Hydrangeas are hearty plants that produce large, round clusters of blooms. The colors of hydrangeas can include white, violet, pink, or blue, depending on the variety. Some varieties of pink hydrangeas can actually turn either pink or blue, depending on soil acidity. Adding aluminum sulfate to the soil can alter the flowers' color. However, this substance can be deadly to the flowers. According to "Hydrangeas for American Gardens," you should give these plants aluminum sulfate treatments in September to decrease the risk of damaging them.
Wash and dry a 1-gal. jug. Add 2.4 oz. of powdered aluminum sulfate to the container. Powdered aluminum sulfate should be available at gardening centers and in the gardening section of your local hardware store.
Fill the container with water and shake or stir to combine the mixture.
Pour the entire jug of aluminum sulfate solution around the base of the plant, completely drenching the soil. Be careful not to splash any up onto the leaves or blooms of the hydrangea shrub because direct contact with the solution can kill it.
Repeat the treatment two weeks later. This will build up the stores in the soil without killing the plant and allow the pink flowers to emerge as blue when they bloom.