The belief that all hydrangeas can change their flower color based on the soil they are planted in is incorrect. While the dynamic shift from pink to blue is a defining characteristic of this ornamental shrub, this ability is unique to only a few species within the genus. The majority of hydrangeas produce blooms whose color is genetically predetermined, remaining stable regardless of the soil’s chemical composition. Understanding which types possess this trait allows gardeners to either enjoy the natural hue or purposefully manipulate the color.
The Types That Do Not Shift Hue
The most reliable and cold-hardy hydrangeas are the ones whose bloom colors are fixed by their genetics, not by soil chemistry. The Panicle Hydrangea (Hydrangea paniculata) is a popular example, known for its large, cone-shaped flower clusters. These blooms typically open creamy white or pale green and gradually fade to shades of pink or deep red as the season progresses.
The Smooth Hydrangea (Hydrangea arborescens), including the widely grown ‘Annabelle’ variety, features large, rounded flower heads that are almost always white. The Oakleaf Hydrangea (Hydrangea quercifolia) produces elongated clusters of white flowers that mature to a rosy pink or burgundy. In these species, the color change that occurs is a natural aging process known as senescence, which is not influenced by soil acidity or alkalinity. These species provide consistent color and do not contain the necessary biological mechanism for a fundamental color shift.
The Species That Change Color
The color-shifting ability is almost exclusively found in two specific species: the Bigleaf Hydrangea (Hydrangea macrophylla) and its close relative, the Mountain Hydrangea (Hydrangea serrata). These species are distinct because their blooms contain a water-soluble pigment called anthocyanin. This pigment is the coloring agent responsible for the pink, purple, and blue hues seen in their flowers.
Within these species, the color produced is a direct reflection of the chemical environment where the plant is growing. The blooms can range from a vibrant blue in one garden bed to a deep pink in another, or even display a mix of lavender and purple shades. This variability is why H. macrophylla and H. serrata are often referred to as “chameleon” plants. White-flowering varieties within these two species are exceptions, as they lack the necessary anthocyanin pigment and will remain white.
The Chemistry of Color Transformation
The mechanism that drives the color change is not the soil’s acidity alone, but its influence on a specific metal element: aluminum. The chemical reaction that produces the blue color requires the presence of mobile aluminum ions (\(\text{Al}^{3+}\)) to bind with the anthocyanin pigments. Hydrangea blooms only turn blue when the plant successfully absorbs a sufficient amount of this aluminum.
Soil acidity, measured by the pH level, dictates the availability of the aluminum. In acidic soil, typically with a pH range of 5.0 to 5.5, the aluminum compounds naturally present become soluble. This solubility allows the aluminum ions to dissolve in the soil water and be taken up by the plant’s roots, where they are transported to the flowers, resulting in a blue color.
Conversely, in alkaline or neutral soil, with a pH of 6.0 or higher, the chemical environment causes aluminum to precipitate and become insoluble. When aluminum is chemically locked up, it is unavailable for the plant to absorb, which means the anthocyanin pigment expresses its native color, pink. The color difference is solely due to the presence or absence of the bound aluminum ion.
Practical Steps for Adjusting Color
Gardeners can intentionally shift the color of their Bigleaf or Mountain Hydrangeas by manipulating the soil’s pH and aluminum availability. To achieve blue flowers, the goal is to lower the soil pH to the acidic range of 5.0 to 5.5. Applying aluminum sulfate is effective because it both lowers the pH and directly introduces the necessary aluminum ions into the soil.
A liquid drench of aluminum sulfate (about one tablespoon per gallon of water) can be applied monthly during the growing season to ensure consistent aluminum uptake. When promoting blue blooms, use a fertilizer that is low in phosphorus, as high levels of this nutrient can chemically bind to aluminum, preventing the plant from absorbing it.
To produce pink flowers, the strategy is to raise the soil pH to the neutral to alkaline range, ideally between 6.0 and 7.0. This is typically achieved by applying garden lime, such as dolomitic lime, which increases the alkalinity and locks up any available aluminum. Using a fertilizer with a high phosphorus content will also assist this effort by further limiting the aluminum’s mobility. Color adjustment takes time, often requiring several months or a full growing season to see the desired result on new blooms.