Hydrangeas are popular flowering shrubs known for their large, showy blooms that can dominate a summer garden. These plants are unique because their flower coloration is not always fixed, but represents an interplay between the plant’s genetics and its surrounding environment. The vibrant spectrum of shades hydrangeas display has long captivated gardeners. Understanding the factors that influence these hues allows for a deeper appreciation of this common garden plant.
The Diverse Spectrum of Hydrangea Colors
The full palette of hydrangea colors includes pink, blue, red, purple, violet, white, and shades of green or lime. The ability to shift color is limited to specific species, primarily the Bigleaf (Hydrangea macrophylla) and Mountain (H. serrata) varieties. These types are often referred to as “color-shifting” because their blooms transition between pink, purple, and blue depending on the soil conditions.
Other common species possess fixed colors that do not change with soil chemistry. Smooth hydrangeas (H. arborescens) and Panicle hydrangeas (H. paniculata) typically produce white or lime-green flowers. These blooms may age to shades of pink or red due to cooler temperatures and natural maturity, not soil manipulation. White-flowering varieties, regardless of species, are incapable of accumulating the necessary pigments and will always remain white.
The purple and violet hues represent a transitional phase between pink and blue. These mixed shades often result from a slightly acidic to neutral soil environment where conditions are not ideal for full expression of either color. Color intensity can also vary significantly, ranging from deep, saturated tones to soft pastels.
The Chemistry Behind Pink and Blue
The dramatic color change between pink and blue is caused by the availability of aluminum ions (Al3+), not directly by soil acidity. All color-changing hydrangeas contain the base pigment, an anthocyanin called delphinidin-3-glucoside, which is naturally red or pink. The blue color develops only when this pigment binds with aluminum inside the flower’s cells.
In highly acidic soil (pH below 6.0), aluminum becomes soluble and is easily absorbed by the roots. The aluminum is transported to the sepals, where it chemically bonds with the anthocyanin pigment and specific co-pigments. This process, known as chelation, stabilizes the pigment and shifts its color from pink to blue.
Conversely, in alkaline soil (pH above 7.0), aluminum remains tightly bound to soil particles and is unavailable for absorption. Without aluminum to chelate with the pigment, the anthocyanin expresses its natural color, resulting in pink or red blooms. The purple range occurs in slightly acidic or neutral soil (pH 6.0 to 7.0), where aluminum is only partially available.
Achieving Desired Shades
Gardeners can influence the color of their Bigleaf and Mountain hydrangeas by intentionally altering the soil environment to control aluminum availability. To achieve blue flowers, the soil must be made more acidic, ideally reaching a pH between 5.0 and 5.5. This is accomplished by applying a soil acidifier, such as aluminum sulfate or elemental sulfur. Aluminum sulfate provides both acidity and necessary aluminum ions, while elemental sulfur works more slowly to reduce the pH.
For pink blooms, the goal is to raise the soil’s pH to an alkaline level (above 7.0) to prevent aluminum uptake. This is achieved through the application of a liming material, such as garden lime or dolomite lime, which contains calcium carbonate. These materials neutralize soil acidity, effectively locking up available aluminum.
Color manipulation is not an instant process; soil amendments should be applied in early spring or late fall before flower buds develop. It is important to test the soil pH regularly and apply amendments consistently, as the effect is not permanent and soil naturally reverts to its original pH over time. Applying excessive amounts of any amendment can severely harm the plant by scorching the roots or causing nutrient deficiencies.