Answer: Soil pH (acidity or alkalinity) can affect the color of big leaf hydrangea flowers. Soil treatments are usually added in the early spring to affect the color of the new flowers because it takes several weeks for the minerals to react with the soil. You may have overdone things and changed the soil's pH too much. Browning leaf edges indicate over watering or over fertilization. I would prune off the fading flowers and adjust the watering. Thoroughly soak the soil then allow it to dry out slightly before you water again. Your hydrangea will adjust and new growth should be a healthy green. Best wishes with your hydrangea.
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