Hydrangeas are prized garden shrubs, celebrated for their globe-shaped or flattened clusters of flowers that display color throughout the summer. Understanding the conditions that govern their appearance is fundamental to successfully growing them. The soil’s pH, which measures its acidity or alkalinity, is a crucial factor. This chemical characteristic plays a complex role in determining the ultimate hue of the flowers.
The Mechanism of Color Change
The ability of some hydrangeas to shift flower color between pink and blue results directly from soil chemistry, specifically the availability of aluminum ions. The pigment responsible for bloom color is anthocyanin, which is naturally red or pink. For the flower to display blue, the plant must absorb aluminum from the soil and transport it to the petals, where it complexes with the anthocyanin pigment.
Soil acidity, measured by pH, determines aluminum availability. When the soil is highly acidic (pH 5.5 or lower), aluminum is readily dissolved and exists in a mobile, soluble form that roots can easily take up. This high availability leads to blue coloration in the flowers.
Conversely, in alkaline or neutral soil (pH 7.0 or higher), aluminum binds tightly with other compounds, forming an immobile compound unavailable to the plant. This results in flowers that remain the natural pink or red hue. In transitional soil conditions (pH 5.5 and 7.0), aluminum is only partially available, often leading to mixed colors like lavender, purple, or a blend of pink and blue. White-flowering hydrangeas are genetically incapable of accumulating these pigments, so their color remains unaffected regardless of soil pH.
Hydrangea Species and pH Sensitivity
The color-changing characteristic is not shared by all plants within the Hydrangea genus; only specific species possess the mechanism to utilize aluminum. The primary species known for its pH-sensitivity is Hydrangea macrophylla, or Bigleaf Hydrangea. This species, which includes Mophead and Lacecap varieties, reliably shifts flower color from pink to blue as the soil pH drops.
Hydrangea serrata, often called Mountain Hydrangea, also exhibits this color-changing response. Its blooms depend on aluminum uptake for blue coloration. These two species are the only reliable candidates for gardeners aiming to manipulate flower color.
Other popular hydrangea species are completely unaffected by soil pH. Hydrangea paniculata (Panicle Hydrangea) and Hydrangea arborescens (Smooth Hydrangea) will not produce blue flowers, even in acidic soil. Their flowers are typically white or cream. While some may age to pink or red later in the season, this change is unrelated to soil pH.
Practical Soil Adjustment Techniques
The first step in attempting any color change is to test the existing soil, ideally with a professional soil test kit. Testing accurately determines the current pH level and reveals if aluminum is present. Understanding the starting point prevents incorrect application of amendments, which can be detrimental to the plant’s health.
Encouraging Blue Flowers (Lowering pH)
To encourage blue flowers, the goal is to lower the pH to a range of 5.0 to 5.5. Gardeners typically achieve this by applying elemental sulfur or aluminum sulfate. Elemental sulfur works gradually, converted to sulfuric acid by soil bacteria, a slow process that can take a year or more. A general application rate is about half a cup per 10 square feet worked into the soil around the plant.
Aluminum sulfate is a faster-acting option because it immediately supplies both aluminum and acidity. It is often applied as a liquid drench, mixing one tablespoon per gallon of water and applying it directly to the soil. This application should be performed multiple times during the spring and early summer. Caution is necessary, as excessive use of aluminum sulfate can burn roots.
Encouraging Pink Flowers (Raising pH)
For those who desire pink flowers, the goal is to raise the soil pH to the alkaline range, ideally between 6.0 and 6.5. This increase in alkalinity makes existing aluminum less available to the plant roots. Garden lime, either agricultural or dolomitic, is the most common amendment used to achieve this effect.
Dolomitic lime also supplies magnesium and is typically broadcast over the soil surface. Like sulfur, lime works slowly, requiring several months for the full effect to be seen. Gardeners must monitor the soil periodically, as natural processes will cause the pH to revert over time. Maintaining a consistent color requires ongoing, measured applications of the appropriate amendments each year.