The color of the blue or pink flowers on bigleaf and mountain hydrangeas is determined by the soil in which they are planted. It’s a common belief that pH, which is the measure of soil acidity, is the main factor that turns flowers blue. While pH plays an important role, it is the aluminum, which is present in most soils, that makes the flowers blue.

How pH effects hydrangea flower color

Think of pH as a faucet in the ground that allows aluminum to flow. A pH of 5.5 or lower (more acidic) opens the faucet, and the aluminum becomes available to the hydrangea root system. The plant moves the aluminum from the roots to the flowers, which turns them blue. As the pH rises (becomes more alkaline), the faucet starts to close and eventually shuts off, making the flowers pink. The aluminum is still in the soil, but it becomes unavailable to the plant because of soil chemistry.

What makes hydrangeas pink?

Many container-grown hydrangeas have little or no aluminum in their soil, which is why you see many red and pink flowers in your garden centers.

In my experience:

• Most red-flowering potted hydrangeas will be dark blue to purple once they are planted in ground with a lower pH.
• Hydrangeas that have lighter pink flowers in pots usually turn light to medium blue once planted in ground with a lower pH.
• White flowers do not change color regardless of soil chemistry.

How to use amendments to change hydrangea flower color

1. Consider getting a soil test before applying these products to know what pH and mineral content you are working with.
2. There are many products available at your garden center that can help you get the color you want.
3. To turn pink flowers blue, look for soil acidifiers that will lower the pH, and add aluminum.
4. To turn blue flowers pink, add lime to raise the soil pH.
5. Keep in mind that application rates of these products can be misleading and are often used to increase sales; less is often more.
6. Whatever your goal, add amendments in fall or spring; while it might take a year to kick in, the effects should last for a few years after that.

Bob Aube is the founder and co-owner of Morningstar Nurseries, a wholesale nursery in West Kingston, Rhode Island.

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