A recently planted hydrangea is a shrub whose root system has been pruned to fit into a plastic pot. As such, it takes the plant a while to develop a bigger root system to deal with summer weather issues. While the plant is for sale at your plant nursery, it gets watered a lot and often. Plus it gets placed in areas with shade cloth so it gets limited amounts of sun and wind. Then you introduce the plant to your garden and it tries to cope with sunlight issues, windy conditions and moisture issues. It begins to wilt. Transplant shock is what some people call this.
So what can you do... try providing it increasing higher levels of sunlight slowly, as opposed to dumping it in its final location. An example: put it where it gets full shade for a week or so... as long as it is "bright" shade. Then introduce it to get 1 or 2 hours of morning sun. And so on. Some people get a chair or umbrella and place it to shade up new plants so you can try this. The shrub should end in a place where it gets morning sun only, say, until 11am-12pm. More or less. Afternoon or evening sun is not good because it is usually hot enough to make wilting episodes look really baaaad. You can go down to 11-2 hours of morning sun only if you want. Or full shade as long as it is bright shade. But keep the plants away of (house) wall surfaces, cement surfaces, rock beds etc that irradiate too much indirect sunlight to the plants and for long periods.
Windy locations make the leaves loose moisture faster than the roots can resupply so this causes leaves to wilt. Not a big deal often. The plants recover on their own at night or by the next morning. In future years, when they have larger root systems, this only happens during the middle of the summer months (or on windy days).
Mulch will help keep the soil moist longer so keep the shrub mulched with 3-4" of mulch up to the drip line. If you see those round fertilizer pellets in the potting soil, you do not need to fertilize this year. Do it starting next year in Spring. Do not fertilize the hydrangeas just as they are having issues with sun, heat and water. You can give them 1/2 to 1 cup of cottonseed meal, composted manure or organic compost. Or you can use a slow-release, general purpose chemical fertilizer. During the rest of the growing season, try some coffee grounds, liquid seaweed and-or liquid fish but stop all fertilizers by the end of June so the plant will go dormant in the Fall and not stay in grow mode.
Hydrangeas love water so each time you water the bushes, give each one 1 gallon of water in Spring and 1.5 in the Summer or as needed (more when the shrubs get larger, etc). Revert back to Spring levels in Fall when the summer temps moderate.
Transplant shock will go away in 1-3 years. You will notice less and less but, it is always good to ck the plants often during the summer months or after windy days when the weather service gives wind advisories.
PS - I have worse looking wilting episodes when I turned off my sprinkler system (it was raining a lot) and forgot to turn it back on!
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