When a hydrangea changes color, it is not due to genetics but rather a reaction to the chemistry within the soil. Only the Bigleaf Hydrangea (Hydrangea macrophylla) and the Mountain Hydrangea (H. serrata) possess the ability to shift their bloom color from pink to blue. This remarkable transformation is entirely dependent on the presence of aluminum in the soil and the plant’s ability to absorb it.
The Core Requirements for Blue Hydrangeas
Achieving a vibrant blue color requires a specific interplay between two factors: aluminum and soil acidity. Aluminum ions are the actual agents that create the blue pigment when absorbed by the plant’s sepals. Without aluminum, a hydrangea will not produce blue flowers, regardless of any other soil condition.
The soil’s pH acts as the master switch, determining whether the aluminum is soluble and therefore available for the roots to take up. Aluminum becomes accessible only when the soil is acidic, meaning the pH is low. To achieve a deep blue color, the target pH range is typically below 6.0, ideally maintained between 5.2 and 5.5. If the soil pH is too high (alkaline), the aluminum binds to other compounds and becomes locked in the soil, resulting in pink blooms. Always perform a professional or home soil test to determine the current pH level before beginning any acidification treatments.
Step-by-Step Guide to Acidifying Soil
The two most common and effective materials for lowering soil pH are Aluminum Sulfate and Elemental Sulfur. Aluminum Sulfate offers a faster result because it immediately lowers the pH while also supplying aluminum ions directly to the soil.
Aluminum Sulfate Application
Aluminum Sulfate should always be dissolved in water before being applied as a soil drench around the root zone. A common recommendation is to mix one tablespoon of granular Aluminum Sulfate into one gallon of water, applying this solution to the plant’s drip line. Water the plant thoroughly beforehand to prevent the concentrated solution from burning the roots. Wear gloves and avoid splashing the solution onto the leaves or stems, which can cause scorching.
Elemental Sulfur Application
Elemental Sulfur is an effective long-term method for reducing soil pH. Sulfur relies on soil bacteria to convert it into sulfuric acid, a process that can take several months. This method provides a gradual, more stable pH reduction and is less likely to cause root burn than Aluminum Sulfate. Broadcast the granular sulfur uniformly over the soil surface and work it into the top two to three inches before watering it in. The optimal time to begin these treatments is in early spring before the buds swell, or immediately after the plant has finished flowering.
Ongoing Maintenance for Deep Blue Color
Maintaining a deep blue color requires ongoing management of soil pH and nutrient levels, since the soil naturally tries to revert to its original pH over time. A major factor to monitor is the phosphorus content in your fertilizer, which can sabotage your efforts. Phosphorus (the middle number in the N-P-K ratio) binds tightly to aluminum in the soil, making it unavailable for the hydrangea to absorb.
Fertilizer Management
Use a low-phosphorus fertilizer, such as one with an N-P-K ratio like 25-5-30 or 12-4-8. Selecting a fertilizer where the middle number is significantly lower than the first and third numbers ensures that the aluminum remains free to be taken up by the plant. Applying a low-phosphorus, acid-loving plant formulation once in the spring is usually sufficient to support healthy growth without interfering with the blue coloration.
Water and Location
Water quality is another influence on soil pH, particularly if your tap water is considered “hard.” Hard water contains high levels of dissolved calcium and magnesium, which are alkaline minerals that raise the soil pH with every watering. If you live in an area with hard water, consider collecting and using rainwater for your hydrangeas or periodically re-applying acidifying amendments. Additionally, avoid planting blue hydrangeas directly next to concrete structures or limestone rock walls, as these materials leach alkaline compounds into the surrounding soil.