Answer: There are several reasons why you might see yellowing. One possibility is the beginning of seasonal change to fall color. If this is a new plant, it could be a response to transplant stress. Another is possibly overwatering, especially if it is happening first at the bottom of the plant and then spreading upward.
These plants need an evenly moist soil like a wrung out sponge; it should not be saturated/sopping wet or dried out. When you water, avoid wetting the foliage. Apply the water at the root area slowly and thoroughly so it soaks down deep to the deepest roots. After watering, wait a few hours and then dig down to see how far it went -- sometimes this can be surprising. It is better to water deeply less often than to sprinkle lightly every day.
Using an organic mulch will help reduce the need for watering. It will also help feed the soil slowly over time as it breaks down. Apply it in a flat layer over the root area about three inches thick.
As far as the wilting, it may be that you have been watering too lightly to fully hydrate the plant. Or, these will wilt in the afternoon if they are in too hot and sunny a location then they will perk up again when it cools off at night. If they are wilted in the morning, then they are too dry. Use your finger to dig down into the soil and see if you need to water. If it is still damp, do not water yet.
The black spots could be fungal or bacterial infection, but this late in the season it is not something to really worry about now. This fall, pick up and remove all of the fallen foliage from the plant and put it in the trash. This should help prevent reinfection next year. Wetting the leaves when you water, planting in a spot with poor air circulation, and overall weather conditions can all contribute to foliage issues. Depending on the weather next year it may or may not recur.
If you see the spots beginning again next season, contact your local county extension for help in identifying the problem and determining how to treat it. If a chemical control is needed, they will have the most up to date information on what to use and how/when is best to apply it for maximum results.
Good luck with your hydrangeas.
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