The genus Hydrangea is renowned for its large, beautiful blooms that can appear in shades of pink, purple, and blue. This color variation reflects the chemical environment in which the plant is grown. For gardeners seeking deep blue blossoms, the answer regarding soil acidity is a definitive yes. Producing blue flowers depends entirely on manipulating the soil’s chemical composition toward a more acidic state.
The Science of Color Change
The blue color in hydrangea flowers comes from aluminum, a metallic element naturally present in many soils. This element must be absorbed by the plant and transported to the flower’s sepals, where it interacts with the native pigment, anthocyanin.
Soil acidity acts as a transport mechanism for the aluminum. When the soil is sufficiently acidic, aluminum ions become mobile and soluble, allowing the roots to take them up effectively. If the soil is neutral or alkaline, the aluminum binds to other compounds, making it insoluble and unavailable to the plant. Without available aluminum, the pigment remains in its default pink form.
Achieving Blue Specific Soil Requirements
Achieving a vibrant blue hue requires a narrow and specific range of acidity. Gardeners should aim for a soil pH between 5.2 and 5.5, which maximizes aluminum solubility and uptake. A pH level higher than 6.0 causes aluminum to become locked up, resulting in pink flowers. Soil in the range of 5.6 to 6.5 often produces lavender or purplish blooms, which are a mix of blue and pink coloration.
Successful blue hydrangea cultivation requires both acidic soil and the presence of aluminum. If the soil is acidic but lacks aluminum, the flowers will remain pink because the necessary metallic element is missing. Conversely, if the soil has plenty of aluminum but is alkaline, the plant cannot absorb it, and the flowers will also remain pink. Therefore, testing your soil is the first step to determine both the native pH and the presence of aluminum.
Practical Steps to Acidify Soil
Lowering the soil pH is a gradual process requiring the careful application of specific amendments. The most common and effective way to achieve both the necessary acidity and aluminum is through the use of aluminum sulfate. This compound acts quickly, providing aluminum ions immediately while simultaneously lowering the soil pH.
Using Aluminum Sulfate
Aluminum sulfate is typically mixed with water and applied as a liquid drench around the base of the plant, often at a rate of about one tablespoon per gallon of water. Do not apply the solution directly onto dry soil or foliage, as this can lead to root damage or leaf burn. Regular applications throughout the growing season are usually necessary to maintain the required acidity, since the soil naturally attempts to return to its original pH.
Using Elemental Sulfur
A more sustainable, but slower, approach involves using elemental sulfur. Soil microbes convert the sulfur into sulfuric acid, which gradually lowers the pH over a period of months. This method is preferred for long-term maintenance or when a significant pH adjustment is needed. The amount of elemental sulfur required varies greatly depending on the soil type; sandy soil requires far less than heavy clay or loam soil to achieve the same pH drop.
Fertilizer Considerations
All amendments must be thoroughly watered in after application to help them dissolve and move into the root zone. High-phosphorus fertilizers should be avoided entirely when seeking blue blooms, as phosphate ions bind tightly to aluminum, neutralizing efforts to make it available to the plant. Use a fertilizer formulated for acid-loving plants, which will typically be low in phosphorus and higher in potassium.
Not All Hydrangas Are Created Equal
The color-changing ability is not a universal trait across the Hydrangea genus. Only specific species possess the genetic capacity to react to the presence of aluminum in the soil. The two main species capable of this color shift are Hydrangea macrophylla (bigleaf hydrangea) and Hydrangea serrata (mountain hydrangea).
Many other popular varieties will not change color regardless of how much aluminum or acid is applied. For example, Hydrangea paniculata (panicle hydrangea) and Hydrangea quercifolia (oakleaf hydrangea) maintain their predetermined white or cream color. Similarly, Hydrangea arborescens (smooth hydrangea), like the ‘Annabelle’ cultivar, remains white or green. True white varieties of H. macrophylla also lack the necessary pigment to react with aluminum, so they always stay white.