The captivating color transformation of certain flowering shrubs is a unique feature in the garden landscape. The bigleaf hydrangea, Hydrangea macrophylla, is known for producing flowers in shades of pink, purple, or blue depending on its environment. Achieving a vibrant blue demands deliberate and sustained management of the soil composition. This involves specific amendments and careful cultivation practices to ensure the plant can access the necessary elements for color expression.
The Chemistry of Color Change
The blue color in bigleaf hydrangeas is caused by the presence of aluminum ions within the flower tissue. The blue pigment forms when aluminum (Al³⁺) complexes with the plant’s natural anthocyanin pigment, delphinidin. If aluminum is not present or cannot be absorbed by the roots, the flower color defaults to pink or red.
Soil acidity, or pH, regulates this process by determining the availability of aluminum. In alkaline or neutral soils, aluminum remains chemically bound and unavailable for root uptake. When the soil becomes acidic, the aluminum dissolves into a form the plant can absorb. For the most intense blue coloration, the soil pH must be maintained within a narrow, strongly acidic range, typically 5.2 to 5.5.
Primary Soil Amendments for Bluing
The most reliable method to ensure both aluminum presence and necessary acidity is the application of aluminum sulfate. This compound serves a dual purpose: it directly supplies aluminum ions and simultaneously lowers the soil’s pH. Applying this amendment provides the quickest path to achieving the desired blue hue, often showing results in the next blooming cycle.
Aluminum sulfate is typically dissolved in water to create a soil drench. A standard recommendation is to mix approximately one tablespoon (or 1/2 ounce) of the powder into one gallon of water. Apply this solution directly to the soil around the base of the plant. Ensure the foliage and blooms are not splashed, as the solution can cause leaf burn.
Thoroughly water the plant before applying the drench to protect the roots from chemical burn. For established plants, start the application in early spring before the flower buds show color. Repeat applications every three to four weeks may be necessary throughout the growing season to deepen the blue and sustain the effect.
Maintaining the Acidic Environment
Sustaining the low pH is a continuous management task separate from the direct aluminum application. Elemental sulfur, often sold as powdered or pelletized garden sulfur, is an effective material for long-term acidification. Unlike aluminum sulfate, elemental sulfur relies on soil microbes to slowly convert it into sulfuric acid, lowering the pH over several months.
This slower, gradual process makes elemental sulfur ideal for maintaining the acidic conditions required for aluminum availability throughout the year. For established plants, apply approximately one-half cup of wettable sulfur per ten square feet, working it lightly into the topsoil. Due to its slow-acting nature, apply this amendment in the fall or early spring to allow time for conversion before the bloom season.
Incorporating certain organic materials can also contribute to a stable, acidic environment. Acidic organic matter, such as peat moss, pine needles, or pine bark mulch, breaks down slowly, naturally conditioning the soil. While these materials are not a substitute for sulfur or aluminum sulfate, they help buffer the soil against pH fluctuations.
Factors That Prevent Blue Color
Several external factors can counteract efforts to achieve blue flowers, even with correct soil amendments. The most significant inhibitor is the use of high-phosphorus fertilizers. Phosphorus binds tightly to aluminum in the soil, forming an insoluble compound that roots cannot absorb, regardless of low pH. Gardeners aiming for blue blooms should select a fertilizer with a very low phosphorus content to ensure aluminum remains available.
The physical environment can also undermine acidification efforts. Planting hydrangeas close to concrete foundations, sidewalks, or patios is problematic because lime leaches from these structures into the surrounding soil. This leached lime significantly raises the soil pH, making it difficult to maintain the necessary acidity.
The quality of irrigation water can also affect soil chemistry. Highly alkaline tap water, often called hard water, continuously introduces basic minerals that gradually raise the pH over time. In areas with hard water, collecting and using rainwater for irrigation can help preserve the established acidic conditions.