Hydrangea flowers are made up of sepals, which are petal-like structures; they are typically 2 to less than 5.5 cm in diameter. Hydrangea sepals can change color depending on the soil’s pH and aluminum content. If Hydrangea is planted in alkaline soil, the sepals will be pink; in acidic soil, the sepals will be blue. Red or pink blooms result from neutral or basic soil (pH 7 and above), whereas blue blooms indicate acidic conditions (pH less than 7).