How do I make a Hydrangea turn from pink to blue?
Hydrangeas need two conditions to turn from pink to blue: they need an acid soil and aluminium or iron salts in the soil.
What is an acid soil?
In chemistry lessons at school, we all learnt that distilled water has a neutral pH of 7. In horticulture, however, a neutral pH is generally classed as 6.5. Any soil with a pH lower than 6.5 is acidic and any soil with a pH higher than 6.5 is alkaline. You can test your soil using a pH testing kit – we stock them at Greenshutters Nurseries & Garden Centre. I do not recommend using the cheap pH meters that are available as they are generally not accurate. A simpler method is often to look at gardens nearby and see what plants are growing locally. If you see plants like Rhododendrons, Camellias and Pieris growing happily in the soil near you, then chances are your soil is acidic.
Acid soils tend to be peaty or sandy soils. In general, clay soils are not very acidic. Chalky soils are alkaline. You can make your soil more acid by adding ericaceous compost or Irish moss peat. Ericaceous compost has a pH of about 5.0 to 5.5, so is acidic, whereas an ordinary multipurpose compost will have a pH of 6.0 to 6.5.
Mulching (putting a layer 5-10 cm thick on top of the soil) with Irish moss peat is particularly effective as Irish moss peat has a pH of about 4.0 – much too acidic for most plants to grow in, but when mixed with a neutral or alkaline soil, it reduces the pH of the soil. If you want to keep the soil around a plant acidic, then mulch with Irish moss peat once a year and let the worms mix the peat into the soil.
Alternatively, you can grow Hydrangeas in a pot. Hydrangeas will grow in either multipurpose or ericaceous compost but if you want them to turn blue, it is best to pot them in ericaceous compost.
How do I add Aluminium and Iron salts to the soil?
If your soil does not naturally have aluminium or iron salts in the soil, it can easily be added by mixing Hydrangea Colourant into the soil. People say that adding rusty nails to the soil or compost will turn Hydrangeas blue, I’ve never tried it so I couldn’t say whether it really works or not. Hydrangea Colourant, on the other hand, is a mixture of Iron salts and Aluminium Sulphate. It can be added to the soil on potting or planting or, for established plants, dissolve the Hydrangea Colourant in water and water your Hydrangeas with it every week during the growing season (March to September). If you do this, then your light pink varieties will turn blue and the dark pink varieties will turn mauve-blue.
However, if you want your hydrangea to stay pink, then grow it in a neutral or alkaline soil.