Answer: Aluminum sulfate is sold in garden centers as a soil conditioner. Bonide is one brand that you'll find locally. Superphosphate is simply a fertilizer that is lacking nitrogen and potassium. Look in the fertilizer section for a product with zero as a first number and zero as a second number such as: 0-10-0 or 0-20-0. It should be identified as a super phosphate fertilizer.
You only need a few tablespoon of either product per plant so buy the smallest quantity possible. You can apply in the fall and again in the spring and a third time in early summer. It may take a year or two for you to see changes in the color of the hydrangea flowers.
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