Answer: Not all hydrangea plants can be manipulated to change flower color, but big leaf hydrangeas that are pink or blue can sometimes be coaxed to change color. 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. If your soil is alkaline, add aluminum sulfate to the soil in February or March and next summer's blooms should be blue. If your soil is already on the acidic side and you plant a pink flowering hydrangea, the flowers will turn blue all on their own. Best wishes with your hydrangea.
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