Why your blue hydrangea turns pink — and what actually fixes it

The color of bigleaf hydrangea (Hydrangea macrophylla) is not a genetic constant. It is a direct indicator of the soil chemistry in the root zone. Gardeners who buy a sky-blue plant at the nursery often watch it bloom an ordinary pink the following year. The variety has not changed — the soil has. The chemistry: aluminum plus acidity Blue sepals require free aluminum ions that bind with anthocyanin pigments. That only happens in acidic soil, roughly pH 4.5 to 5.5. When pH rises toward neutral or alkaline, aluminum converts to an insoluble form the roots cannot take up, and the flowers inevitably turn pink. Igor Lytkin, soil scientist: "If on one plant you see both pink and blue heads at the same time, that is a sure sign that acidity in the root zone is distributed in patches. To even out the color you need a uniform substrate composition around the entire crown perimeter."