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.
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