The vibrant blue hue of a hydrangea is often the ultimate prize for gardeners, transforming the shrub into a striking landscape feature. This dramatic color shift has led to many home remedies and myths, such as burying copper pennies around the base of the plant. Understanding the science behind the color change, however, reveals why such simple solutions are ultimately ineffective.
Why Pennies and Copper Don’t Work
The myth of using copper pennies to turn hydrangea blooms blue misunderstands the required chemical reaction. The theory suggests copper from the coins leaches into the soil, making it more acidic or providing the necessary element for bluing. This belief is flawed, starting with the composition of modern currency.
Since 1982, U.S. pennies have a zinc core and only a thin copper coating, making the copper available to the soil negligible. Even pre-1982 pennies (95% copper) corrode too slowly to release a significant amount of ions. The minimal copper that oxidizes does not contribute the specific element the plant requires for blue pigmentation.
The actual element responsible for the blue color is aluminum, not copper. Copper does not participate in the color-changing mechanism of the hydrangea flower. Therefore, introducing copper to the soil does nothing to facilitate the necessary uptake of aluminum ions. Gardeners who believe the penny method works are likely seeing the result of naturally acidic soil or another unknown factor.
The Role of Soil pH and Aluminum
The blue color in hydrangeas is a display of soil chemistry, directly caused by the presence of aluminum ions (\(\text{Al}^{3+}\)) within the flower sepals. These ions interact with the plant’s natural pigment, an anthocyanin. If aluminum is present in sufficient quantity, the flowers will be blue; if absent, they remain pink or red.
Soil \(\text{pH}\), which measures acidity or alkalinity, controls the availability of aluminum to the plant’s roots. Aluminum is naturally present in most soils, but its chemical form changes depending on the \(\text{pH}\) level. In alkaline or neutral soils (\(\text{pH}\) 6.0 or higher), aluminum ions are tightly bound to other compounds and are chemically immobile, preventing absorption.
Only when the soil becomes sufficiently acidic (typically \(\text{pH}\) 5.0 to 5.5) does the aluminum release from these bonds and become soluble. This soluble form is then readily absorbed by the hydrangea, transported to the blooms, and results in the blue color. For intermediate colors, such as purple or mauve, the \(\text{pH}\) is usually between 5.5 and 6.0, indicating partial aluminum absorption.
Effective Methods for Turning Hydrangeas Blue
Achieving blue hydrangea blooms requires a two-pronged approach: ensuring aluminum is present and creating an acidic environment to make it accessible. The most direct and reliable method is the application of aluminum sulfate (\(\text{Al}_2(\text{SO}_4)_3\)). This compound simultaneously lowers the soil \(\text{pH}\) and provides the necessary aluminum ions in a single application.
Aluminum sulfate is typically mixed with water and applied as a soil drench around the root zone, usually at a rate of about one tablespoon per gallon of water. This liquid application should be done in early spring as the plant begins to grow, well before the flower buds form color. Applying the solution to dry soil should be avoided, as the plant must be watered thoroughly beforehand to prevent root burn.
For highly alkaline soils, elemental sulfur may be needed to significantly lower the \(\text{pH}\) first. Sulfur works more slowly than aluminum sulfate because soil microbes must convert it to sulfuric acid, a process taking several months. A low-phosphorus fertilizer is also advised, since high levels of phosphorus can chemically bind aluminum, making it unavailable even in acidic soil.
Monitoring and Maintaining Blue Color
The ability to change color is limited to specific species: the Bigleaf Hydrangea (Hydrangea macrophylla) and the Mountain Hydrangea (Hydrangea serrata). White-flowering varieties cannot be turned blue, as they lack the necessary anthocyanin pigment to react with aluminum. Before beginning any treatment, a professional soil test should be performed to determine the current \(\text{pH}\) level and establish a baseline for amendments.
Maintaining the desired blue hue is an ongoing commitment, not a one-time fix. The soil naturally resists \(\text{pH}\) changes, often reverting toward its original level over time. Additionally, the alkalinity of local irrigation water can slowly raise the soil \(\text{pH}\) with every watering.
Annual or semi-annual applications of an acidifying agent are necessary to keep the \(\text{pH}\) consistently within the target range of 5.0 to 5.5. Fertilizing blue hydrangeas with a formula low in phosphorus and high in potassium minimizes the chance of phosphorus interfering with aluminum uptake.