Hydrangeas turn blue mainly because of soil chemistry : the plant must be able to absorb aluminum from the soil, and that happens best in acidic conditions. Blue color is usually seen in certain hydrangea types, especially bigleaf and lacecap varieties, not all hydrangeas.

What actually causes blue

For the flower to look blue, three things generally need to line up: the soil has aluminum, the aluminum is available to the roots, and the hydrangea variety can change color. In acidic soil, aluminum stays more available to the plant, which pushes blooms toward blue; in more alkaline soil, the same flowers tend to stay pink.

What helps

Gardeners who want blue blooms often lower soil pH and add aluminum sulfate. Some sources also mention keeping phosphorus low, since high phosphorus can reduce aluminum uptake.

What to know

White hydrangeas usually won’t turn blue, and already-open flowers usually won’t change color much; the treatment works best on developing buds over time. Even in the right soil, some varieties only shift to lavender or pale blue rather than a deep blue.

Quick scoop

If you want blue hydrangeas, think: acidic soil + available aluminum + the right variety.

Information gathered from public forums or data available on the internet and portrayed here.