Changing Hydrangea ColorLearn how to change the color of your hydrangea flowers
The color of a hydrangea can change dramatically, and sometimes they even exhibit multiple colors on the same shrub. Photo by: Ulrich Doering / Alamy Stock Photo.
Flower color can be changed only on specific species of hydrangea, mainly those related to H. macrophylla or H. serrata. The common thought is that flower color is related to the pH of the soil. However, as Michael Dirr explains in his book Hydrangeas for American Gardens, the color is not determined by the pH, per se, but the amount of aluminum a plant can access in the soil — which is determined by pH and phosphorus levels. A lower pH allows aluminum to be soluble and absorbed by plants (unless phosphorus is high), and the flowers tend to be blue. A higher pH locks up the aluminum and the flowers tend to be pink. Shades of cream to green can also be seen when blooms first open, and tones can change as flowers age.
Here’s how you can adjust your soil to affect the bloom color:
- 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.
Can coffee grounds be used to change the color of hydrangeas?
Some gardeners report success in turning their hydrangeas blue by applying coffee grounds to the soil. The coffee grounds make the soil more acidic, allowing the hydrangea to more easily absorb aluminum. In addition, fruit peels, lawn clippings, peat moss and pine needles, are thought to have a similar effect.
Can eggshells be used to change the color of hydrangeas?
If you want pink hydrangeas, crushed eggshells may be one way to get them. Eggshells will slowly break down and reduce the acidity of your soil—making it harder for hydrangeas to absorb aluminum.