Hydrangea Flower Color: The Chemistry Behind Blue and Pink Blooms

We know life depends on chemistry—we may not understand it, but life goes on.

For hard-core understanding of why hydrangeas are blue or pink:

Maintaining Blue in Blue Hydrangeas

Carl Rosen, Extension Soil Scientist, University of Minnesota

Popularity of hydrangeas in Minnesota increased with winter-hardy zone 4 cultivar 'Endless Summer'.

Like most hydrangeas, flower color blue in acid soil, converts to pink in slightly acid to alkaline soil. During conversion, flowers show various blue/pink shades.

Blue/pink color due to anthocyanin delphinidin 3-monoglucoside. In presence of aluminum, blue forms from aluminum binding with anthocyanin. Blue under acid conditions due to increased aluminum availability/uptake.

As soil pH decreases, aluminum availability increases. Conversely, as pH increases, aluminum availability decreases. Ideal pH ~5.0 for adequate aluminum for blue flowers.

Aluminum not essential for plant growth; high levels toxic to most plants (stunted roots, death). Usually soil pH <4.5 for toxicity.

Eastern U.S. gardeners struggle maintaining pink flowers—native soils acid with sufficient aluminum for blue. Upper Midwest soils more alkaline—difficult to maintain blue.

Minnesota soil pH highest in west (calcareous), more acid east. Western gardeners harder time maintaining blue hydrangeas. Highly calcareous soils (pH 7.8+) may cause yellow leaves from iron deficiency; raised beds with acid soil mix may be only solution.

Potted hydrangeas grown in artificial media. For blue, media kept acid with sphagnum peat moss and acidifying amendments. When transplanted to high pH soils, root zone pH eventually increases, flowers turn pink unless acid conditions maintained.