Q: I’ve been thinking about the notion of applying Epsom salts to hydrangeas to enhance the blue color. Since I have an old box of Epsom salts, I would like to try this, but the word “salt” sends up a red flag. How much should I apply?
A: The name “Epsom salt” comes from a bitter saline spring at Epsom, England. The water there contains magnesium sulfate in solution. “Salt” is just a chemistry word for the product of a reaction between an acid and a base.
When the ions of Epsom salt disassociate, they have a negligible effect on the pH of the soil. Since the blue color of a hydrangea is formed by aluminum made available in an acid soil, adding Epsom salt would not make your flowers change color. Alum (aluminum sulfate) is acidifying and would give the effect you want at two tablespoons per gallon per plant per year.