How to Change Hydrangea Colours in Australia

One of the most fascinating features of hydrangeas is their ability to change flower colour based on soil conditions. Unlike most plants, hydrangea blooms are directly influenced by soil pH and nutrient balance.

Blue Flowers: Develop in acidic soils (pH below 6) where aluminium is available to the plant.
Pink Flowers: Form in neutral to alkaline soils (pH 7 and above), where aluminium is locked away.
Purple Flowers: Occur when soil pH is between 6 and 7, creating a mix of both pigments.

White hydrangeas remain white regardless of soil pH, though they may show slight colour tinges in extreme conditions. In Australian gardens, colour changes are most noticeable in mophead and lacecap varieties (Hydrangea macrophylla).

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Changing Hydrangea Colours

Colour change is a gradual process and may take one or two growing seasons. Use these steps to achieve the shade you want:

How to Make Hydrangeas Blue

Lower soil pH to below 6 using soil acidifiers or aluminium sulfate.
Apply a potassium-rich fertiliser with low phosphorus, such as CompleteGrow 4-18-38 + CaMg, which helps intensify blue shades.
Mulch with pine needles or peat moss to gradually acidify soil.
Water with rainwater if possible—tap water in some regions contains lime, which raises pH.

How to Make Hydrangeas Pink

Raise soil pH to above 7 using garden lime or dolomite.
Choose a balanced fertiliser with higher phosphorus, such as CompleteGrow NPK 20-20-20, which reduces aluminium uptake.
Mulch with composted wood chips or straw to maintain alkalinity.
Avoid acidifying fertilisers, as they encourage blue pigments.

To achieve purple blooms, maintain soil pH between 6.0–6.5 and alternate between neutral and acidic feeding practices.

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Colour Care Tips & FAQ

Tips for Success

Test your soil pH before making changes. Over-adjusting can harm roots.
Fertiliser type matters—high phosphorus prevents blue, while low phosphorus enhances it.
Changes happen slowly. Expect gradual shifts over one or two blooming seasons.
Container hydrangeas are easier to control since soil can be adjusted quickly.

FAQ

Q: How long does it take to change hydrangea colour? A: Usually 6–12 months, though some changes show within a single season.
Q: Will fertiliser alone change hydrangea colours? A: No. Fertiliser supports colour change, but soil pH is the main factor.
Q: Can all hydrangeas change colour? A: No. Only macrophylla (mophead & lacecap) varieties respond strongly. White hydrangeas stay white.
Q: What’s the best fertiliser for blue hydrangeas? A: Use a low-phosphorus, high-potassium feed such as CompleteGrow Advanced 4-18-38 + CaMg.

Final Recommendation: To control hydrangea colour in Australia, adjust soil pH carefully and support the process with CompleteGrow fertilisers. Use NPK 20-20-20 for pink shades or 4-18-38 + CaMg for deep blues and purples.