How to Change the Colour of Hydrangea Flowers Using Aluminum Sulfate
Hydrangeas are one of the most beloved garden plants across the UK, known for their lush blooms and striking colours. While many gardeners enjoy their natural shades of pink, blue, purple and white, some seek to control or alter these hues – especially in soil-sensitive flowers like hydrangeas. One of the most reliable and widely used methods to influence hydrangea flower colour is by adjusting soil chemistry with aluminum sulfate.
In this comprehensive guide, we’ll explore:
- Why hydrangeas change colour
- The role of soil pH and aluminum availability
- How aluminum sulfate works
- Step‑by‑step application methods
- Safe handling practices
- When and how often to apply
- Real examples involving grams and litres
- Mistakes to avoid
- Practical gardening tips
- 10 FAQs at the end
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Understanding Hydrangea Flower Colours
Hydrangeas (Hydrangea macrophylla and related species) display an astonishing variety of colours, but their pigment expression is not entirely genetic. The colour is strongly influenced by soil conditions – especially pH (acidity vs alkalinity) and the availability of aluminium ions (Al3+).
Natural Variations
- Blue hydrangeas are typical where the soil is acidic and aluminium is available.
- Pink or red hydrangeas are more common in alkaline soils where aluminium is less available.
- Purple hues often occur in intermediate conditions.
White hydrangeas generally lack the pigments that respond to soil aluminium and rarely change colour.
The Science Behind Colour Change
Hydrangea petals contain anthocyanins, pigments that can appear blue, purple, or red depending on chemical interactions within the plant cells. Aluminium binds with anthocyanins in acidic conditions and stabilises a blue pigment complex. If aluminium is unavailable – for example, in alkaline soils – the pigments express more reddish or pinkish tones.
| Soil pH | Flower Colour | Description |
|---|---|---|
| < 5.0 | Deep Blue | Very acidic soil; high aluminium availability |
| 5.0 – 5.5 | Blue to Violet-Blue | Acidic soil; good aluminium uptake |
| 5.5 – 6.0 | Purple to Blue-Purple | Slightly acidic; moderate aluminium uptake |
| 6.0 – 6.5 | Purple to Pink-Purple | Near-neutral; aluminium availability drops |
| 6.5 – 7.0 | Pink | Mildly alkaline; aluminium unavailable |
| > 7.0 | Bright Pink to Red | Alkaline soil; no aluminium uptake |
Key Soil Factors
-
Soil pH
pH below 6.0 encourages aluminium solubility; above 7.0 suppresses it. -
Aluminium availability
Only absorbed in dissolved form in acidic soil. -
Soil structure, organic matter, and moisture
These all impact pH and aluminium movement.
What Is Aluminum Sulfate and How Does It Work?
Aluminum sulfate (Al2(SO4)3) is a white crystalline salt used to acidify soil. When mixed with water, it releases aluminium ions (Al3+) and sulfate ions (SO42−), which lower pH and increase aluminium uptake, turning hydrangeas blue (if the cultivar allows).
Note: Only coloured hydrangeas (not white) respond to aluminium-induced colour change.
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Step‑by‑Step Guide: How to Change Hydrangea Colour
Step 1 – Test Your Soil
Use a soil pH testing kit. Ideal pH for blue: 4.5–5.5. For pink: 6.5–7.5.
Step 2 – Calculate Dosage
| Soil pH | Aluminum Sulfate per Plant | Soil Depth Treated |
|---|---|---|
| 6.0–6.5 | 30–60 g | 15–25 cm |
| 6.5–7.0 | 60–90 g | 25–30 cm |
| >7.0 | 90–120 g | 30 cm |
Step 3 – Prepare and Apply
- Dissolve 40–80 g per 5 litres or 80–160 g per 10 litres of water.
- Pour slowly around dripline (not stem).
Step 4 – Water the Area
Water after application to help the solution soak in and protect roots.
When to Apply
- Early spring – best time for changes.
- After flowering – for maintenance.
- Reapply every 4–6 weeks if needed.
Safety Tips
- Wear gloves and goggles.
- Avoid contact with edible plants.
- Keep away from children and pets.
Common Mistakes
- Using too much product at once.
- Not checking soil pH regularly.
- Applying during drought without watering in.
- Expecting overnight results.
Example: Calculated Application
5 hydrangeas with soil pH 6.8 → apply 70 g per plant in 8 litres of water.
Total needed: 70 g x 5 = 350 g of aluminum sulfate.
Alternative Amendments
- Elemental sulfur – slower acting
- Acidic mulches – pine needles, bark
Conclusion
Aluminum sulfate is an effective way to control hydrangea colour by acidifying the soil and providing aluminium. Regular testing and careful application will produce striking blue blooms over time.
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FAQs: Changing Hydrangea Colours with Aluminum Sulfate
- Can all hydrangeas change colour? No, only certain varieties respond to aluminium.
- How often should I apply? Every 4–6 weeks in growing season, depending on pH.
- Is it harmful to the plant? Not if used correctly; overuse may damage roots.
- How long until I see colour change? Weeks to months, visible in new blooms.
- Can I use it near vegetables? Best kept to ornamental beds only.
- Does rain help or hinder? Light rain helps; heavy rain may wash it away.
- Should I test pH regularly? Yes, every few weeks if you’re adjusting soil.
- Is it safe for pets? When used properly and watered in, yes. Keep pets away during application.
- Can I mix with fertiliser? Apply separately for best results.
- What if my soil is very alkaline? Apply gradually and consider organic acidifiers too.
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