Answer: The most common cause of spots on hydrangea flower petals is water. If drops remain on the petals when the sun is shining, the sunlight can be magnified through the water and burn the petal, or bleach the color out in those spots. Of course you can't control the rain, but you can control overhead sprinkling. Try to keep water from splashing on flowers and foliage, especially during the hottest months of summer.
It is normal for some of the leaves to turn yellow on your hydrangea in mid-summer. Those affected are usually the lowest leaves. You can remove them, especially if they've developed brown spots. It won't be long before the fall season arrives and your hydrangea loses all of its leaves. At that time be sure to rake them up and remove them from the garden - you won't want any fungal spores hanging around over the winter months!
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