Answer: 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 can fail to color, or, more often, turn greenish-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. Since your soil is probably alkaline already, add superphosphate to the soil in February or March and next summer's blooms should be pink. You might want to test your soil's pH - 7.8 will provide the pinkest coloring. Best wishes with your hydrangea!
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