Answer: It may be too late for this season's bloom but you can start adjusting the pH earlier next year and next season's blooms should be pink. Hydrangea bloom color can be affected by soil pH. A somewhat acidic 5.5 should turn them blue, a more alkaline 6.0 should turn them pink with the effect being more intense the more extreme the 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 a while for the chemicals to react so apply early - before the flower buds begin to develop.
Best wishes with your hydrangea!
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