How to Change Hydrangea Colors: The Complete Guide

The Science Behind Hydrangea Colors

Not all hydrangeas have the chameleon-like ability to change colors. The color-changing magic primarily belongs to the bigleaf hydrangeas (Hydrangea macrophylla) and mountain hydrangeas (Hydrangea serrata). These varieties can shift from pink to blue or purple depending on specific soil conditions.

The key factor in this color transformation is aluminum availability in the soil and the plant’s ability to absorb it. When aluminum is present in the soil and accessible to the plant, the flowers turn blue. When aluminum is absent or inaccessible, the flowers remain pink.

The soil’s pH level determines whether aluminum is available to the plant:

• In acidic soils (pH below 5.5), aluminum becomes soluble and available for the plant to absorb, resulting in blue flowers.
• In alkaline soils (pH above 6.5), aluminum becomes insoluble and unavailable to the plant, resulting in pink flowers.
• In soil with a pH between 5.5 and 6.5, hydrangeas may produce purple blooms or a mix of pink and blue flowers on the same plant.

Which Hydrangeas Change Color?

Before attempting to alter your hydrangea’s color, it’s essential to know which varieties can actually change and which will maintain their color regardless of soil conditions.

Hydrangeas That Change Color Based on Soil Chemistry

These varieties can shift from pink to blue depending on soil pH and aluminum availability:

• Bigleaf Hydrangeas (Hydrangea macrophylla) – Both mophead and lacecap types within this species can change color.
• Mountain Hydrangeas (Hydrangea serrata) – These more delicate relatives of bigleaf hydrangeas also change color based on soil conditions.

Hydrangeas That Change Color Based on Age

Some hydrangeas change color as their blooms mature, regardless of soil conditions:

• Panicle Hydrangeas (Hydrangea paniculata) – Many varieties begin white or green and gradually develop pink or reddish tones as they age in late summer and fall.
• Oakleaf Hydrangeas (Hydrangea quercifolia) – Some varieties of oakleaf, like Gatsby Pink, transform from white to pink as the season progresses.

Hydrangeas That Don’t Change Color

These varieties maintain their color regardless of soil pH:

• White Hydrangeas – White-flowering varieties of bigleaf hydrangeas stay white regardless of soil pH.
• Smooth Hydrangeas (Hydrangea arborescens) – Popular varieties like ‘Annabelle’ remain white regardless of soil conditions.
• Some Panicle Hydrangeas – Varieties that stay white until frost, when they turn brown.

Testing Your Soil

Before making any adjustments to change your hydrangea’s color, it’s crucial to know your starting point by testing your soil’s pH level. There are several ways to do this:

Professional Soil Testing

For the most accurate results, send a soil sample to your local cooperative extension service. This comprehensive test will provide detailed information about your soil’s pH, nutrient levels, and recommendations for amendments.

Home Testing Kits

Inexpensive soil testing kits are available at garden centers and online. While not as precise as professional testing, these kits can give you a general idea of your soil’s pH level.

DIY Vinegar and Baking Soda Test

For a quick, rough estimate:

1. Collect two soil samples from your garden
2. Add vinegar to one sample – if it fizzes, your soil is alkaline
3. Add baking soda and water to the other sample – if it fizzes, your soil is acidic
4. No reaction to either test suggests a neutral pH

How to Turn Hydrangeas Blue

If your soil test reveals alkaline conditions (pH above 6.5) and you’re dreaming of blue hydrangeas, you’ll need to increase soil acidity and ensure aluminum is available to your plants. This process isn’t immediate—it may take several months to see results, and you’ll need to maintain these conditions over time.

Step 1: Lower Soil pH

To achieve blue hydrangea blooms, aim for a soil pH between 5.0 and 5.5. Here are effective ways to increase soil acidity:

• Aluminum Sulfate: This provides an immediate source of aluminum and lowers pH. Mix 1 tablespoon with a gallon of water and apply to the soil around your hydrangeas.
• Garden Sulfur: A more gradual approach that lowers pH over time. Follow package instructions for application rates.
• Coffee Grounds: Used coffee grounds worked into the soil can gradually increase acidity.
• Pine Needles or Oak Leaf Mulch: These acidic mulches slowly break down and help maintain lower pH levels.
• Acidic Fertilizers: Products labeled for acid-loving plants like azaleas or rhododendrons work well for blue hydrangeas.

Step 2: Add Aluminum

Even with acidic soil, you may need to supplement aluminum:

• Dissolve 1 tablespoon of aluminum sulfate in a gallon of water
• Apply this solution to the soil around your hydrangeas every 4-6 weeks during the growing season
• Always water thoroughly after application to prevent root burn

Step 3: Maintain Blue Color

Blue hydrangea color requires ongoing maintenance:

• Reapply acidifying amendments in spring and fall
• Use rainwater for irrigation when possible (tap water can be alkaline)
• Avoid applying lime or wood ash near hydrangeas
• Continue using acidic mulch like pine needles

How to Turn Hydrangeas Pink

If your soil is naturally acidic but you prefer pink hydrangeas, you’ll need to raise the pH to make aluminum less available to the plant. This process typically takes longer than turning hydrangeas blue, so patience is key.

