Answer: You can change the color of hydrangea blooms with a simple soil treatment. Pink and red bydrangeas often turn blue or purple in acid soils, and plants can be made red by liming the soil or applying superphosphate. For blue flowers, treat the soil with 2 Tablespoons of aluminum sulphate per plant, making 4-6 applications at 2 week intervals, starting in the early spring before the plants bloom. (Sulfur will acidify the soil. The desired pH range would be between 5.5 and 6.) For pink flowers, add lime to the soil, to raise the pH above 7.8). Test your soil to measure the pH range.
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