Pink to Blue

• If your blue hydrangea has turned pink, it's because your soil is alkaline or has a high pH. Adding aluminum sulfate to the soil will turn the flowers blue again. The aluminum sulfate 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 aluminum to the soil than to subtract it.
  
  

Aluminum Sulfate

• Use 1/2 oz. of aluminum sulfate per gallon of water. It should be applied several times throughout the growing season. Water the plant first, then slowly pour the aluminum sulfate solution onto the soil around the hydrangea. Adding the mixture too quickly can damage the roots.
  
  

Low PH

• 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 aluminum 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 fertilizer 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 fertilizer with high levels of phosphorus to prevent aluminum from entering the plant's system again.
  
  

Potted Hydrangeas

• Dolomite lime and aluminum sulfate are available at garden centers. Gardeners who want to change the color 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.