What is the Best Fertiliser for Hydrangeas?

Hydrangeas demand consistent feeding for healthy leaves, strong stems, and abundant flowers. The best fertiliser depends on the season and whether you want to enhance blue, pink, or purple blooms.

Core Nutrients

Nitrogen (N): Drives leafy growth in spring. Too much creates leaves but fewer flowers.
Phosphorus (P): Strengthens roots and supports bud initiation. High phosphorus reduces aluminium uptake, helping pink flowers.
Potassium (K): Vital for large, colourful, long-lasting blooms. Deficiency leads to pale or weak flowers.
Calcium & Magnesium: Prevent yellowing leaves and strengthen stems.
Micronutrients (Iron, Boron, Zinc): Correct minor deficiencies and support consistent colour.

How Fertiliser Affects Colour

Blue Hydrangeas: Low-phosphorus, potassium-rich fertilisers (e.g. CompleteGrow 4-18-38 + CaMg) allow aluminium uptake in acidic soil, deepening blue tones.
Pink Hydrangeas: Balanced fertilisers with moderate to high phosphorus (e.g. CompleteGrow NPK 20-20-20) reduce aluminium uptake, keeping blooms pink.
Purple Hydrangeas: Occur when pH is neutral (6–6.5). Balanced fertilisers maintain colour intensity while soil chemistry controls the mix of blue and pink pigments.

Fertiliser itself does not directly “change” flower colour. Instead, it supports the plant’s health so soil pH adjustments show more strongly. This means fertiliser choice and soil management must work together.

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Best Fertiliser Schedule for Hydrangeas in Australia

Feeding hydrangeas regularly across the year keeps them blooming reliably. Here’s a seasonal program tailored to Australian conditions:

Spring (September–October): Use CompleteGrow NPK 20-20-20 every 4–6 weeks. This balanced formula builds leafy growth and strong stems.
Early Summer (November–December): Switch to CompleteGrow Advanced 4-18-38 + CaMg. The low phosphorus allows aluminium uptake for blue flowers, while high potassium enhances all bloom colours.
Mid–Late Summer (January–February): Reapply potassium-rich fertiliser to extend flowering, especially in warmer states.
Autumn (March): Apply a light dose of NPK 20-20-20 to strengthen roots before dormancy. Avoid heavy nitrogen late in the season.

For potted hydrangeas, fertilise every 2–3 weeks with a diluted liquid formula, as nutrients wash out quickly with watering. Always water before fertilising to protect roots.

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Fertiliser Tips, Mistakes & FAQ

Mistakes to Avoid

Overusing nitrogen-heavy fertilisers—this produces leaves at the expense of flowers.
Feeding late in autumn—encourages soft new shoots that frost can damage.
Skipping potassium—lowers flower quality and colour intensity.

FAQ

Q: What is the best all-round fertiliser for hydrangeas? A: CompleteGrow NPK 20-20-20 is ideal for balanced growth in spring and early development.
Q: Which fertiliser should I use for blue hydrangeas? A: Use CompleteGrow Advanced 4-18-38 + CaMg with acidic soil (pH below 6) to intensify blue colour.
Q: Which fertiliser helps keep hydrangeas pink? A: CompleteGrow NPK 20-20-20, combined with alkaline soil (pH above 7), helps maintain pink blooms.
Q: Can fertiliser make white hydrangeas change colour? A: No—white varieties remain white, though fertiliser improves bloom size and health.
Q: How often should hydrangeas be fertilised? A: Every 4–6 weeks in the ground, or every 2–3 weeks in pots with diluted liquid feed.

Final Recommendation: For the best results in Australia, use CompleteGrow NPK 20-20-20 during spring growth, then CompleteGrow 4-18-38 + CaMg in summer to intensify flower colour and extend blooming. This seasonal combination ensures healthy plants and vibrant blue, pink, or purple hydrangeas year after year.