Hydrangeas are admired garden shrubs known for their spectacular blossoms, which display a spectrum of colors ranging from vibrant blues to deep pinks. Gardeners are often fascinated by the ability to influence this color display. Sodium bicarbonate, commonly known as baking soda, is a simple household substance often cited as a way to manipulate this color. This white powder is a mild alkaline compound with a high pH, sometimes used to change the soil environment around the plant. This article explores the specific effects of introducing baking soda to hydrangeas.
Understanding Hydrangea Color and Soil pH
The striking color of Hydrangea macrophylla (bigleaf hydrangeas) is determined by the availability of aluminum ions in the soil, not the plant’s pigment alone. The soil’s pH level regulates this aluminum availability. When the soil is acidic (typically pH 5.5 or lower), aluminum is soluble and mobile, allowing the plant to absorb it readily. This uptake of aluminum chemically reacts within the bloom to produce a blue coloration.
Conversely, if the soil is neutral or alkaline (pH 7.0 or higher), the aluminum ions become bound up and immobile. Aluminum precipitates as aluminum hydroxide, making it inaccessible to the hydrangea’s root system. A lack of available aluminum results in the blossoms displaying pink or red hues. Transitional colors, such as lavender or purple, emerge when the pH is between 5.5 and 7.0, allowing for partial aluminum uptake.
The Alkalizing Action of Baking Soda
The premise behind using baking soda on hydrangeas is to intentionally shift the soil’s pH toward the alkaline side. Baking soda, or sodium bicarbonate, is a base that typically has a pH of approximately 8.3 when dissolved in water. When this solution is applied to the soil, the bicarbonate ion acts as a neutralizing agent.
The bicarbonate ions react with and neutralize the acid components present in the soil. This chemical process raises the soil’s pH level, which reduces the solubility of aluminum. Gardeners use this method to lock up aluminum in the soil, preventing the plant from absorbing the element that creates blue blooms and encouraging pink or red flowers. However, the effect of baking soda on soil chemistry is often minor and temporary due to the soil’s natural buffering capacity.
Safe Application Methods and Dosages
For those who choose to experiment with baking soda, a measured dilution is required to minimize the risk of plant damage. A widely suggested mixture involves dissolving one tablespoon of baking soda into one gallon of water. This solution is intended to alter the soil environment, not the foliage.
The solution should be applied as a drench directly to the soil around the base of the hydrangea. Ensure the liquid does not splash onto the leaves, as applying the mixture to the leaves can cause leaf burn or spotting. This drench is typically applied once a month during the growing season, but only after the soil pH has been tested to confirm that an increase in alkalinity is needed.
Potential Drawbacks and Alternative pH Management
While baking soda offers a quick, mild increase in soil pH, it presents a significant drawback due to its composition. Sodium bicarbonate is a salt, and repeated application leads to a buildup of sodium ions in the soil. This sodium accumulation can be toxic to the plant, causing symptoms like leaf burn, stunted growth, and interference with water and nutrient absorption.
The temporary nature of the pH change also makes baking soda an inefficient long-term solution. Professional horticulturists recommend using more stable and safer materials for sustained pH management. To reliably encourage pink blooms, dolomitic lime or garden lime are preferred, as these amendments slowly and consistently raise the soil pH without the risk of sodium toxicity. For gardeners seeking blue blooms, elemental sulfur or aluminum sulfate are the standard compounds used to safely increase soil acidity.