Hydrangeas are beloved flowering shrubs, largely due to their magnificent, globe-shaped or lacy blooms. These plants are unique because their flower color is not permanently fixed but can often be manipulated by the gardener. While many believe the color is purely genetic, adjusting the growing environment can transform the blooms from shades of pink to vibrant blue. This ability to change color is a chemical process accessible to any home gardener.
The Chemical Mechanism of Color Change
The vibrant hues in many hydrangea varieties are determined by anthocyanins, pigments that naturally produce a red-to-pink color. Blue coloration requires the availability of aluminum ions (\(\text{Al}^{3+}\)) in the soil. Soil acidity, or pH, dictates whether aluminum is soluble and accessible to the roots. When the soil is acidic (pH below 6.0), aluminum dissolves, is absorbed by the plant, and binds with the anthocyanin pigment to register as blue. Conversely, when the soil is alkaline (pH above 6.5), aluminum is locked up and insoluble, causing the uncomplexed anthocyanin pigment to express its pink or red color.
Identifying Varieties That Respond to Treatment
Not all hydrangeas can change their flower color in response to soil amendments. The color-shifting phenomenon is primarily observed in Hydrangea macrophylla (Bigleaf Hydrangeas), including Mophead and Lacecap cultivars. Hydrangea serrata (Mountain Hydrangeas) also share this trait, making them responsive to soil chemistry manipulation. However, certain popular types are colorfast. Pure white cultivars of H. macrophylla lack the necessary pigment precursors, and species like Hydrangea paniculata and Hydrangea quercifolia change color due to aging and temperature, not soil pH.
Practical Steps for Achieving Blue Flowers
To encourage deep blue blooms, the goal is to lower the soil pH to an acidic range, ideally between 4.5 and 5.5, to maximize aluminum availability. The first step involves testing the soil, which can be done with a simple kit, to establish the current pH level. Aluminum Sulfate is a widely used material that immediately lowers the pH and supplies the necessary aluminum. Apply about one tablespoon of Aluminum Sulfate per gallon of water to the soil around the plant’s drip line multiple times during the growing season, starting in early spring as flower buds form. Gardeners should also use fertilizers low in phosphorus, as high levels of this element can bind to aluminum, making it unavailable for absorption.
Practical Steps for Achieving Pink Flowers
Shifting the color toward pink requires raising the soil pH into a neutral to alkaline range, typically above 6.0 to 6.5, which restricts aluminum absorption. The most common material used to increase alkalinity is garden lime, also known as dolomitic or agricultural lime. Lime contains calcium carbonate and works to neutralize soil acidity over time. Apply lime to the soil surface around the plant in early spring and again in the fall, following product instructions. Another effective strategy is to use a high-phosphorus fertilizer, as phosphorus ions readily bind to available aluminum, locking it away from the root system.