Answer: Pink and red hydrangeas often turn blue or purple in acidic soils. You can control the flower color by controlling the soil pH. If the blooms were purple, the plant was growing in very acidic soil. Your soil is probably alkaline, and the higher pH made the blooms come out blue this year. To change the flower color back to purple, apply superphosphate to the soil in the spring, prior to the development of the flower clusters. Test the pH of your soil to see how much treatment is necessary to lower the pH to about 5.5 or 6.0.
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