Hydrangeas are valued for their large, striking blooms, which can display a wide range of colors. The ability to intentionally alter a plant’s color, shifting blooms to pink, blue, or purple, fascinates many gardeners. However, not every white hydrangea variety can be manipulated. Successfully changing the color requires understanding the specific genetic limitations and the underlying soil chemistry of the plant.
Genetic Limits: Which White Hydrangeas Cannot Change Color
A hydrangea’s ability to change color depends entirely on its genetics and the presence of specific pigment compounds. Color shifting is only possible in Hydrangea macrophylla (Bigleaf) and Hydrangea serrata (Mountain) varieties, which naturally contain anthocyanin pigments. These pigments allow the blooms to display pink, blue, or purple in response to soil treatments. True white hydrangeas, such as Hydrangea paniculata (Panicle) and Hydrangea arborescens (Smooth), completely lack anthocyanin compounds. Since there is no pigment to alter, these shrubs will remain white regardless of soil amendments.
A different category exists for “blushing white” varieties, which are a type of Hydrangea macrophylla. These start out white but develop a pink or reddish tint as the blooms age later in the season. Because these varieties possess the necessary anthocyanin pigments, they may be successfully shifted to blue or pink using soil treatments.
The Role of Soil pH in Pigmentation
The color shift from pink to blue is caused by the availability of aluminum ions, not directly by soil acidity or alkalinity. Hydrangeas containing the anthocyanin pigment delphinidin-3-glucoside exhibit blue sepals when they absorb aluminum ions. Soil pH acts as the facilitator for this absorption process.
When the soil is acidic (pH 5.5 or below), aluminum becomes highly soluble, allowing the roots to absorb it easily. This absorption transports the aluminum to the flowers, where it complexes with the pigment to display a blue color. Conversely, in alkaline soil (pH 6.5 or above), aluminum precipitates and binds to other compounds, making it unavailable for root uptake.
When aluminum is unavailable, the anthocyanin pigment remains in its original form, resulting in pink or red blooms. A soil pH in the neutral range (between 5.5 and 6.5) often results in aluminum being only partially available. This partial availability produces purple or mixed shades of pink and blue on the same plant.
Practical Steps for Color Modification
Before attempting modification, test your soil to determine the current pH level, which provides a baseline for treatment. Adjusting the color of a responsive hydrangea requires applying specific amendments to increase or decrease the availability of aluminum ions. Application should begin in early spring before the flower buds have fully developed, since the color change only appears on new growth.
To shift the color toward blue, the soil pH must be lowered to 5.5 or below to release aluminum. The most direct method is applying aluminum sulfate, which both lowers the pH and provides the necessary aluminum ions. A common application is a liquid drench using one tablespoon of aluminum sulfate dissolved per gallon of water, applied around the plant base. Elemental or garden sulfur can also be used to acidify the soil, but this process is more gradual and takes several months to show results.
To achieve pink or red coloration, the goal is to raise the soil pH to 6.5 or above to lock up the aluminum. This is accomplished by incorporating dolomitic or agricultural lime into the soil. The lime supplies calcium and magnesium, which reduce soil acidity and bind the aluminum, making it inaccessible to the roots. Naturally alkaline wood ash can also be used to help raise the pH.
Maintaining the Desired Color Over Time
Achieving a color change is not a one-time event, as the soil’s pH is constantly subject to change due to rainfall and watering. Maintaining the desired coloration requires consistent, long-term management of the soil chemistry. The initial application of amendments should be followed by regular re-applications throughout the growing season.
In areas with heavy rainfall, acidifying or alkalizing compounds may leach out of the soil more quickly, necessitating seasonal re-applications. For blue flowers, soil testing should be performed every few months to ensure the pH remains below 5.5. For pink flowers, the pH should be checked to confirm it stays above 6.5.
The full color transformation may take an entire growing season or even two years to become fully apparent. Consistency in applying the correct amendments, typically in early spring and again in the fall, is the most reliable strategy for sustained color control. Over-application must be avoided, as excessive amounts of any soil amendment can potentially harm the plant.