Expert Advice

Tips for Using pH to Change Hydrangea Color

Authored by:

Earth Science

We’ve all seen gardens with large, beautiful displays of flowers, in colors that sometimes defy the imagination. Most of us probably assume that the color of the flower is always determined by the genetics of the plant—and that’s true. But did you know that the genetics of one particular plant actually changes the color of its blossoms under certain conditions? That’s right. Some hydrangeas change blossom color depending on the pH of the soil in which they are planted.

What is pH?

pH is a scientific term that describes the amount, or “power” (hence the “p”), of hydrogen (the “H”) that’s available within the soil. The hydrogen ions are what enable a plant’s roots to absorb the various nutrients from the soil. The pH of soil ranges from 0 to 14, and we speak of low pH soil as being “acidic” and high pH soil as being “alkaline.” Generally speaking, areas with heavy rainfall and a lot of forests have acidic soil, and drier areas of desert terrain have alkaline soil. Plains with less rainfall and lighter cover tend to have neutral soils.

Hydrangeas and pH

Some hydrangeas are very sensitive to the pH of the soil in which they are grown, and this sensitivity is reflected in the color of their blossoms. Most French hydrangeas (Hydrangea macrophylla) have, over the centuries, developed this sensitivity. In acidic soil (below a pH of 6), their flowers will be blue. In alkaline soil (with a pH above 7), their flowers will be pink, or even red. In neutral to slightly acid soil, these hydrangeas can have purple blooms, or even a mixture of pink and blue blooms on the same plant.

There are exceptions to every rule, of course, and there are some exceptions here. Hydrangeas with white blossoms will always bloom in white, no matter the pH of the soil. Some individual cultivars stick to their color in all but the most extreme conditions. For example, Ami Pasquier remains red unless the soil is extremely acidic, in which case, it turns purple, but never truly blue. Purple Tiers remains purple regardless of the pH of the soil.

Other Factors that Impact Hydrangea Blossoms

Other elements can affect hydrangea blossoms. For example, some say that hot weather will prevent any hydrangea from turning a true crimson or red, no matter how alkaline the soil may be. The depth and intensity of the blossom color is impacted by factors that you will have less control over, such as the weather conditions, the amount of humidity, and the health of the plant. It is also possible that the amount of aluminum in the soil has as much effect as acidic soil on the tendency for hydrangeas to bloom in blue.

Changing the pH of Your Soil

If you do want to control the color range of your hydrangea blossoms, changing the pH of your soil is the most straightforward method.

Increasing the pH of your soil

In order to promote pink blossoms on your hydrangea, you will want to add lime to your soil. However, this does not mean a trip to the grocery store for a bag of those acidic green limes. In this case, we are speaking of the mineral lime, which is actually the finely ground mineral calcium carbonate. Earth Science’s Fast Acting™ Lime product quickly raises soil pH, as well as nourishes the soil environment.

Decreasing the pH of your soil

In order to promote blue blossoms on your hydrangea, you will want to add sulfur to your soil. Earth Science’s Fast Acting Sulfur® is an effective solution to lowering the soil’s pH, and again, fostering a better soil environment. Both sulfur and lime are effective for in-ground and potted hydrangeas.

Some newly developed hydrangea varieties have an extended bloom period. A few even naturally produce flowers in multiple colors, without the need to adjust pH.

By creating various soil pH levels, you can test out the range of hydrangea colors- a fun way to change things up in your garden. From pink to blue, and even a mishmash of the in-between, hydrangeas have color changing down to an art. For more information, contact us today!