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 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. Since your soil is probably alkaline, add aluminum sulfate to the soil in February or March and next summer's blooms should be blue.
With the exception of Oakleaf hydrangea, which can be pruned to the ground each autumn, hydrangeas produce flowering shoots in the spring on last season's wood. So, to prune for flowering, reduce the old wood by one-half to one-third after bloom. The shrub will develop new flowering wood the following spring.
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