How do I keep my garden hydrangea blue?
To maintain the beautiful blue colour every year, sprinkle aluminium sulphate on the soil during the first half of September. This product is available at most garden centres. Make sure that the sulphate does not come into contact with the leaves or flowers. Is your blue garden hydrangea not staying blue, even after adding aluminium sulphate? Then use slightly acidic (possibly sandy) peat soil with a pH of 4 to 4.5. Use rainwater when watering if possible: tap water contains a lot of lime which makes the flowers pink.