The answer to whether hydrangeas change color based on soil conditions is yes, but this unique characteristic is limited to specific types, primarily the Bigleaf Hydrangea (Hydrangea macrophylla). Its bloom color is not solely determined by genetics; the hue displayed by its flowers is a direct visual indicator of the soil’s chemical composition. This interaction allows gardeners to actively participate in determining the final color of the blossoms.
The Science Behind Color Change
The ability of certain hydrangeas to shift color relies on how soil pH controls the availability of aluminum ions. The color is determined by the presence or absence of this metal within the flower sepals. The vibrant pigment responsible for the color is an anthocyanin, which changes its structure when it binds with aluminum.
In acidic soil (pH 5.0 to 5.5), aluminum becomes highly soluble and is easily absorbed by the roots. Once transported to the flower buds, the aluminum ions complex with the anthocyanin pigment, resulting in blue coloration.
Conversely, when the soil is alkaline (pH 6.0 or higher), aluminum is chemically bound to other compounds in the soil, making it insoluble and unavailable for the plant to absorb. When aluminum is absent, the anthocyanin pigment remains unbound, and the flowers display shades of pink or red. Intermediate colors, such as purple or lavender, appear when the pH falls between 5.5 and 6.0. The soil pH therefore acts as a gatekeeper, dictating whether the available aluminum can be taken up.
How to Encourage Blue Blooms
To encourage blue blooms, the goal is to make the soil more acidic, which in turn increases the availability of aluminum ions for the plant. Gardeners should first conduct a soil test to determine the current pH level, as knowing the starting point is important for calculating the correct amount of amendment. The target pH range for true blue coloration is typically 5.0 to 5.5.
The most common and effective amendments used to lower the pH are aluminum sulfate or elemental sulfur. Aluminum sulfate works quickly because it provides both the acidity and the necessary aluminum simultaneously. It is often applied as a liquid drench around the base of the plant in the spring, before the flowers begin to bud.
Applying too much aluminum sulfate, however, can be detrimental to the plant’s roots and may cause foliage damage if splashed onto the leaves. Elemental sulfur is a slower-acting but gentler alternative that gradually lowers the soil pH over several months by converting to sulfuric acid through microbial action. Regular reapplication may be necessary, especially in regions with naturally neutral or alkaline soil, to maintain the desired acidic environment.
How to Encourage Pink Blooms
Achieving pink blooms requires the opposite approach by raising the soil pH to a level where aluminum becomes insoluble and cannot be absorbed by the plant. The target pH for pink flowers is 6.5 or higher, which effectively binds the aluminum in the soil.
The most common amendment for increasing alkalinity is garden lime, which can be applied as either calcitic or dolomitic lime. Dolomitic lime is often preferred as it also supplies magnesium, which is beneficial for plant health. Another option is hydrated lime, which acts more quickly, or wood ash, though wood ash should be used sparingly due to its high salt content.
These amendments are typically broadcast over the soil surface and watered in, and the color change may take a full growing season or longer to become apparent. Maintaining a strong pink color can be challenging in regions where the native soil is highly acidic, as the alkalinity introduced by the lime will slowly be neutralized by the soil’s natural chemistry and rainfall. Persistent effort and regular monitoring of the pH are necessary to keep the aluminum unavailable.
Hydrangea Varieties That Do Not Change Color
While the color-shifting ability is a famous trait, it is not universal across all hydrangea species. The mechanism is primarily limited to Bigleaf Hydrangeas (Hydrangea macrophylla) and Mountain Hydrangeas (Hydrangea serrata). Other popular species lack the genetic makeup or the necessary pigment to react to aluminum uptake.
For instance, Smooth Hydrangeas (Hydrangea arborescens), such as the popular ‘Annabelle’ cultivar, and Panicle Hydrangeas (Hydrangea paniculata), like ‘Limelight,’ have fixed white blooms. Their flowers may age to shades of cream, pink, or red late in the season, but this change is a natural part of the aging process and is completely independent of the soil pH. Oakleaf Hydrangeas (Hydrangea quercifolia) also have a fixed color, with their white flowers aging to pink or burgundy in the fall due to temperature changes.
Furthermore, white-flowered cultivars of the Bigleaf Hydrangea itself, such as ‘White Wedding’, will also remain white regardless of how acidic the soil becomes. These white varieties simply lack the anthocyanin pigment needed to form a complex with the aluminum, making them immune to the color-changing effect.