Hydrangeas are garden shrubs cherished for their large, striking blooms that display colors ranging from bright pink to deep blue. This color variability is not determined by genetics alone but by the plant’s chemical environment. The color is not fixed at planting but depends on the availability of a specific metal in the soil. Understanding this chemical relationship explains why certain hydrangeas can shift their blooms to purple and blue shades.
The Essential Role of Anthocyanins and Aluminum
The basic color pigment in the sepals of color-changing hydrangeas is a flavonoid called anthocyanin, specifically delphinidin-3-glucoside. This pigment naturally appears red or pink when unbound within the plant’s cells. To change color to purple or blue, the anthocyanin must chemically interact with aluminum, which the plant absorbs from the soil as a free ion (\(\text{Al}^{3+}\)).
When the aluminum ion reaches the sepal, it forms a complex with the anthocyanin pigment and a co-pigment, altering the pigment’s structure to reflect blue light. Pink sepals contain a low concentration of aluminum (less than 10 micrograms per gram of tissue), while blue sepals contain over 40 micrograms per gram. The purple color is a direct visual indicator of an intermediate level of aluminum uptake, typically between 10 and 40 micrograms per gram.
Understanding the Soil pH Connection
Although aluminum causes the color change, the soil’s acidity level (pH) controls whether the plant can absorb it. Aluminum is present in most soils, but its solubility depends entirely on the pH. When the soil is acidic (low pH), aluminum is released from soil particles, becoming soluble and mobile for root absorption.
Highly acidic soils (pH below 5.5) favor blue blooms because they maximize aluminum availability. In neutral or alkaline soils (pH 6.5 or higher), aluminum ions bind to hydroxide ions, forming immobile aluminum hydroxide. This compound is unavailable for absorption, causing flowers to remain pink.
The purple hue appears when the soil pH is in the transitional range, usually between 5.5 and 6.5. This range allows for limited, but not maximal, aluminum uptake. This middle-ground pH ensures a partial complexing of the anthocyanin, resulting in the blend of red and blue tones perceived as purple.
Practical Steps for Color Manipulation
Gardeners can manipulate the soil environment to encourage a specific flower color, though this requires patience. To shift pink flowers toward purple and blue, the goal is to increase aluminum availability by lowering the soil pH. The most direct method is applying aluminum sulfate, which both acidifies the soil and supplies aluminum ions. A common application is a liquid drench of one tablespoon of aluminum sulfate per gallon of water, applied monthly during the growing season.
Lowering pH
For a gradual, long-term approach, elemental sulfur can be broadcast around the plant and converted by soil microbes into sulfuric acid. Incorporating organic materials like pine needles or composted oak leaves can also slowly acidify the soil. These amendments only affect the color of new flower buds, and a complete color change often takes a full growing season or more.
Raising pH
To encourage pink flowers and prevent blue or purple transition, the objective is to raise the soil pH, which locks up the aluminum. This is accomplished by applying a liming material, such as garden lime or dolomitic lime. Dolomitic lime is often preferred because it also supplies magnesium, which benefits plant health.
Dry applications of about one cup of dolomitic lime per 10 square feet are practical for a lasting effect. Gardeners must monitor the soil pH regularly using a test kit to ensure the desired range is maintained, as soil chemistry changes are not instantaneous.
When Color Change Isn’t Possible
The ability to change color is not universal; only the Bigleaf Hydrangea (Hydrangea macrophylla) and the Mountain Hydrangea (Hydrangea serrata) possess this trait. Other common varieties, such as Smooth Hydrangea (H. arborescens) and Panicle Hydrangea (H. paniculata), cannot be manipulated regardless of soil pH. Their flowers may change color as they age, typically from white to rosy pink, but this is due to maturity, not soil chemistry.
Even within the color-changing species, cultivars that produce pure white flowers will not shift to blue or purple. This is because these white varieties lack the necessary anthocyanin pigment in their sepals. Since anthocyanin must bind with aluminum to produce the blue color, its absence makes chemical manipulation ineffective.