To change the colour of hydrangea flowers, you primarily manipulate the soil pH, which directly affects the availability of aluminum to the plant. This determines whether the flowers will be blue, pink, or purple.
Understanding Hydrangea Colour Changes
The vibrant colours of Bigleaf Hydrangeas (Hydrangea macrophylla) and Oakleaf Hydrangeas (Hydrangea quercifolia) are largely determined by the soil's acidity or alkalinity, specifically the presence of aluminum.
Turning Hydrangeas Blue
For hydrangeas to produce blue flowers, the soil needs to be acidic (pH 5.0-5.5) and have sufficient aluminum available. Aluminum is naturally present in most soils, but its availability to the plant is enhanced in acidic conditions.
- How it Works: Both aluminum sulfate and elemental sulfur work by making aluminum available to your hydrangeas. This reaction acidifies the soil, and your plant produces blue flowers.
- Recommended Amendments:
- Aluminum Sulfate: This is a popular and fast-acting choice. It quickly lowers soil pH and provides readily available aluminum. It is usually the preferred choice for most gardeners due to its speed.
- Elemental Sulfur: A more budget-friendly option, elemental sulfur acidifies the soil more slowly. It requires microbial activity to convert it into a usable form, so results may take longer to appear.
- Application: Apply these amendments around the base of the plant, according to product instructions, typically in spring or autumn.
Changing Hydrangeas to Pink
To achieve pink flowers, the soil needs to be more alkaline (pH 6.0-7.0), which limits the plant's ability to absorb aluminum.
- Recommended Amendments:
- Dolomitic Lime: Adding dolomitic lime or garden lime increases the soil's pH, making it more alkaline. This binds the aluminum in the soil, preventing the plant from absorbing it and leading to pink blooms.
- Bone Meal: Some gardeners also use bone meal as a slow-release phosphorus source that can contribute to pinker blooms by slightly increasing pH.
- Application: Apply lime in late fall or early spring, ensuring it's well-mixed into the topsoil.
Achieving Purple Blooms
Purple hydrangeas often result from a soil pH that is somewhere in between the acidic conditions for blue and alkaline conditions for pink. A pH of approximately 5.5-6.0 tends to produce a mix of blue and pink pigments, resulting in purple or mauve shades.
Hydrangea Colour Chart
This table summarizes the relationship between soil pH and hydrangea flower colour for Bigleaf and Oakleaf varieties:
| Desired Flower Colour | Soil pH Range | Aluminum Availability | Common Amendment(s) (to achieve) |
|---|---|---|---|
| Blue | Acidic (5.0-5.5) | High | Aluminum Sulfate, Elemental Sulfur |
| Purple/Mauve | Slightly Acidic (5.5-6.0) | Moderate | Balanced approach, slight acidification |
| Pink | Alkaline (6.0-7.0) | Low | Dolomitic Lime, Garden Lime |
What About White and Green Hydrangeas?
It's important to note that not all hydrangeas change colour with soil pH. White hydrangeas (e.g., Hydrangea paniculata varieties like 'Limelight' or 'Annabelle') do not contain the pigments that react to aluminum, so their flower colour remains white, though some may fade to a blush pink or green as they age. Similarly, green hydrangeas are typically bred to stay green.
Practical Tips for Colour Change
- Test Your Soil: Before applying any amendments, it's crucial to perform a soil pH test. This will tell you your current pH level and help you determine how much and what type of amendment is needed.
- Patience is Key: Changing soil pH takes time, especially with slow-acting amendments like elemental sulfur. Visible colour changes may not appear until the next blooming season.
- Consistent Application: Maintaining the desired pH often requires ongoing applications, as natural soil conditions and watering can alter pH over time.
- Water Quality: If your water supply is very hard (alkaline), it can counteract your efforts to acidify the soil for blue flowers. Using rainwater or distilled water can help maintain the desired pH.
- Container Plants: Changing the colour of hydrangeas in containers is often easier than in garden beds because you have more control over the soil medium.
By understanding the role of soil pH and aluminum, you can effectively manage and change the beautiful colours of your hydrangea flowers.
