The color of certain hydrangea blossoms is uniquely sensitive to the soil in which they grow, unlike most flowering plants whose color is genetically fixed. Soil pH measures acidity or alkalinity on a scale from 0 (most acidic) to 14 (most alkaline), with 7.0 being neutral. The ability of Hydrangea macrophylla (bigleaf) and Hydrangea serrata (mountain) varieties to change flower color based on pH makes managing soil chemistry important for their appearance.
The Ideal Soil pH Range for Hydrangeas
For general plant health and vigorous growth, hydrangeas prefer a slightly acidic to neutral soil environment. A pH range between 5.5 and 6.5 is typically considered optimal for the overall well-being of the plant, ensuring nutrients are readily available for root uptake. This range allows the plant to efficiently absorb necessary nutrients like nitrogen, phosphorus, and potassium.
Not all hydrangeas are equally affected by soil pH. While bigleaf and mountain species respond dramatically, others like Hydrangea paniculata (panicle) and Hydrangea arborescens (smooth) typically flower in white or pale green. These varieties maintain their inherent color regardless of soil acidity or alkalinity. Therefore, pH manipulation focuses primarily on cultivating the color-changing species.
How Soil pH Influences Flower Color
The color change in sensitive hydrangeas is an indirect result of how pH affects the availability of aluminum. The flower petals contain anthocyanin, a pigment responsible for the red or pink color. When aluminum is present and absorbed by the plant, this metal binds with the anthocyanin, shifting the color from pink to blue.
To achieve a vibrant blue, the soil must be sufficiently acidic, ideally pH 5.5 or lower, to free aluminum ions from soil particles. In this acidic state, aluminum becomes soluble and is taken up by the roots and transported to the flowers. Conversely, for pink or red flowers, the soil must be neutral or alkaline, with a pH of 7.0 or higher. Under alkaline conditions, aluminum binds tightly to the soil, making it unavailable for plant absorption. A purple or lavender hue indicates a transitional pH, typically between 5.6 and 6.5, where aluminum is only partially available.
Testing and Adjusting Soil pH
Determining the starting point of soil chemistry is the first step toward controlling flower color. A soil test kit, purchased from a garden center, offers a convenient way to check pH at home using a color-coded strip or meter. For a more precise analysis of nutrient levels and pH, sending a soil sample to a professional laboratory is recommended. When collecting samples, take multiple cores from the root zone and mix them to obtain an accurate average reading.
To shift the color toward blue, which requires a lower, acidic pH, amendments can be added. Elemental sulfur gradually lowers the pH as soil microbes convert it to sulfuric acid. For faster results, aluminum sulfate can be applied, as it immediately provides both the necessary acidity and aluminum ions for blue coloration. However, this substance should be used carefully, as excessive amounts can lead to aluminum toxicity in the plant.
To encourage pink flowers, gardeners apply garden lime to raise the pH to a more alkaline level. Dolomitic or calcitic lime neutralizes the soil’s acidity over time, effectively locking away available aluminum. These initial adjustments are best made in the fall or early spring. It can take six to twelve months for the soil chemistry to stabilize and for the color change to become visible in new blooms.
Maintaining the Desired Soil pH
Achieving the correct pH is a starting point, but maintaining that level requires ongoing attention because natural processes constantly change soil chemistry. The water source is an often-overlooked factor, as hard tap water is typically alkaline and can gradually reverse acidification. Watering with rainwater or distilled water helps preserve the lower pH needed for blue flowers.
Strategic use of mulch also contributes to long-term stability. Acidic mulches, such as pine needles, peat moss, or shredded bark, slowly break down and help sustain the low pH required for blue flowers. Conversely, alkaline mulches, like limestone gravel, should be avoided where a blue color is desired. Since soil pH naturally drifts, especially with heavy rainfall or watering, retesting the soil annually in the spring or fall is good practice. Based on yearly test results, smaller maintenance applications of sulfur or lime can be applied to counteract the drift and ensure the plant continues to produce the desired pink or blue flowers.