Hydrangeas with pom-poms of blue or pink are a common sight during summers in the southern United States. Unlike most other flowers, these blooms can change colour, depending on the type of soil in which the hydrangea is planted. Hydrangea flowers turn blue in acid soil and pink in alkaline soil. In neutral soil, they are bluish-pink or mauve.
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Pink to Blue
If your blue hydrangea has turned pink, it's because your soil is alkaline or has a high pH. Adding aluminium sulphate to the soil will turn the flowers blue again. The aluminium sulphate changes the soil to acidic, or low pH. It is easier to change pink flowers blue than vice versa. That's because it's easier to add aluminium to the soil than to subtract it.
Use 14.2gr of aluminium sulphate per gallon of water. It should be applied several times throughout the growing season. Water the plant first, then slowly pour the aluminium sulphate solution onto the soil around the hydrangea. Adding the mixture too quickly can damage the roots.
For blue hydrangeas, a pH level of 5.2 to 5.5 is optimum. Adding organic matter such as grass clippings, coffee grounds or fruit and vegetable peels to the soil lowers the soil's pH. If the soil has a naturally low pH, it contains aluminium and acid and will produce blue or purple hydrangeas without being amended. Douglas Bailey, a floraculture expert at the University of Georgia, recommends using a fertiliser low in phosphorus and high in potassium for blue blooms.
Blue to Pink
If you would like to change your blue hydrangea blooms pink, add dolomite lime to the soil around the plant several times per year. This will raise the pH. A level of 6.0 to 6.2 is ideal for pink hydrangeas. Use a fertiliser with high levels of phosphorus to prevent aluminium from entering the plant's system again.
Dolomite lime and aluminium sulphate are available at garden centres. Gardeners who want to change the colour of their hydrangeas may find it easier to grow them in pots. It is easier to alter the pH of potting soil in containers than the pH of soil in your garden. White hydrangeas cannot be changed to pink or blue.
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