Although the colour range depends on the individual variety of plant, blue shades are usually produced on soils with a greater acidity. The content of aluminium in the soil is directly responsible for these oceanic hues, and is found in increasing quantities as a soil becomes more acidic. Crimson flowers are usually produced on neutral or alkaline soils but it is not unusual to find a range of blue and red flowers on the one plant; we will let you wonder at the reasons for the multi-coloured flower heads.
Blue flowers are only possible on acid soil, on varieties that can carry blue flowers. In neutral soil, they take up less aluminium and the flowers are pink.
All young blue-flowered plants carry pink flowers, and it may take between one and three years for them to turn blue. If the plant is moved, even in the same garden, the flowers will be pink the first year and then turn blue again with time.
To help achieve a blue colour on a limey soil, special blueing compounds composed of aluminium sulphate can be used. (See below). However, the hydrangeas will be unlikely to capture the azure blue of plants grown in naturally acidic soils.
Another tactic is to grow a compact variety such as ´ Blue Bird ´ in a large container filled with lime-free compost and supplement its liquid feeding with a blueing compound.
On white flowers, only the eye colour of the male flowers will change. Plants grown in soil with a pH level higher than 7 (alkaline) can also become chlorotic because of a lack of iron and must be treated accordingly.
It is still not possible to predict the colour of hydrangeas even if we know the soil type but we try our best.
A deep blue variety in an acid soil will turn pink if moved to a limier spot and vice versa, in a similar effect to litmus paper. Many also change flower colour as they mature so identifying varieties with any certainty can be confusing.