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What pH turns hydrangeas blue?

Hydrangeas are a beloved flower known for their lush, full blooms that come in a variety of colors. One of the things that makes hydrangeas so unique is that their flower color can be changed depending on the pH of the soil they are grown in. Hydrangeas grown in acidic soil with a lower pH will produce blue flowers, while those grown in more alkaline soils with a higher pH will bloom pink. But what exactly is the pH range that results in blue hydrangeas? Let’s take a closer look.

How Hydrangeas Change Color

Hydrangeas are unique in their ability to “self-color” their flowers based on the pH of the soil. This is because the color of hydrangea blooms is determined by the presence of anthocyanin, a type of pigment that flowers produce. In hydrangeas specifically, the anthocyanin pigment turns the flowers blue in acidic conditions, while a lack of anthocyanins results in pink flowers.

The availability of aluminum in the soil is key to the hydrangea’s color change. In acidic soils, aluminum is more soluble and available to the plant. The anthocyanins in the hydrangea bonds with these aluminum ions, resulting in a blue pigmentation. In more alkaline soils, the aluminum is less available, so without aluminum to bond with, the flowers remain pink.

Ideal pH for Blue Hydrangeas

In order to produce blue hydrangea blooms, the soil needs to have a lower pH, usually between 4.5-5.5. This acidic environment allows for the maximum availability of soluble aluminum that the hydrangea needs to turn blue. A soil pH outside of this range will result in hydrangea blooms that are more pink or even purple.

Soil pH Flower Color
4.5-5.5 Blue
5.6-6.0 Lilac-Purple
6.1-6.5 Reddish Purple
6.6-7.0 Pinkish-Purple
7.0+ Pink

As you can see from the table, the most acidic pH range between 4.5-5.5 will produce true, vivid blue blooms. A pH above 7.0 is too alkaline for blue flowers.

How to Lower Soil pH for Blue Hydrangeas

If your hydrangea flowers are not the blue hue you desire, it is possible to lower the pH of the soil to achieve blue blooms. Here are some methods to help acidify the soil:

  • Add aluminum sulfate – Mixing aluminum sulfate into the soil provides accessible aluminum and lowers pH
  • Incorporate peat moss – Peat moss is highly acidic and will help acidify soil for hydrangeas
  • Use coffee grounds – The acidity in coffee grounds makes them effective for lowering soil pH
  • Mulch with pine needles – As they break down, pine needles release acids that lower pH
  • Add elemental sulfur – Sulfur combines with water and oxygen to form sulfuric acid to lower pH

It is best to test your soil’s pH before attempting to alter it. Aim to gradually lower the pH to somewhere between 4.5-5.5 for best results. Going below 4.5 can potentially harm the plant.

When working to acidify soil, it is easier to do so before planting. But even after hydrangeas are established, incorporating acidic materials into the soil annually can produce blue blooms over time.

Factors that Affect Hydrangea Color

While soil pH is the primary determining factor of hydrangea color, there are some other things that can influence the hues:

  • Variety – Some hydrangea varieties are bred to be be more fixed in their coloring. For example, Endless Summer hydrangeas will typically always bloom pink or blue.
  • Age – Older hydrangea shrubs often get less colorful. The pigments become less intense year after year.
  • Climate – Cooler climates produce deeper, more vivid hydrangea colors.
  • Water – Adequate water is needed for hydrangeas to bloom their best colors.
  • Pruning – When and how hydrangeas are pruned impacts bud development and subsequent blooms.
  • Nutrients – Proper fertilization ensures hydrangeas have nutrients needed to develop rich colors.

While these factors can influence color, the pH of the soil remains the key determining element for blue hydrangea flowers.

Best Practices for Growing Blue Hydrangeas

Follow these tips to successfully grow vivid blue hydrangeas in your garden:

  • Test soil pH yearly and adjust as needed to maintain acidity
  • Choose a hydrangea variety suited to your climate zone
  • Plant in partial to full sun for best growth
  • Water 1-2 inches per week, increasing in hot weather
  • Prune lightly in late winter, removing only spent blooms
  • Mulch annually with acidic material like pine fines or oak leaves
  • Fertilize in early spring and midsummer with a balanced fertilizer
  • Monitor for pests like aphids and diseases like powdery mildew

With proper care focused on acidic soil, you can enjoy the beautiful blue blooms of hydrangea in your garden for years to come.

Frequently Asked Questions

Why are my hydrangea flowers turning purple?

A purple hydrangea color is often an indicator that the soil pH falls within the transitional range around 6.0-6.5. This pH level results in the aluminum ions reacting with anthocyanins to produce reddish-purple or lavender blooms.

Can you change hydrangea flower color?

Yes, for most hydrangea varieties it is possible to change their bloom colors over time by adjusting the pH of the soil. Lower pH levels lead to blue flowers, while higher pH causes pink blooms.

How often should you test hydrangea soil?

Test hydrangea soil pH yearly to determine if any adjustments in acidity need to be made. Checking pH annually allows you to modify the soil to maintain the ideal acidic range for blue blooms.

Should hydrangeas be pruned?

Light pruning is beneficial for hydrangeas. In late winter, remove only the dead or damaged growth from the previous season. Avoid heavy pruning, as it can remove next season’s flower buds.


For vibrant blue hydrangea blooms, a soil pH between 4.5-5.5 provides the right conditions. This acidic environment supplies the aluminum ions that react with anthocyanin pigments to turn the flowers blue. Test soil yearly and incorporate acidic materials as needed to lower pH into the target range. With proper care focused on soil acidity, colorful blue hydrangea flowers can be achieved.