There are certain types of hydrangeas that lend themselves to changes of color, specifically H. macrophylla or H. serrata. Most people consider the color of the flower to be associated with the soil pH. Michael Dirr, author of Hydrangeas for American Gardens, would like to set the matter straight. He says it is not fully associated with the pH, but rather, with the aluminum that is accessible in the soil. That factor is determined by pH and phosphorus levels.
If the pH in the soil is lower, the aluminum is more soluble and able to be absorbed by plants more easily. In that case, you tend to see blue flowers. If the pH is higher, the aluminum is not as readily accessible and the flowers are pink. You may also see a variety of other colors, including cream and green when the blossoms first open and as the flowers age, the tone can change.
You can adjust the soil to adjust the bloom color in the following way:
– Soil pH 5.0 to 5.5 = Blue
– Soil pH 6.0 to 6.5 = Pink/purple
– Soil pH 5.5 to 6.5 = Purple, or both blue & pink
– To make sepals bluer, add aluminum sulfate to soil and maintain low levels of phosphorous, moderate levels of nitrogen, and high levels of potassium.
– To make sepals pinker, add lime to soil plus maintain high levels of nitrogen and low levels of phosphorous.
This should be done well in advance of flowering, in late autumn or early spring.
Another common question is if the color of hydrangeas can be changed using coffee grounds. Some success has been noted by gardeners in making hydrangeas blue in color when they use coffee grounds in the soil. Since the soil is more acidic when coffee grounds are used, the hydrangea absorbs more aluminum. You might also want to try fruit peels, lawn clippings, peat moss, and pine needles for a similar effect.
You might also be wondering if eggshells can be used to change hydrangea colors. Using crushed eggshells can, at times, change the hydrangea to pink. As the eggshells slowly break down, the acidity of the soil is reduced, making it more difficult for the hydrangeas to absorb aluminum.SKM: below-content placeholder