Answer: Hydrangeas turn blue in acidic soils. If your soil is alkaline, they will turn pink. To keep blue hydrangeas blue in alkaline soil you need to acidify the soil by adding sulfur (aluminum sulfate). It takes several months for this to work its way into the soil and change the pH so applying it now won't change the color of this years blossoms. But, applying now will help next years blooms. It takes only about 3 TBS per foot of soil to make the change; sprinkle on top of the soil and then water it in well.
Best wishes with your garden!
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