Step 1: Raise Soil pH

For pink hydrangeas, aim for a soil pH of 6.5 to 7.0. Here’s how to increase alkalinity:

• Garden Lime: Apply dolomitic lime according to package instructions, typically in fall for results the following season.
• Wood Ash: Sprinkle a thin layer of wood ash around your hydrangeas to gradually raise pH.
• Crushed Eggshells: These decompose slowly and add calcium while raising pH.

Step 2: Minimize Aluminum

To achieve vibrant pink blooms:

• Use a phosphorus-rich fertilizer (phosphorus helps prevent aluminum uptake)
• Avoid using acidic mulches like pine needles
• Consider planting in a container with soilless potting mix for better control

Step 3: Maintain Pink Color

To keep your hydrangeas pink year after year:

• Reapply lime every 1-2 years as needed
• Use tap water for irrigation (many municipal water supplies are slightly alkaline)
• Apply a fertilizer with a higher phosphorus content (middle number)

Container-Grown Hydrangeas

Growing hydrangeas in containers offers greater control over soil conditions, making it easier to achieve and maintain your desired flower color.

For Blue Hydrangeas in Containers

• Use an acidic potting mix formulated for acid-loving plants
• Incorporate pine bark or peat moss into your potting mix
• Fertilize with acid-forming plant food
• Water with rainwater or distilled water when possible

For Pink Hydrangeas in Containers

• Choose a potting mix with little to no aluminum
• Add a small amount of garden lime to your potting mix
• Use tap water for irrigation
• Fertilize with a phosphorus-rich, low-aluminum fertilizer

Special Cases and Color Variations

Purple Hydrangeas

Purple hydrangea blooms typically occur when soil pH is between 5.5 and 6.5, creating a partial availability of aluminum. This middle ground can produce striking lavender to purple flowers. If you’re aiming for purple blooms, test your soil regularly to maintain this precise pH range.

White Hydrangeas

White-flowering hydrangea varieties, such as ‘Annabelle’ or ‘Limelight,’ remain white regardless of soil pH. Their color is genetically determined and won’t change with soil amendments. Some white varieties may develop green, pink, or burgundy tints as blooms age, but this is due to the natural aging process rather than soil chemistry.

Reblooming Hydrangeas

Modern reblooming hydrangea varieties like the Endless Summer® series can produce multiple rounds of blooms throughout the season. When changing the color of these hydrangeas, be consistent with your soil amendments to ensure all bloom cycles display your desired color.

Troubleshooting Common Issues

Color Not Changing

If you’ve applied soil amendments but haven’t seen a color change, consider these factors:

• It can take several months or even a full growing season to see results
• Some cultivars are more resistant to color change than others
• Soil amendments may not be reaching the root zone
• Your soil may have a strong buffering capacity that resists pH changes

Mixed Colors on One Plant

If your hydrangea displays both pink and blue flowers, this indicates uneven soil conditions or a pH level in the transitional range. For consistent color, continue applying amendments evenly around the plant and be patient as the soil equilibrates.

Flowers Turning Brown or Green

Hydrangea blooms can turn brown due to drought stress or excessive heat. Green tints in aging flowers are normal for many varieties. Ensure consistent watering and provide afternoon shade in hot climates to maintain vibrant colors longer.

Frequently Asked Questions (FAQs)

Q: How quickly will my hydrangeas change color?

A: Color changes usually don’t happen immediately. Typically, you’ll need to apply soil amendments for 2-3 months before seeing results, and sometimes it may take until the next blooming season to see the full effect.

Q: Can all hydrangeas change color?

A: No, only certain varieties have this ability. Primarily bigleaf hydrangeas (H. macrophylla) and mountain hydrangeas (H. serrata) change from pink to blue based on soil chemistry. Panicle and some oakleaf hydrangeas change from white to pink/red as the blooms age, but this is not influenced by soil pH.

Q: Will coffee grounds turn my hydrangeas blue?

A: Coffee grounds can help increase soil acidity over time, which promotes blue color, but they alone are usually not sufficient to create a dramatic color change. They work best as part of a comprehensive approach to lowering soil pH.

Q: Is it possible to have both blue and pink hydrangeas in the same garden?

A: Yes! You can plant different hydrangeas in separate areas with different soil treatments, or grow some in containers with controlled soil conditions while others are in the ground.

Q: Are there any risks to adding aluminum sulfate to my soil?

A: Yes, excessive aluminum can be toxic to plants. Always follow package directions, apply aluminum sulfate sparingly, and water thoroughly after application to prevent root burn. Consider using organic methods like pine needle mulch as a gentler alternative.

Final Thoughts

Changing your hydrangea’s color is a fascinating garden project that demonstrates the dynamic relationship between plants and soil chemistry. Whether you prefer the cool tranquility of blue blooms or the warm charm of pink flowers, with patience and consistent soil management, you can influence your hydrangeas to display your color of choice.

Remember that these color changes require ongoing maintenance, as the natural conditions of your soil will eventually reassert themselves. Think of it as an ongoing conversation with your garden, where your hydrangeas provide beautiful feedback about the invisible chemistry happening beneath the soil surface.

Whichever color you choose, enjoy the journey of working with these remarkable plants that offer gardeners the rare opportunity to paint with living flowers.

Author

medha deb

 

Medha Deb is an editor with a master's degree in Applied Linguistics from the University of Hyderabad. She believes that her qualification has helped her develop a deep understanding of language and its application in various contexts.

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