As we enter the days of balmy spring weather, we can expect to see many of our favorite flowers once more. Daffodils that come in the early days of the season or honeysuckle in the hot days of summer are some classic staples, but perhaps the most anticipated floral debut is that of the hydrangea macrophylla, or the big-leafed hydrangea.
A common sight in many gardens, the plant is known for its large, spherical blooms comprised of what appear to be smaller flowers. In shades of blue, pink, red or purple, the hydrangea is appreciated throughout for its beauty and vibrance. However, something else makes this plant stand apart. The hydrangea is a backyard pH test!
A hydrangea’s hue indicates the pH level of the soil in which it grows. Blue blooms mean that the shrub is growing in acidic soil that has a pH less than seven. When the blooms turn red or pink, one can expect the plant to be growing in neutral or basic soil, where the pH is greater than six.
This process occurs because the color of hydrangeas is determined by the availability of aluminum ions in the soil. The presence of aluminum in the hydrangea is responsible for changing the color. In acidic soil, aluminum ions can easily be absorbed by the flower, and there are plenty of ions available to interact with, turning the blooms blue. In neutral or basic soil, on the other hand, aluminum ions combine with hydroxide ions, which, when absorbed by the plant, keep the blooms red.
Typically, aluminum is taken in through the roots, which changes the color. However, scientists are experimenting with new ways to change the hydrangea’s color without changing the soil. The plant will still change from red to blue if the aluminum is taken in through the petals, although this method is not effective in changing the blooms from blue to red.
Because the color of the flower is determined by the pH levels in the soil, many gardeners attempt to manipulate their blooms by adding different compounds to the soil. Dolomitic lime can be introduced to the soil to turn blue blooms pink, as well as phosphorus-based fertilizers. For a gardener who prefers blue, aluminum sulfate, as well as fertilizers with low levels of phosphorus and high levels of potassium, are sufficient in changing the color. Coffee grounds and compost can also be an effective way to lower the pH levels over time, leading to consistently blue hydrangeas.
It is easiest to change a hydrangea’s color when it is planted in a pot, as opposed to open soil. Regardless of where it’s planted, it will likely take at least one growing season to see the full effects of the color change. The hydrangea is a beautiful addition to any garden, but it is so much more than just a pretty thing to look at. Next time you find yourself staring at a blue hydrangea, take a moment to consider exactly what that means!