Changing the color of a hydrangea is a deliberate horticultural action that requires patience and planning. This manipulation is only possible with bigleaf hydrangeas (Hydrangea macrophylla) and mountain hydrangeas (H. serrata), whose pigments are chemically sensitive to soil conditions. The process involves amending the soil to shift the chemistry, and results are only visible on new blooms that form after the change takes effect. Successfully altering the color requires applying the appropriate amendments well in advance of the blooming season.
The Role of Soil pH and Aluminum
The vibrant color of these hydrangeas is not directly determined by the soil’s acidity or alkalinity, but rather by the availability of aluminum ions within the soil. The color-changing compounds in the flowers, called anthocyanins, turn blue only when they can bond with aluminum ions. If the plant absorbs aluminum, the bloom will be blue; if aluminum is absent, the flower will be pink.
The soil’s pH level acts as the facilitator for this chemical process. In acidic soil (pH generally below 6.0), aluminum naturally becomes soluble and is easily absorbed by the plant’s root system. Conversely, in alkaline soil (pH above 7.0), the aluminum is chemically “locked up” and remains unavailable to the plant. Neutral soil (pH between 6.0 and 7.0) often results in blooms that are a mix of blue and pink, or a shade of purple.
The Specific Timeline for Visible Changes
The timeline for a visible color change is a matter of months to a full growing season, and sometimes even two, for established plants. Amendments require sufficient time to dissolve and permeate the root zone, shifting the bulk soil pH. Since the color is set when flower buds develop, changes applied to the soil will not affect blooms that have already opened.
For most in-ground shrubs, the ideal time to apply amendments is in late fall or very early spring before new growth begins. This timing ensures the plant can absorb or block aluminum uptake while new buds are forming for the summer bloom. Changing a blue-flowering hydrangea to pink is generally quicker and easier than changing a pink one to blue, especially if the native soil is highly alkaline. Container-grown hydrangeas typically show results faster, sometimes within a few months, because their small soil volume is easier to manage and saturate.
Methods for Achieving Blue or Pink Blooms
Achieving the desired bloom color begins with testing the soil, which provides the starting pH and indicates how much amendment is needed.
Encouraging Blue Blooms
To shift the color toward blue, the goal is to lower the pH to an acidic range, ideally between 5.0 and 5.5, and ensure aluminum availability. This is most effectively accomplished by applying aluminum sulfate. Aluminum sulfate both lowers the pH and directly provides the aluminum ions needed for blue pigment formation. Elemental sulfur can also be used to lower the pH, though it works more gradually.
Encouraging Pink Blooms
To encourage pink blooms, the objective is to raise the soil pH to an alkaline level, typically above 6.5, which restricts aluminum uptake. Garden lime or dolomitic lime is the standard material used for this purpose, as it raises the pH over time.
Application Methods
Amendments should be spread around the plant’s drip line and lightly worked into the top layer of soil, followed by watering. Liquid soil drenches are a faster method for both color shifts. However, drenches require repeated applications throughout the spring and early summer to maintain the desired pH.