Except for genetically pink cultivars and pure white cultivars, an acid pH soil reaction will produce blue flowers and an alkaline pH soil reaction will produce pink flowers. When attempting to shift the plant's flower color, it may take two to three blooming cycles to attain the desired color and color intensity. Flower buds develop during mid- to late summer, so it is important to apply soil amendments in early spring to begin changing the soil pH. The bloom cycle after the first application may result in "muddy" flowers, flowers that have no distinct color. Soil pH below 5.5 will result in blue flowers with the addition of 1/4 cup of aluminum sulfate per mature plant. Soil pH above 6.5 will result in pink flowers with the addition of 1/4 cup of either agricultural or dolomitic limestone per mature plant. Repeat the application in June. Water well after each application to the soil. If the desired color is not produced, repeat the following year.
Hydrangea color may be especially slow to change if the plant root zone is close to a "fresh" or new concrete wall, foundation, or slab. These structures can affect soil pH for lengthy periods.
