Blue flowers are enhanced by aluminum in the soil. To add aluminum, apply aluminum sulfate, which is available at garden centers. Be sure to apply at the recommended rate. Too much can burn your plant.
Test your soil's pH to see if it's in the ideal range: 4.5-5. If the pH is too high (alkaline) you'll never get blue flowers--even if there’s plenty of aluminum in the soil. Alkaline soil tends to "lock up" the aluminum, making it unavailable to the plant. (However, you can grow fabulous pink hydrangeas!) If your soil is pretty close to the ideal range, you can use a soil acidifier, such as pH Down, to create a better environment for blue hydrangeas. It may take a season or two before you see the pH change.
If your pH is high, you might want to consider growing pink hydrangeas. Or grow blue hydrangeas in a pot. It's much easier to control pH in a container.
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