Garden Q&A: Why have my hydrangeas turned from blue to pink?

Whether you're a gardening beginner or expert, Irish Examiner columnist Peter Dowdall has the answer to your questions

Hydrangeas can shift from blue to pink. File picture

Question

Why aren’t my hydrangeas blue any more? I bought them in flower when they were a lovely blue colour, but now they are a dirty pink.

Answer 

I love your expression “dirty pink” and I understand what you are describing. The reason for this is all down to soil chemistry. 

The vivid blue colour in Hydrangea macrophylla relies on aluminium being available to the plant, which only happens in acidic soils with a pH below about seven. If your soil is more alkaline, the aluminium gets locked up and the flowers shift to pink. 

It can also happen over time if you add lime or your soil naturally becomes more alkaline. To restore blue tones, try feeding with aluminium sulphate and mulching with ericaceous compost. Using rainwater, which is often more acidic than tap water in many areas, can also be beneficial.

In very limey ground, you’re often better growing your hydrangea in a pot of ericaceous compost where you can control the pH more easily.

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