How do I change hydrangea color using soil pH?
Written by
Nguyen Minh
Reviewed by
Prof. Martin Thorne, Ph.D.Hydrangeas develop their colour purely depending on what is going on in the soil. An acid soil, that is, one with a reaction below 5.5, admits the absorption of the aluminium salts contained in the earth; an alkaline soil, above 6.5, absolutely rejects it, and consequently pink flowers appear. This transmutation occurs naturally if the proper agents are used. I have the answer to the production of my own garden flowers through the exercise of these scientific factors.
Soil Testing
- Test pH every 60 days using digital meters or kits
- Sample from four locations six inches deep near roots
- Acidic readings below 5.5 favor blue blooms
Blue Bloom Method
- Apply one tablespoon aluminum sulfate per gallon water monthly
- Use rainwater exclusively to maintain low pH
- Add pine needle mulch to sustain acidity
Pink Bloom Method
- Apply one cup dolomitic lime per ten square feet annually
- Use high-phosphorus fertilizer every six weeks
- Water with tap water to elevate pH naturally
Timing is of utmost importance while making the colour changes. Apply your aluminium sulphate in early spring, as the buds are just forming. Apply the lime in the autumn, and you will have pink flowers the following year. I missed this once and did not achieve the required result for a whole twelve months. Careful testing of the soil would save me this wretched aspect of waiting.
Water can greatly affect your results. Blue hydrangeas require rainwater to maintain their acidity. The minerals in your tap water will slowly trend the water toward pink as the pH increases. I added rain barrels solely for my blue hydrangea bed. That simple switch has stabilized the blue color more effectively than any soil amendment ever could.
Dispel the myth that you can achieve an instant color change without work and rust a rusty nail, only to find that you won't be able to use the aluminum! Don't bother with coffee grounds; they won't significantly alter the pH. Real change requires six to twelve months. My neighbor wasted a season trying vinegar before succumbing to the right way.
Read the full article: Soil pH for Hydrangeas: Color Control Guide
