Why did my pink hydrangeas turn blue?
Written by
Michael Sullivan
Reviewed by
Prof. Samuel Fitzgerald, Ph.D.When pink hydrangeas change to blue, it is an indication that notable changes in soil chemistry are occurring below your plants. The color change occurs when aluminum is made soluble and available for roots to uptake. Acidic conditions unlock aluminum, allowing it to bind with the pigments that color your flowers. Understanding this process will ultimately help you regain control over your blooms.
Soil Acidification Causes
- Acidic rainwater: Natural precipitation lowers pH over time
- Sulfur applications: Accelerates aluminum release in soil
- Pine needle mulch: Decomposes into mild acidifiers gradually
Pink Restoration Methods
- pH testing: Check levels monthly with digital meter
- Lime application: Use dolomitic lime to raise pH
- Water adjustment: Irrigate with alkaline water sources
Aluminum solubility is the driving force of this color shift. Below a pH of 5.5, this metal leaches into the soil solution. The roots absorb it, transporting aluminum to the growing flowers, where it binds with anthocyanin pigments, causing the shift from pink to blue. I have measured aluminum concentrations that double after a heavy rain season.
To restore pink color, it is necessary to inhibit the uptake of aluminum. Raise the soil pH to approximately 6.0-6.2 using dolomitic limestone. These pH levels provide conditions that prevent aluminum from remaining insoluble. The addition of phosphorus also plays a significant role in the process, as it changes the chemical bonding with aluminum. I add bone meal around my pink hydrangeas once every three months.
It is important to maintain regular water chemistry to avoid regression. Use tap water over rainwater during key biased periods of the growing season. Test your runoff for chemistry once a month and maintain a pH above 5.8. My hydrangea bed, located next to the gutter downspout, is always the first to turn blue, demonstrating the influence of water.
When selecting fertilizers for color stability, I look for formulations that indicate suitability for pink hydrangeas and are high in phosphorus. Be sure to avoid those containing aluminum sulfate. My personal preference is to clearly mark those containers to avoid accidental blueing.
Read the full article: Hydrangea Color Change: Ultimate Guide
