Answer: Changeable hydrangeas should be blue in soil with a somewhat acid pH. Usually a pH of 5.5 or a bit lower is enough to obtain a blue coloring in varieties that are capable of it. In your area I would expect the soil to be naturally acidic enough to bring out the blue, unless for example lime had been applied to raise the pH. In that case, the lime will leach away in a few years and the pH will naturally drift lower again. Aluminum sulphate is sometimes suggested for acidiying the soil but I would not recommend it as the aluminum can be toxic if it builds up in the soil. Sulphur can also be used to lower pH. Sometimes simply using a fertilizer formulated for acid loving plants per the label directions is enough to make the difference. I would suggest you check with your local county extension for help in testing the soil and determining the best way to lower the pH to accentuate the blue in hydrangeas.
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