Answer: In general, hydrangeas are pink in alkaline soil, and blue in acid soil. To change the colors, you will need to change the pH balance of your soil, four to six months before the blooming season (i.e. at the end of the previous season). Turn alkaline soil into acid soil by adding aluminium sulfate. Use 3-4 tablespoons in a gallon of water, and apply twice, a month apart.
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