The Q&A Archives: aluminum sulfate

Question: I want to be sure that the hydrangeas I purchased will have blue blooms. The attached tag says the plant will have colored blooms based on the soil acidity. After reading through the answers from your experts on this topic, I assume I'll need to be sure the soil I use has the proper PH...adding aluminum sulfate will drop the PH and result in blue blooms. I purchased Miracle Grow tree/shrub garden soil to plant my hydrangeas. How much alumimum sulfate should I add (if any) to this soil to ensure blue blooms? Where can I purchase aluminum sulfate? Do I mix it into the soil before planting? Thanks!

Answer: The changeable color varieties of hydrangea should be blue in a pH range of about 5.0 to 5.5 and turn pink when the pH is a bit higher, say between 6.0 and 6.5 or so. In your area, the soil may naturally be acidic enough (if it has not been limed) for the hydrangeas to stay blue. Using an acidic mulch such as pine needles or oak leaves, and feeding with an acidified fertilizer such as Miracle Gro should be enough to naturally maintain the pH in the range you need for the blue coloration. If you apply sulfur to lower the pH, you could lower it enough that no plants will be happy, so test your soil before adding anything like sulfur or aluminum sulfate.

Best wishes with your hydrangeas!

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