Certain hydrangea plants can produce pink or blue blooms. The colour of Hydrangea macrophyllia depends on the specific cultivar and aluminium availability, according to the U.S. National Arboretum. Only H. macrophylla and Hydrangea serrata bloom in colours other than white and can change colour based on soil acidity, according to the University of Maryland Extension. Florist hydrangeas are hybrid plants and won't change colour.
Water the hydrangea daily for a week so that the plant is well hydrated.
Mix 1 tbsp of aluminium sulphate per gallon of water. Use up to 2 gallons per plant. Drench the soil in a 2-foot radius at the base of the hydrangea with the mixture beginning in February. Keep the aluminium sulphate mixture off the leaves to avoid damage to the hydrangea's foliage, Professor of Environmental Horticulture Gary W. Knox at the University of Florida website advises.
Repeat the process of drenching the soil under the hydrangea with the aluminium sulphate mixture in March and April.
Reapply the aluminium sulphate treatment every year to maintain your blue hydrangeas.
Hydrangea macrophylla is also known as a French hydrangea, bigleaf hydrangea or big leaf hydrangea. Hydrangea serrata comes from Japan and Korea. H. serrata is a version of Bigleaf Hydrangea with a finer texture, according to the Norfolk Botanical Gardens. The blooms are the flat, lacecap shape. Garden centres carry aluminium sulphate. Aluminium sulphate is also known as alum. Avoid increasing the amount of aluminium sulphate you use; too much aluminium can be toxic to other plants.
Don't use lime near your hydrangeas -- lime encourages hydrangeas to turn pink. Keep fertilisers out of reach of children.