The Q&A Archives: Adjust pH on Hydrangeas

Question: Does the soil pH need to be adjusted to be more acidic
before buds form on hydrangea for blue flowers, or can
the color be changed after the bloom opens? My blue
hydrangea are just beginning to bloom, but they look
pink instead of blue. I have fertilized with a water
soluble acid fertilizer.

Answer: Bloom color will change quickly once the plant roots are in acid soil, but soil pH change can take time. Therefore the sooner you start the better. If you have added enough acidifying material to the soil, you should see some bloom color change later this season. Just keep up the acid fertilizing (though don't overdo it--follow label directions.)

You may have to add sulfur dust or aluminum sulfate to the soil to bring the pH down enough to change bloom color. Some soils are too buffered to drop in pH by acid fertilizing alone. A good starting point is 1 or 2 pounds of sulfur dust or aluminum sulfate per 100 square feet of soil surface. A heavy, Gulf Coast clay soil will take twice that much.

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