Answer: Soil pH usually dictates the color of hydrangea macrophylla flowers, although some hydrangeas will remain blue regardless of the soil pH. Pink and red hydrangeas often turn blue or purple in acid soils. Blue hydrangeas can turn pink in alkaline soils. Plants can be made (or kept) blue by applying aluminum sulfate to the soil; plants can be kept pink by liming the soil or applying superphosphate. It takes several months for soil treatments to change the flower color so additives should be incorporated into the soil in the fall and again in the spring.
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