The ability of Bigleaf hydrangeas (H. macrophylla) to change color is a unique natural indicator. Contrary to popular myths, hydrangeas don't turn blue simply from 'acidic soil.' The secret lies in aluminum. The plant turns blue only when there are free aluminum ions in the soil that it can absorb. And it can only absorb them in a highly acidic environment.
Important Limitation: Genetics vs. Chemistry
You can only change the color of PINK and RED varieties of Bigleaf hydrangea. White varieties (e.g., 'Madame E. Mouillere') will never turn blue—they lack the pigment delphinidin-3-glucoside responsible for the metamorphosis. It is also useless to try 'painting' Panicle or Smooth hydrangeas.
Step 1. Acidity Control (pH)
To achieve a blue color, the soil pH must be in the range of 5.0 – 5.5. If the level is above 6.0, aluminum becomes bound to the soil and unavailable to the roots. If it's below 4.5, the plant may suffer from mineral toxicity.
Do not use regular white vinegar for acidification—it provides only a temporary effect and kills beneficial soil microflora. Use elemental (colloidal) sulfur for a long-lasting effect or specialized citric-acid-based soil acidifiers.
Step 2. Adding Aluminum
If your soil is acidic but the hydrangea remains pink, there is simply no aluminum in the ground. It must be added externally.
Supplements for Blueing:
- Aluminum Alum (Potassium Aluminum Sulfate): Available at pharmacies or garden centers. Solution: 30-40g per 10 liters of water. Water every 2 weeks from the start of the growing season.
- Aluminum Sulfate: The most effective professional method. Apply in dry form (up to 500g per 1 sq.m. annually) or as a drench.
- Specialized Fertilizers: Labeled 'For Blue Hydrangeas.' These have a pre-balanced composition.
Step 3. The Phosphorus Mistake
This is the most common mistake for beginners. High phosphorus content in the soil blocks aluminum, turning it into insoluble salts. If you want a blue hydrangea, use fertilizers with low phosphorus content (NPK ratios like 25-5-30).
Never use bone meal or superphosphate for blue hydrangeas—they will instantly push the plant back toward a pink color.
Transformation Schedule (Calendar)
When to Begin Procedures:
- March/April: Begin acidification (apply sulfur or drench with acidifier).
- May: First application of alum. The soil must already be moist.
- June: Monitor pH. If the buds turn a muddy purple, the acidity is insufficient.
- Blooming Period: Continue to maintain acidity but reduce the alum dosage by half.
Achieving a pure blue or deep violet color is a long-term commitment. It often takes two seasons to completely change the chemical composition in the root zone. Be patient and always test the pH before adding a new dose of chemicals to the soil.
