Hydrangeas, particularly the popular bigleaf (Hydrangea macrophylla) and mountain (Hydrangea serrata) varieties, can shift their bloom color from vibrant pink to deep blue. This phenomenon is not determined by the plant’s genetics alone, but is an environmental expression resulting from chemical interactions within the soil and the plant’s tissues.
The Core Chemistry of Color
The color-changing mechanism begins with the plant’s natural pigment, an anthocyanin called delphinidin-3-glucoside, which is the primary color source in the sepals. In the absence of aluminum, this pigment typically appears pink or reddish, which is its default expression.
The shift to blue requires the presence of aluminum. Blue coloration occurs when the delphinidin pigment forms a complex with aluminum ions (\(\text{Al}^{3+}\)) within the flower’s vacuole. This chemical structure, often stabilized by co-pigments, alters the light absorption properties of the anthocyanin molecule. To achieve a true blue hue, sepals must accumulate aluminum concentrations greater than \(40 \text{ \mu g}\) per gram of fresh tissue.
The Role of Soil Acidity (pH)
While aluminum is the coloring agent, soil acidity controls its availability to the plant. Soil \(\text{pH}\) is a measure of hydrogen ion concentration, rated on a scale from \(0\) (most acidic) to \(14\) (most alkaline). Aluminum is naturally abundant in many soils, but its solubility is highly dependent on the \(\text{pH}\) level.
When the soil is acidic (\(\text{pH } 5.2\text{–}5.5\) or lower), aluminum becomes soluble and is readily taken up by the roots. This available aluminum is then transported and stored in the flower buds, where it bonds with the anthocyanin, resulting in a blue bloom.
Conversely, in alkaline soil (above \(\text{pH } 6.5\)), aluminum is chemically bound to soil particles, forming insoluble compounds. In these higher \(\text{pH}\) conditions, the aluminum remains unavailable to the plant, and the flower’s natural pink pigment remains dominant. The soil \(\text{pH}\) acts as the gatekeeper, controlling whether the aluminum is mobile enough to be absorbed and trigger the blue coloration.
Practical Steps to Achieve Blue Flowers
The first step in attempting a color change is to obtain a professional soil test to determine the exact \(\text{pH}\) level. Knowing the starting \(\text{pH}\) dictates how much amendment is necessary. To successfully achieve blue flowers, the goal is to consistently maintain the soil \(\text{pH}\) within the acidic \(5.2\text{–}5.5\) range.
Gardeners can introduce readily available aluminum by applying aluminum sulfate, which both lowers the \(\text{pH}\) and supplies the necessary mineral. A common application rate is \(1 \text{ tablespoon}\) of aluminum sulfate dissolved in a gallon of water, applied as a soil drench around the root zone. This application should be repeated every \(6\text{–}8\) weeks during the growing season, starting in early spring before the flower buds form.
Applying aluminum sulfate requires caution, as over-application can cause root burn or leaf scorch. To support the blue color, use a fertilizer low in phosphorus, such as a formula with a ratio like \(25\text{-}5\text{-}30\). High-phosphorus fertilizers, often marketed as “bloom boosters,” will chemically bind with the aluminum in the soil, making it unavailable to the plant, regardless of the \(\text{pH}\).
Maintaining the low \(\text{pH}\) is a long-term process that often takes months or a full season to show results. Gardeners should also be mindful of their irrigation source, as hard water (containing high levels of calcium and magnesium) can gradually raise the soil \(\text{pH}\) over time.
Genetic Limits and Other Colors
Not all hydrangeas can change color. Pure white varieties of Hydrangea macrophylla and all cultivars of the popular panicle hydrangea (Hydrangea paniculata) lack the necessary delphinidin anthocyanin pigment. These plants will remain white regardless of aluminum presence or soil \(\text{pH}\).
If a pink or red flower is the desired outcome, the soil \(\text{pH}\) must be raised to an alkaline level, typically \(\text{pH } 6.5\text{–}7.0\) and above, which locks up the aluminum. This is achieved by applying amendments such as garden lime or dolomitic limestone.
The purple coloration sometimes seen on blooms is the result of a transitional stage, occurring in the slightly acidic to neutral \(\text{pH}\) range of \(5.6\text{–}6.5\). In this range, the plant absorbs some, but not enough, aluminum to fully convert the pink pigment to blue.