The color variation in bigleaf hydrangeas (Hydrangea macrophylla) is primarily governed by soil chemistry, specifically the availability of aluminum ions and the soil's pH level. Aluminum is naturally present in most soils, but its accessibility to the plant's roots is highly dependent on pH. In acidic soils (pH below 6.0), aluminum ions are soluble and can be absorbed by the plant. This aluminum complexes with anthocyanin pigments in the sepals, resulting in blue flowers. In alkaline soils (pH above 7.0), aluminum becomes insoluble and locked in the soil, making it unavailable to the plant. Without aluminum, the same hydrangea cultivar will produce pink flowers.
Vinegar, typically a 5% solution of acetic acid, is a fact in its ability to lower pH, but its application is a gardening myth in practice. Pouring vinegar directly onto the soil or plant is not recommended. Acetic acid is a non-selective herbicide that can easily damage or kill beneficial soil microbes, earthworms, and even the hydrangea's delicate feeder roots upon contact. Furthermore, its effect is extremely short-lived. It acts quickly but is then rapidly broken down by soil bacteria, leading to a sharp, temporary pH spike followed by a quick return to the original pH. This volatility provides no lasting change and can cause significant stress to the plant and soil ecosystem.
Used coffee grounds are often cited for their slight acidity. From the plant's perspective, fresh coffee grounds can be acidic, but used grounds are generally close to neutral (pH ~6.5-6.8). Their value is not as a potent acidifier but as a source of organic matter. As coffee grounds decompose, they improve soil tilth, water retention, and drainage. The slow decomposition process can foster a healthy, slightly acidic environment beneficial for nutrient uptake, including aluminum. However, the effect on pH is minimal and very gradual. They are not a reliable or fast-acting method for significantly altering soil pH to change bloom color. Excessive, uncomposted grounds can also form a barrier that repels water and impedes air flow to roots.
For reliable and safe results, plants respond best to consistent, controlled amendments. To encourage blue flowers, a sustained acidic environment is required. This is best achieved by applying elemental sulfur or aluminum sulfate to the soil following package instructions. Elemental sulfur is metabolized by soil bacteria to slowly and persistently lower the pH. Aluminum sulfate provides both the necessary aluminum ions and acidity. For pink flowers, garden lime is used to raise the soil pH and lock up aluminum. These amendments work gradually, allowing the plant to adjust without root shock, and provide a stable change in soil conditions that the hydrangea's root system can reliably respond to over the growing season.
