Turning Hydrangeas Blue: Aluminum Sulfate Timing and Tips

How Fast Does Color Change?

A gardener applied a couple of tablespoons of aluminum sulfate around each hydrangea to turn pink blooms blue. The plants already had large, bright magenta flower heads. The question: how long until color shifts, and will current blooms be affected?

Expert Advice: Increase Dosage, Watch Phosphorus

Color change can begin within a couple of weeks, but the initial application was likely too light. For a 3-foot-tall plant, use 1 to 2 cups of aluminum sulfate. Also, apply a low-phosphorus fertilizer. Phosphorus can bind aluminum, making it unavailable to the plant and preventing the blue coloration.

Key Points

• Aluminum sulfate lowers soil pH and provides aluminum ions for blue pigments.
• Current blooms may not change fully; new growth will show the effect.
• Use 1–2 cups per mature plant, not tablespoons.
• Avoid high-phosphorus fertilizers (e.g., bloom boosters).