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Epsom Salt Won’t Blue Your Hydrangeas—Use Alum Instead

Epsom salt (magnesium sulfate) is pH-neutral; it doesn’t acidify soil. Blue hydrangeas need aluminum availability in acid soil. Apply 2 Tbsp aluminum sulfate (alum) per plant to lower pH and supply Al³⁺. Epsom salt is fine as a magnesium supplement, but not for color change.

Hydrangea with blue flowers

About Gardening: Epsom Salt Overdose on Hydrangeas: Rescue and Prevention; Food Coloring in Roses: Instant Color Change, Not Genetic; Can Hydrangea Florets Survive in Wet Foam After Separation?; Epsom Salt for Pepper Plants: Recipe and Timing; Epsom Salt on Lawns: Better Alternatives; Fire Ant and Termite Control: Orange Oil, Epsom Salt, and Cedar Mulch Timing; Timing Orange Oil/Epsom Salt for Fire Ants in North Texas; Epsom Salt on Lawns: Only If Soil Test Shows Magnesium Deficiency; Goldfish Hemorrhagic Septicemia: Salt & Water Quality; Epsom Salt for Roses: The Magnesium Boost Your Bushes Crave

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