You can change the color of certain hydrangeas, but this is not true “dyeing.” Instead, the bloom color is shifted from pink toward blue or blue toward pink by manipulating the plant’s natural pigment through soil chemistry. This color change relies entirely on adjusting the soil’s acidity or alkalinity.
The Science Behind Hydrangea Color
The vibrant colors of hydrangeas are produced by water-soluble pigments called anthocyanins, which are naturally red or pink in the plant’s cells. The flower’s hue is determined by how the soil’s pH controls the availability of aluminum ions. These aluminum ions bind with the anthocyanin pigment.
When aluminum is absorbed by the plant and transported to the flower sepals, it creates a stable complex molecule with the pigment that reflects blue light. In acidic soil, aluminum is highly water-soluble and easily absorbed by the plant’s root system. Conversely, in alkaline soil, aluminum combines with hydroxide ions to form a solid, insoluble compound, making it unavailable for the plant to absorb.
The soil pH determines whether the aluminum is mobile and accessible or bound and inaccessible. To produce a true blue color, aluminum must be present in the soil, and the soil must be acidic enough to release it. Without available aluminum, the pigment reverts to its default pink or red color.
Identifying Color-Changing Species
Not all varieties of this shrub are capable of changing color in response to soil amendments. Only two main species possess the specific genetic makeup required for this chemical reaction.
The species that can shift color are the Bigleaf Hydrangea (Hydrangea macrophylla) and the Mountain Hydrangea (Hydrangea serrata). Bigleaf hydrangeas are commonly known as mopheads or lacecaps, and many modern cultivars are bred to be color-changeable.
Other common types, such as Smooth Hydrangea (H. arborescens), Panicle Hydrangea (H. paniculata), and Oakleaf Hydrangea (H. quercifolia), do not change color with soil pH adjustments. White cultivars lack the anthocyanin pigment entirely, so they will remain white regardless of soil chemistry.
Achieving Blue Hues: Acidifying the Soil
To shift flower color from pink toward blue, the soil pH must be lowered to a strongly acidic range, ideally between 5.0 and 5.5, to maximize aluminum availability. This process requires a soil test to determine the starting pH and the amount of amendment needed. A common and effective product for this purpose is aluminum sulfate.
A typical application involves dissolving about one tablespoon of aluminum sulfate in a gallon of water and applying it as a soil drench around the base of the plant. It is important to pre-water the plant thoroughly before applying the solution, as this prevents the aluminum sulfate from burning dry roots. This liquid application should begin in early spring and continue monthly through the growing season for the best results.
For a longer-term solution, elemental sulfur can be sprinkled around the plant to lower the soil pH permanently. This process is slower but safer than frequent aluminum sulfate drenches, aiming for the 5.0 to 5.5 pH range. Gardeners should also use fertilizers low in phosphorus and high in potassium (e.g., 25-5-30), because phosphorus can bind aluminum and prevent its uptake.
Organic matter like pine needles or coffee grounds can serve as mild, supplementary acidifiers, though they are usually not strong enough to achieve a deep blue on their own. Since the color change happens in the developing flower buds, the entire process takes time, often several months or even a full season, to fully manifest in the blooms.
Achieving Pink Hues: Alkalizing the Soil
To encourage pink or red blooms, raise the soil pH to an alkaline range, typically between 6.0 and 6.5. This higher pH locks up the aluminum ions, making them unavailable for the plant to absorb and allowing the anthocyanin pigment to express its natural pink color. Soil tests are important to avoid raising the pH above 6.4, which can lead to iron deficiency and yellowing leaves.
The most common and practical way to raise the soil pH is by applying garden lime or dolomitic lime. Dolomitic lime is preferred because it also supplies magnesium, which is beneficial for plant health. This granular material is typically worked into the soil around the plant in late fall or early spring, following package directions for application rates.
Another method involves using wood ash from a fireplace, which also increases soil alkalinity, but it must be applied sparingly to avoid over-alkalizing the soil. For fertilization, gardeners should choose a formula that is high in phosphorus, such as a ratio close to 25-10-10, as the phosphorus helps bind any residual aluminum already in the soil.
If the soil is naturally highly acidic, maintaining the pink color can be a continuous effort, requiring repeated applications of lime every few months. Growing the hydrangea in a container with a neutral potting mix is a simpler alternative in areas with naturally acidic soil, as it allows for easier control over the soil chemistry.