The hydrangea (Hydrangeaceae) is a deciduous shrub producing large, showy blossoms in pink, blue or white. While enjoying a shady site in well-drained soil, the bloom is more prolific in full sun as long as ample moisture is available. This tall shrub, some varieties to 25 feet, has the unique characteristic of sometimes being purchased with pink flowers and, after a year in the soil, changing to display blue blossoms.
The make-up of the soil is the most important factor in what keeps the hydrangea blue, or changes it from pink to blue. For the bloom to be blue, there must be aluminium in the soil. In addition to the presence of aluminium, the pH of the soil must be acidic (5.5 or lower) so the plant can absorb the aluminium. If these conditions are at the planting site, the hydrangea naturally turns blue.
Aluminium sulphate is available at gardening centres and can be added at a rate of 1 tbsp to one gallon of water every time the hydrangea is watered. Wait until the shrub is 2 to 3 years old, and always water it well before the application so the roots will not be burnt by the aluminium sulphate.
Creating an Acid Soil
Fertilise using a formula of 25-5-30, which is low in phosphorus and high in potassium. Any rhododendron fertiliser is a good choice. Soil amendments and mulch from materials such as pine needles, peat moss and shredded pine bark will aid the soil toward becoming more acid or remaining so.
Test the Water
Have the water being used to irrigate the hydrangea tested. It should be low in pH, no higher than 5.6, to keep the colour of the hydrangea blue. Flowers on a hydrangea growing in a pH range of between 5.5 and 6.5 can be purple, or a combination of pinks, blues and purples.
Grow in Containers
If the natural soil is highly alkaline with no aluminium present, it will be difficult to keep the hydrangea blue. In this case, grow the shrub in a large container with a potting soil sold for acid-loving shrubs, such as azaleas and rhododendrons. Dilute the fertiliser further as it concentrates more in the contained medium.
No Blue Allowed!
Some pink cultivars will not turn blue no matter the soil composition. They will turn a dull, dirty pink rather than clear, vibrant rose or pink if planted in acid soil with aluminium. Some cultivars that will not turn blue are Kardinal, Pia and Alpengluhen. To keep them their true shades of pink, add lime to the soil. White hydrangeas also refuse to turn blue.