Do multi-coloured hydrangeas have to be grown in special compost?
Paul Albert Cleeter, by email
Stefan says: You know the colour of many of the mop-head types of hydrangea is influenced by the pH (acidity or alkalinity) of the soil in which they’re growing. In an alkaline soil, they’ll be red; in an acidic soil, blue; and on a truly acid soil, some varieties can be an almost electric blue. This blue can be intensified by using commercial ‘bluing powder’ containing aluminium.
You have multi-coloured varieties, however, on which there is a range of colour from red through purple to blue – although the extremes (the red and the blue) are never as intense as those on the ‘pure’ single-coloured kinds.
For you to achieve the optimum results, I suggest that you aim to have a soil midway between acidic and alkaline: a pH of around 7, therefore.