How to Get Hydrangeas to Turn Pink or Blue

Hydrangeas come in beautiful shades of white, blue, pink, and green. And unlike other flowering shrubs, hydrangeas have the unusual ability to change their flower color. The determining factor of hydrangea color is the level of soil acidity: in alkaline soil, hydrangea flowers turn pink, while in acidic soil, they turn blue.

If the soil is not naturally alkaline or acidic, you can gradually amend it in order to change the flower color of your hydrangea. But not all hydrangea varieties can change their flower color from pink to blue and vice versa. Learn how to change the color of your hydrangea blooms below.

What Determines Hydrangea Color

The color of hydrangeas comes from anthocyanins, or water-soluble pigments, in what is commonly called the "flower heads" or bracts. When the pigment molecules are closely stacked together in the plant tissue of the bracts, they appear blue, and when they are sparse and scattered, they appear pink.

Whether the hydrangea blooms pink or blue is determined by two interdependent factors: the availability of the micronutrient aluminum to the plant and the soil pH. The more aluminum the plant can absorb from the soil, the closer the pigment molecules move together, thus turning the color blue. It is the soil pH that determines how much aluminum is available to the plant because aluminum is more soluble in acidic soils. Most soils have enough aluminum; however, it is not available to the plant if the soil pH is high. 

Here's how soil pH affect the hydrangea flower color: 

• Acidic soil (pH 5.0-5.5): Blue flowers
• Neutral to alkaline soil (pH 5.5– 6.5): Mauve, purple, or a mixture of blue and pink flowers
• Alkaline soil (pH 6.5-7.0): Pink flowers 

Which Types of Hydrangeas Change Colors

Before you take any steps to change your hydrangea colors, you need to identify what kind of hydrangea you have. Of the many different varieties, only bigleaf hydrangeas (Hydrangea macrophylla), also known as French, mophead, or lacecap hydrangeas, and mountain hydrangeas (Hydrangea serrata) can change the color of their flowers.

The color of oakleaf hydrangeas (Hydrangea quercifolia) and peegee hydrangeas (Hydrangea paniculata) with white or cream-colored flowers cannot be changed.

How to Change Hydrangea Color 

Step #1: Do a Soil Test

A soil test to determine the pH of your soil is a must to know whether you need to increase or decrease the pH to get your desired flower color. For the most accurate results, get a soil pH testing kit from a garden center or your local Cooperative Extension Office. Adding amendments to the soil based on merely guessing the soil pH can harm not only the hydrangea but also other plants in its vicinity.

Step #2: Select Your Soil Amendment

Once you have the pH test results in hand, you can do one of the following:

• To turn the flowers blue, lower the soil pH (make it more acidic) by adding aluminum sulfate or wettable sulfur to the soil.
• To turn the flowers pink, raise the soil pH (make it more alkaline) by adding limestone to the soil.

The exact amounts of aluminum sulfate, sulfur, or lime you need depends on the soil pH and soil type. Carefully follow the directions on the bag and don’t use more of the product than indicated because it will injure the plant. 

Step #3: Use Granular Products or the Liquid Drench Method

Granular products can be applied any time of the year. Apply the product around the base of the plant and work it into the soil with a rake or a cultivator. It can take up to a year for the soil pH change to take effect and show in the flower color of your hydrangea.

A faster option is to do a repeated liquid soil drench in March, April, and May. Dissolve 1 tablespoon aluminum sulfate for blue flowers or 1 tablespoon of hydrated lime for pink flowers in 1 gallon of water. Drench the soil around the hydrangea with this solution and make sure not spill any of it around the leaves.

Maintaining the Soil pH

Keep in mind that the bigger the change you need to make in the soil pH to get the desired flower color, the more difficult and lengthier the process will be. Changing the soil pH is a gradual process that cannot be done at once. And even if you can get the soil to the desired pH, it will naturally revert back to its original soil pH over time so changing the flower color of your hydrangea might be an uphill battle.

If your soil is naturally alkaline, you might be better off accepting the fact that your hydrangea blooms pink instead of blue.