When hydrangea flowers are planted in alkaline soil, they usually turn pink or pinkish-red. In alkaline soils with a pH greater than 7, aluminum ions become less accessible for plant uptake. As a result, hydrangea blossoms grown in alkaline soil may appear pink, or pinkish-red. The absence or restricted availability of aluminum ions causes the formation of various pigments in flower petals, resulting in these warmer colour tones.