How do you change hydrangea colors?
Written by
Michael Sullivan
Reviewed by
Prof. Samuel Fitzgerald, Ph.D.The change in color of hydrangeas intrigues gardeners around the world. These flowers alter color based on soil chemistry unlike any other species. Understanding two important aspects will determine your success: pH and aluminum availability. The blue version requires acidic soil and pink requires alkaline. This natural astrological phenomenon only occurs in select varieties of hydrangeas, such as bigleaf hydrangeas.
Blue Blooms Setup
- Soil pH: Must be 4.5-5.5 for aluminum solubility
- Water Source: Use rainwater or acidified water below pH 6.0
- Fertilizer: Select low-phosphorus formulas like 25-5-30 NPK
Pink Blooms Setup
- Soil pH: Maintain 6.0-6.2 to block aluminum absorption
- Amendment: Apply garden lime quarterly
- Fertilizer: Use high-phosphorus 25-10-10 formula monthly
In acidic soils, aluminum promotes the development of blue pigments. It interacts with flower pigments to create the cobalt blue shades you want. If you want pink, you need to prevent that interaction. Phosphorus will prevent that interaction if the pH exceeds 6.0. I have seen gardens destroyed by the use of aluminum fertilizers in pink attempts.
It is essential to recognize that water quality can significantly impact results, more than most people realize. Rainwater typically has just the right amount of acidity to produce blue tones. Tap water is a hard water source and contains minerals that interfere with aluminum uptake. Always check your water pH before starting your experiment, especially if you plan to use fertilizer or soil with a different pH level. I have several rain barrels for rainwater collection in my blue hydrangea bed.
Patience is your best friend in this process. Rushed applications can create distress in plant and soil biology. Begin testing several months before your desired bloom season arrives. Please keep track of the pH level weekly, as I do in my gardening journal. Small, steady adjustments are always best for large, drastic changes.
Be cautious of the common pitfalls that can ruin color projects. Never use vinegar or coffee grounds for long-term acidification. Vinegar or coffee grounds can lead to unreliable pH swings due to their rapid decomposition and inconsistent additions. Eggshells decompose too slowly to see any useful change. For color shifts, rely on aluminum sulfate for blue or dolomitic lime for pink.
Read the full article: Hydrangea Color Change: Ultimate Guide
