The color of hydrangea blooms is directly influenced by the soil’s pH level, a measurement of its acidity (below 7) or alkalinity (above 7). For the most commonly cultivated species, the color of the blooms is directly influenced by this soil chemistry. The answer to whether hydrangeas prefer acidic soil for blue flowers is yes, but this preference involves a complex chemical process.
The Mechanism of Color Change
The true driver of the color shift is the availability of aluminum ions (\(\text{Al}^{3+}\)) to the plant’s root system. Hydrangea flowers contain a pigment called anthocyanin, which is naturally red or pink. It is the presence of aluminum that chemically alters this pigment, causing the flowers to display a blue hue.
In highly acidic soil, generally with a pH range between 5.2 and 5.5, the aluminum compounds naturally present become soluble. This solubility allows the plant roots to easily absorb the aluminum ions, which are then transported to the developing flower buds.
Conversely, when the soil is alkaline, typically at a pH of 7.0 or higher, the aluminum becomes “locked up” and immobile. Aluminum ions combine with hydroxide ions to form insoluble aluminum hydroxide, which the plant cannot absorb. When aluminum is unavailable, the native pigment is expressed, resulting in pink or red blooms. Transitional soil pH levels (5.5 to 7.0) often result in mixed coloration, leading to purple or lavender flowers as the aluminum is only partially available.
Different Hydrangea Types and Their Needs
Not all hydrangeas participate in the color-changing process, as the ability is genetically limited to certain species. The two main species whose flower color is sensitive to soil pH are Hydrangea macrophylla (Bigleaf Hydrangea) and Hydrangea serrata (Mountain Hydrangea). If a cultivar of these species has pigmented blooms, its color can be manipulated by adjusting the soil chemistry.
Other popular hydrangeas maintain their genetically determined color regardless of the soil’s acidity or alkalinity. For instance, Hydrangea paniculata (Panicle Hydrangea) and Hydrangea arborescens (Smooth Hydrangea) will not turn blue even in the most acidic soil. The blooms of these species may change color as they age, but this is a natural aging process, not a pH response.
Adjusting Soil Acidity for Desired Color
Before attempting any color change, determine the soil’s current pH level using a professional test kit or laboratory analysis. Knowing the starting point is essential for calculating the correct amount of amendment needed. Making drastic changes too quickly can harm the plant, and the desired color change may take a full growing season or longer to become fully apparent.
Achieving Blue Flowers
To achieve blue flowers, the goal is to lower the pH to the target range of 5.2 to 5.5 and ensure aluminum is present. Elemental sulfur is a slow-acting amendment that acidifies the soil over several months. A faster method involves applying aluminum sulfate, which immediately lowers the pH while simultaneously supplying the necessary aluminum ions.
Aluminum sulfate is often applied as a liquid drench, diluted at a rate of approximately one tablespoon per gallon of water. This drench is applied monthly from early spring through the summer. To maximize bluing, one should also use a fertilizer low in phosphorus, as high phosphorus levels can bind with aluminum, making it unavailable even in acidic conditions. Repeated applications are necessary to maintain a low pH, especially in areas where the native soil or irrigation water is naturally alkaline.
Encouraging Pink Flowers
To encourage pink blooms, the objective is to raise the soil pH to a level of 6.0 to 6.2 or higher, which locks up the aluminum. This is accomplished by incorporating garden lime, such as dolomitic lime, into the soil around the plant. Lime takes several months to fully dissolve and alter the pH, so applications are typically made in the fall or very early spring.
The use of a high-phosphorus fertilizer will also support the pink coloration by chemically binding with any residual aluminum in the soil. Regardless of the amendment chosen, it should be applied to the soil surface around the plant and watered in thoroughly, taking care to avoid contact with the leaves. Consistent monitoring and re-application are the only ways to sustain the desired flower color over time.