One of the most interesting quirks of some hydrangeas is the ability of the flowers to change colour. This is not true of all hydrangeas, but most often seen in the mophead and lacecap cultivars of Hydrangea macrophylla. Colour changes can also be seen in flowers of others such as H. involucrata and H. serrata
This colour change is due to the soil pH which affects aluminium availability. Those with blue or pink flowers tend to be blue in acid soil conditions (high available aluminium levels), mauve in acid to neutral soil conditions, and pink in alkaline conditions. To get the best flower colour, choose cultivars that give the best colours for the pH in question.
White flowers, and also green-flowered cultivars, remain white or green regardless of soil pH.
Try to use rainwater to water hydrangeas, since mains hard water can affect the flower colour, turning blue flowers mauve or pink.
Cultivars with blue flowers can be kept blue by growing the plants in acidic soil (pH 4.5-5), or by acidifying soils. Where keeping hydrangeas are not reliably blue use 'hydrangea blueing compounds' according to the manufacturer’s instructions. These compounds contain aluminium sulphate and are available from most garden centres. If the soil is very alkaline, or if there is any obvious chalk in the soil, this treatment will not work, but can be very effective for container-grown plants.
If you wish to enhance red or pink flowers, apply a dressing of ground limestone or chalk at a rate of 75-100g per sq m (2-3oz sq yd) in winter.
Colour changes in late summer
The flower colour apparent on many hydrangeas often changes by late summer (August-September). Blue and pink flowers fade to become combinations of green and pink/red. White flowers typically become a more vibrant pink. Indeed, Hydrangea paniculata Vanille Fraise is noted for its change from white to a strawberry-pink.
This change is considered a normal part of the ageing process and can't be halted or reversed for the individual blooms. However, new flowers will return to being the original colour for the plant.