How to Make Hydrangeas Blue in Australia

Hydrangeas change colour depending on soil conditions. Blue blooms form in acidic soil, where aluminium is available for uptake by the plant.

Soil pH below 6: Encourages blue flowers.
Aluminium availability: Acidic soil frees aluminium, which turns petals blue.
Fertiliser choice: Low-phosphorus, potassium-rich fertilisers support blue tones.

In Australia, mophead and lacecap hydrangeas (Hydrangea macrophylla) are most responsive to colour changes. White varieties do not change colour.

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Steps to Turn Hydrangeas Blue

Follow these steps over one to two growing seasons for rich blue blooms:

Test Soil pH: Use a garden pH kit. If above 6.5, the soil needs acidifying.
Acidify Soil: Apply aluminium sulfate or sulphur to gradually lower pH.
Choose the Right Fertiliser: Use a low-phosphorus formula like CompleteGrow 4-18-38 + CaMg. High phosphorus blocks aluminium uptake.
Mulch & Water: Add pine needles or peat moss as mulch. Water with rainwater instead of alkaline tap water where possible.
Be Patient: Colour changes are gradual and may take one or two flowering seasons.

Container-grown hydrangeas change colour faster, as pH is easier to adjust in pots.

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Tips, Mistakes & FAQ

Tips for Deep Blue Blooms

Fertilise regularly with potassium-rich feed to enhance flower colour.
Check pH twice yearly—soil naturally drifts back toward neutral.
Avoid over-liming or alkaline fertilisers, which push blooms toward pink.
Combine good watering practices with feeding—dry soil slows nutrient uptake.

FAQ

Q: How long does it take to turn hydrangeas blue? A: Usually one full season, but strong shifts may take 1–2 years.
Q: Can I make white hydrangeas blue? A: No. Only coloured varieties (mophead and lacecap) respond to pH changes.
Q: What’s the best fertiliser for blue hydrangeas? A: Use CompleteGrow Advanced 4-18-38 + CaMg—low phosphorus and high potassium, ideal for enhancing blue tones.
Q: Do I need to repeat the treatment each year? A: Yes. Soil pH drifts over time, so reapply acidifiers and fertiliser each season.

Final Recommendation: To achieve vivid blue hydrangeas in Australia, maintain acidic soil, provide steady moisture, and fertilise with CompleteGrow 4-18-38 + CaMg. This ensures long-lasting, richly coloured blooms season after season.