The dramatic color change in certain hydrangeas is one of the most fascinating aspects of garden chemistry. Only specific types, primarily the bigleaf (Hydrangea macrophylla) and the mountain (Hydrangea serrata), possess the necessary compounds to achieve this color change. Gardeners can influence the bloom color of these varieties by modifying the soil’s chemistry, turning a naturally pink flower into a deep blue one.
Understanding the Role of Soil pH
Color-changing hydrangeas contain the water-soluble pigment anthocyanin, which causes pink or red coloring. For flowers to appear blue, the plant must absorb aluminum ions from the soil, which then form a complex with the anthocyanin pigment within the flower’s cells.
The soil’s pH level acts as a gatekeeper, controlling whether the aluminum is soluble for the plant’s roots to take up. When the soil is acidic (pH 5.0–5.5), aluminum is highly soluble and readily absorbed. If the soil is alkaline (pH above 6.0–6.5), the aluminum binds to other compounds, rendering it insoluble and unavailable to the plant. Without the aluminum complex, the anthocyanin remains in its pink or red form.
Essential Amendments for Blue Color
To feed hydrangeas for blue flowers, the goal is either to directly supply aluminum or to lower the soil pH to free up native aluminum. The most direct method is the application of aluminum sulfate. When aluminum sulfate dissolves in water, it immediately provides the soluble aluminum ions needed for blue color and simultaneously reduces the soil’s pH, increasing the availability of both the added and existing aluminum.
A second material used to promote bluing is elemental sulfur, which works more slowly but is often considered a safer alternative for long-term soil modification. Soil bacteria must first convert the elemental sulfur into sulfuric acid, which gradually lowers the soil pH. This acidification process then frees up any aluminum naturally present in the soil, allowing the plant to absorb it. Because this biological process is dependent on soil temperature and microbial activity, elemental sulfur takes weeks or even months to show noticeable results.
Practical Application Methods
Before applying any amendment, conduct a soil test to determine the existing pH level and aluminum content. This helps calculate the appropriate amount of product and avoid over-acidifying the soil, which can harm the plant. For established plants, a liquid drench of aluminum sulfate is a common method, providing quicker results than granular applications.
A typical solution involves mixing one tablespoon of aluminum sulfate powder into one gallon of water. This solution should be applied directly to the soil, ideally around the plant’s drip line, which is the outer edge of its branches. It is imperative that the soil is already moist before application, as applying the drench to dry soil can cause root burn due to the high salt content of the product.
Gardeners should repeat this drench application every two to four weeks throughout the growing season, beginning in early spring. When applying, take care to avoid splashing the solution onto the leaves or flowers, as the acidic mixture can cause chemical burn. The color change is not instant; the plant will only display the new color on the blooms that develop after the aluminum has been absorbed and transported into the flower buds, which can take a full season or more.
Varieties That Remain Unaffected
Not every hydrangea variety is capable of color transformation. The ability to change color is limited to Hydrangea macrophylla (bigleaf) and Hydrangea serrata (mountain) varieties that naturally produce pink or blue flowers. Other common hydrangea species lack the specific pigments or mechanisms to react to aluminum, making attempts to turn them blue unsuccessful.
This includes all white-flowered macrophylla cultivars, which do not contain the anthocyanin pigment needed to form the aluminum complex. Other species, such as the panicled hydrangeas (Hydrangea paniculata) and the oakleaf hydrangeas (Hydrangea quercifolia), will not change color based on soil pH. These varieties typically feature white flowers that may transition to shades of pink or red as they age, but this is a natural aging process unrelated to aluminum or soil acidity.