Blue or Pink flowering Hydrangea varieties have the ability to change their flower colour depending on soil conditions. Hydrangeas that are planted in Acidic soils, (less than 7 pH) will develop blue flowers. Hydrangeas planted in Alkaline soils (higher than 7 pH) will develop pink to red flowers.
Technically speaking, it is not the soil's pH alone that changes the flower colour. Rather, it is the presence of Aluminum and the plants ability to absorb it in different soil conditions. Aluminum is more available in acidic soils. When the soil is alkaline, it is harder for the plant to access the aluminum that is present. If your soil does not have aluminum or it is sparse, the effect may be minimal. Annual applications of Aluminum Sulfate in spring can be used to keep your Hydrangeas blue. Inversely, applications of lime can change your flower colour to pink.
Note that this is slow process, it does not happen instantly and it does not happen with white coloured hydrangeas.
If you have any other questions about Hydrangeas, call us at 604.882.1201 or visit Arts Nursery in person.