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. You can start by testing the pH of your soil with a cheap 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.
Another tip is to try 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’s growing in your garden if your soil pH is too alkaline but at least for now, you have a few tricks to try to keep it blooming in blue!