Hydrangeas are popular flowering shrubs, distinguished by their large, rounded flower heads and the vibrant spectrum of colors they display. Unlike many other garden plants, the color of some varieties is not fixed solely by genetics, but can change dramatically depending on the surrounding environment. This unique ability to shift hue has made them a source of fascination for gardeners. Understanding the relationship between the plant and its soil is key to influencing its colorful display.
The Full Spectrum of Hydrangea Colors
The color range of hydrangeas is much broader than the classic pink and blue shades. Hydrangeas can be found in pure white, creamy ivory, vibrant pink, deep rose-red, lavender, purple, and various shades of true blue. The most famous color variation occurs in the Hydrangea macrophylla (Bigleaf Hydrangea) species, where the flowers can transition anywhere between pink and blue.
Purple blooms often appear during this transition, representing a balance between the conditions that produce pink and blue. White or cream varieties are an exception to the color-changing rule because their pigment structure does not react to soil chemistry. Many varieties also exhibit a natural color change as they age, often turning shades of antique green or deep rose regardless of the soil’s composition.
How Soil Chemistry Determines Hydrangea Hue
The color-changing capability in Bigleaf and Mountain Hydrangeas (H. serrata) is a direct result of how the plant absorbs aluminum from the soil. The true color pigment, called anthocyanin, is naturally pink in these plants, but it binds with aluminum ions to create the blue shade. The availability of this aluminum is indirectly controlled by the soil’s acidity or alkalinity, which is measured by its pH level.
When the soil is acidic, with a pH of 5.5 or lower, the aluminum is highly soluble and easily absorbed by the plant’s roots. This high absorption leads to the vibrant blue coloration in the flowers. Conversely, when the soil is alkaline, with a pH of 7.0 or higher, the aluminum becomes chemically bound and unavailable to the plant.
The lack of available aluminum results in the expression of the plant’s natural pink pigment. Purple or mauve flowers occur in a transitional, near-neutral pH range, typically between 5.5 and 6.5, where some, but not all, of the aluminum is absorbed. This chemical interaction, rather than the pH itself, is the precise mechanism driving the color shift.
Controlling Color Through Soil Amendments
Gardeners can actively influence the color of their hydrangeas by adjusting the soil’s pH and aluminum availability, a process that requires patience and a soil test. To encourage blue flowers, the soil must be made more acidic, ideally reaching a pH between 5.0 and 5.5. This is most effectively achieved by applying aluminum sulfate, which lowers the pH and supplies the necessary aluminum ions.
Alternatively, elemental sulfur can be used to acidify the soil, but it works more gradually than aluminum sulfate. For established plants, a liquid drench of aluminum sulfate can be applied several times throughout the growing season for faster results. Gardeners should also use fertilizers low in phosphorus, as high phosphorus levels can bind with aluminum, preventing its uptake.
To promote pink or red blooms, the goal is to raise the soil’s pH into the alkaline range, typically above 6.0, to lock up the aluminum. This is done by incorporating garden lime, such as dolomitic lime, into the soil. The application of lime should be done in the late fall or early spring, as it can take several months for the change to become noticeable.
Using a high-phosphorus fertilizer also supports pink coloration by blocking residual aluminum absorption. Avoid raising the pH excessively (above 6.4), as this can lead to iron deficiency in the plant, causing the leaves to yellow. Regular soil testing is recommended to ensure the pH remains within the desired range for the preferred color.
Hydrangea Varieties That Do Not Change Color
Not all hydrangeas participate in the pink-to-blue color drama; several common species have genetically fixed colors immune to soil chemistry manipulation. The most prevalent exceptions are white-flowered Bigleaf Hydrangeas, which lack the anthocyanin pigment that reacts with aluminum. These remain white regardless of soil pH.
The Smooth Hydrangea (Hydrangea arborescens), including cultivars like ‘Annabelle,’ blooms white or green and is unaffected by pH. Similarly, the Panicle Hydrangea (Hydrangea paniculata), which features cone-shaped flower clusters, opens white or chartreuse and naturally matures to pink or red without soil intervention. The unique Oakleaf Hydrangea (Hydrangea quercifolia) also maintains its white flowers, which turn deep rose or red as they age, regardless of the soil’s pH level.