From our perspective as hydrangeas, particularly the *Hydrangea macrophylla* (bigleaf) varieties, our bloom color is not a fixed trait but a direct response to our soil environment. The key player in this color transformation is the element aluminum. For our roots to uptake aluminum, it must be soluble and available in the soil solution. This solubility is almost entirely governed by the soil's pH level. In very simple terms, acidic soil allows us to access aluminum, turning our blooms blue. Alkaline soil locks the aluminum away, resulting in pink blooms. It is a fascinating survival mechanism, allowing us to thrive in a wider range of conditions.
To produce those coveted, vibrant blue blooms, we require acidic soil with a pH range of 5.2 to 5.5. At this acidity, aluminum ions (Al³⁺) become soluble. Our root systems can efficiently absorb them. Once inside our plant tissues, the aluminum forms complexes with the anthocyanin pigments (specifically delphinidin-3-glucoside) present in our sepals (the colorful part of the bloom). This chemical interaction is what creates the stunning blue coloration. A pH higher than this range begins to reduce aluminum availability, leading to less intense or muddy purple-blue flowers.
To showcase beautiful pink blossoms, we need a soil environment that is neutral to slightly alkaline, with a pH of 6.0 to 6.2. In these conditions, aluminum remains tightly bound to soil particles in an insoluble form. Our roots are physically unable to absorb it. Without aluminum, the natural pink or red pigments in our sepals, which are always present genetically, can express themselves fully. It is not that the blue pigment is removed; it is simply never formed due to the lack of the key aluminum component. A pH much above 6.5 can start to cause other nutrient uptake issues, such as iron deficiency, which can harm our overall health.
You will often see blooms in a transitional purple or lavender shade. This occurs in the pH "twilight zone," roughly between 5.5 and 6.0. In this range, aluminum is only partially available. Our roots uptake a moderate amount, resulting in a blend of the blue and pink pigments and creating the lovely purple hues. To lower pH for bluer flowers, gardeners can amend the soil with elemental sulfur or aluminum sulfate. To raise pH for pinker flowers, the addition of garden lime (calcitic or dolomitic) is effective. These adjustments must be done gradually and based on a soil test, as a sudden, drastic change in pH is stressful to our root systems.
It is important to note that this color chemistry primarily applies to the mophead and lacecap varieties of *Hydrangea macrophylla*. If you are caring for a white hydrangea cultivar, such as *Hydrangea arborescens* 'Annabelle' or *Hydrangea paniculata* varieties, our blooms will remain white or cream-colored regardless of soil pH. Our genetics simply do not produce the anthocyanin pigments that react with aluminum. For us, soil pH only affects our ability to access essential macro and micronutrients for healthy green growth and strong blooms.
