The versatile hydrangea comes in colours and varieties to fit almost any garden. Many varieties change colour during the long season of bloom. One type will even change colour from pink to blue when it finds aluminium in the soil.
One variety of hydrangea, the mophead, or Hydrangea macrophylla, contains cultivars with pigments containing delphinidin 3-monoglucoside. Not all mopheads contain this pigment but the ones that do can change colour according to soil acidity or pH and available aluminium.
Soil pH must be amended with peat or organic sulphur to lower soil pH to between 5.0 and 5.5 before planting the hydrangea. Thereafter, apply aluminium sulphate monthly by irrigation or broadcasting. The University of Minnesota Extension recommends a rate of two cups of aluminium sulphate to five gallons of water for watering the shrub until the soil around it is just saturated.
Applying aluminium sulphate beginning in early spring should result in blue blooms, according to the University of Massachusetts. Aluminium sulphate acidifies soil and provides nitrogen for hydrangeas but too much can be toxic. Plant non-acid-loving plants away from hydrangeas to avoid damaging them.