beginning, but too-frequent proactive watering can overwhelm roots and deprive them of needed oxygen. It's a balance, but one which gardeners can get used to pretty quickly by checking the soil periodically when rains are intermittent.

Overall, though, the hydrangea group looks to be in decent condition. It's possible the petal and leaf browning is just a combination of minor Botrytis (due to surface wetness) and sun/heat stress. You mentioned they are east-facing, so as long as the hydrangeas get some shade from the house (or taller nearby plants) in the midday/afternoon hours, they should be fine with that exposure. If not, they do tolerate more sun, but can become a bit bleached-out looking in summer.

When mulch next needs replenishing in the future, you can first put down about an inch of compost (Leafgro is perfectly fine) and then a light ~2" layer of mulch on top of that. This will help amend the soil over time as it gradually works its way into the root zone of the plants, improving the soil's texture and organic matter content. Mixing it in when planting is also fine, but use caution not to include too much in proportion to the native soil, since this can lead to water percolation irregularities, causing uneven moisture levels around roots. You don't need to correct that at this point and the newest hydrangea looks fine, but it's something to be aware of going forward. The slow breakdown of the mulch will also add organic matter and improve the soil over time. Fortunately, soil life is very diverse and earthworms aren't a necessary contributor for the benefit of mixing and aerating soil layers.

The other shrubs look like a variety of Japanese Holly (Ilex crenata); they are very commonly used because they are spineless and have a Boxwood-like look and are easy to trim for gardeners  who like to prune (though it's not needed). They are generally hardy but can be susceptible to root rot issues in wet soils. If your soil test results show a high pH, the lack of acidity may be contributing to nutrient deficiencies in all three of these shrub species. Typical leaf colors for Japanese Holly, with the exception of odd varieties known for golden leaves, is a deep, dark green. Their color here is a bit yellower than normal, and they are definitely not a golden variety. Japanese Holly are not immune to spider mites, but they rarely contract them at any damaging level, and we don't see signs of pest damage to the foliage. Issues with Japanese Holly usually center around root health.

The upright shrub with developing berries is a variety of Doublefile Viburnum (Viburnum plicatum tomentosum). The berries, if they ripen fully, will eventually turn black or disappear via bird feeding. This too looks a bit off-color, though sometimes they display this leaf bronzing due to nutrient deficiencies. Over-watering is again possible. They appreciate not drying out, but perhaps either the sprinkler system is keeping this one too wet or a downspout in the area is adding to its soil wetness. We can't tell if what looks like a downspout behind it empties into that bed or is buried with its outlet further away.

Miri

Thank you so much.  I lived on a heavily wooded lot for nearly 20 years; it's been a long time since I tried to grow anything but moss, pachysandra, and lariope.

Step 1:  sprinklers are on a timer.  At a minimum, water less often.  I may turn it off entirely and put down a soaker hose instead.  I assume the soaker hose can sit on top of the mulch?  Is it better to run am or pm on a morning exposure?  

Step 2:  get the soil tested.

After I get the soil samples, would it be OK to push the mulch out of the way, sprinkle down leafgro, and then mulch back over?  Obviously not the most efficient approach, but I doubt I'll need to add mulch until next year.

Yes, soaker hoses can sit on top of mulch, though many gardeners use them under a mulch layer so the hose material isn't exposed to as much UV radiation (in case it degrades the material over time) and so the water inside stays a bit cooler; probably also for aesthetics of hiding it, though that's just personal preference.

Regardless of sun exposure, morning watering - via any device - is best as it allows surfaces to be dry by nightfall, which minimizes infection risk. It also makes sure plants have a supply of moisture to get them through the day (though a good soaking should last them several days apiece).

You could wait to add Leafgro until next year's mulching, or you could add it now (or autumn) underneath the current mulch layer. It's up to you and the timing details likely won't have a significant impact either way.

If using the sprinklers is easiest or most cost-efficient way to water for now, you can try the "tin can" technique to monitor how long the sprinklers should run to put down an inch of "rain." This is mentioned at the bottom of the Watering Trees and Shrubs page.

Miri