Hydrangeas are such a fascinating plant as they have the ability to change the colour of their blooms based on the chemistry of the soil. This means that you could buy a beautiful, blue hydrangea only to find the next year it blooms in your garden with lovely, pink flowers. This colour change is due to the soil pH: when grown in alkaline soil, the flowers are pinker; when grown in acidic soil, the flowers are bluer.
You can alter the colour of a hydrangea’s blooms by manipulating your soil’s pH level and aluminium content throughout the growing season. Start by testing the pH of your soil with an inexpensive kit which you can buy for this purpose.
To encourage blue hydrangea flowers, grow the plant in soil that has a pH of 5.2-5.5. If the pH of your soil is naturally higher and therefore, alkaline, it will be very difficult to get blue flowers. You can however, lower the pH of the soil by applying an acidic organic mulch, such as pine needles, pine bark, sawdust or oak leaves. Coffee grounds are also slightly acidic if your local coffee shop lets you collect them. Alternatively, you can buy a commercial soil acidifier at your local garden centre or nursery.
It is easier to manipulate the growing conditions for your plant if you grow it in a pot. Use ericaceous soil in the pot, a soil suited to other plants like rhododendrons and azaleas which also thrive in acidic conditions. Hydrangeas grow well in pots and containers but do bear in mind they need lots of regular watering.
Another tip is to use rainwater to water your hydrangeas, since water from a mains tap can affect the flower colour, turning blue flowers mauve or pink.
Remember though that even if you purchase a blue hydrangea in bloom, you cannot be sure the plant will produce the same colour of flowers once it is growing in your garden if your garden soil pH is too alkaline but at least for now, you have a few tricks to try to keep it blooming in blue!