What makes the black hydrangea black?

Answer by  hydrangeamom (13)

The color of the hydrangea is determined by the pH in the soil that it grows in. The deepest, darkest blue bloom is produced by growing in a strong acid soil and softer pastel color hydrangea blooms are grown in neutral to alkaline soils.

Answer by  InquiringAli (89)

The stems are black if its a Nigra variation of the hydrangea then its the partial shade and just the color that they naturally are. It should be noted though that if the flower has black leaves or black spotted leaves then it's an actual fungal infection that is commonly seen in these flowers.