Garden Q&A: Can I change the colour of my hydrangeas by altering the soil?

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

Blue mophead hydrangea in bloom in a Kerry reader's garden this summer.

Question

Can I change the colour of my hydrangeas by adding something to the soil?

Answer

Yes, hydrangeas are pH-sensitive, so in some cases you can change their colour by altering the soil. 

This mainly applies to mophead and lacecap Hydrangea macrophylla types. 

In acidic soil, where aluminium is more available, the flowers tend to be blue. In more alkaline soil, they usually turn pink. 

You can encourage blue flowers by using a hydrangea colourant or aluminium sulphate, and by keeping the soil acidic with ericaceous compost or sulphur-based products. 

To encourage pink, garden lime can raise the pH.

However, not all hydrangeas behave this way. Hydrangea paniculata types, such as ‘Limelight’, and Hydrangea arborescens types, such as ‘Annabelle’, are not changed from pink to blue by soil pH. 

Their colour changes are part of their natural ageing process, not something you can control with soil additives.

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