Hydrangeas are one of the most celebrated flowering shrubs, known for their ability to display different bloom colors. This unique characteristic, particularly prominent in varieties like the Bigleaf Hydrangea (Hydrangea macrophylla), leads many gardeners to experiment with various soil additives hoping to influence the shade of their flowers. This article explores the precise chemistry behind hydrangea color and clarifies the specific, non-color-related role of Epsom salt (magnesium sulfate) in plant care.
The Direct Answer: Epsom Salt and Hydrangea Color
Epsom salt, chemically known as magnesium sulfate, does not directly change the color of hydrangeas. The transformation from pink to blue, or blue to pink, requires the plant to absorb aluminum, a process strictly governed by soil acidity, or pH. Since magnesium sulfate has a nearly neutral impact on the overall soil pH, it cannot facilitate the chemical reaction necessary for color change.
The misconception often arises because gardeners confuse magnesium sulfate with aluminum sulfate, the latter being a compound that is effective for color manipulation. Epsom salt contains magnesium, which slightly raises soil pH, and sulfur, which slightly lowers it, causing these effects to essentially cancel each other out. Therefore, adding Epsom salt to the soil will not make the aluminum available for the plant to absorb, which is the necessary step for the development of blue flowers.
The Role of Magnesium Sulfate in Plant Health
While Epsom salt does not influence bloom color, it is a valuable soil amendment for addressing a specific nutritional requirement in plants. Magnesium is a central component of the chlorophyll molecule, making it essential for photosynthesis and the plant’s ability to produce energy. Sulfur, the other element in the compound, is necessary for the synthesis of proteins and certain vitamins.
Hydrangeas that are deficient in magnesium display a pattern of chlorosis, where the older, lower leaves turn yellow while the veins remain distinctly green. Applying magnesium sulfate can correct this specific deficiency, restoring the foliage to a healthy, dark green. This improvement in plant health can lead to more vigorous growth and robust blooms, but it is a general nutritional benefit, not a color-altering one.
pH and Aluminum: The Real Determinants of Hydrangea Hue
The color of Hydrangea macrophylla and Hydrangea serrata blooms is determined by the concentration of aluminum ions absorbed by the plant and the soil’s pH level. Blue flowers occur when aluminum is present and absorbed into the petals’ pigment cells. Pink flowers result when aluminum is unavailable or its uptake is blocked.
Soil pH is the mechanism controlling aluminum availability. In highly acidic soil, with a pH between 5.0 and 5.5, aluminum is highly soluble and easily absorbed by the roots, resulting in blue flowers. To encourage or maintain blue coloration, gardeners often apply aluminum sulfate, which both provides aluminum and lowers the soil pH.
Conversely, in neutral to alkaline soil (pH 6.5 to 7.0 or higher), aluminum binds tightly to soil particles and cannot be taken up by the plant, leading to pink or red blooms. To achieve pink flowers, gardeners can raise the soil pH by incorporating garden lime (calcium carbonate) or dolomitic lime, which locks the aluminum in the soil.