Hydrangea plants have always been popular for landscaping and flower gardens because of their vibrant color and long-lasting blooms. The color of your hydrangea is determined by the plant genetics, the pH level of the soil and the availability of aluminum in the soil. White hydrangeas will never be able to change to a color because white plants do not have the genetic marking for color. But, by manipulating the chemical balance of your soil, you can retain or change pink or blue flowers on your hydrangea plants.
Use a soil pH tester or have your soil tested at your local cooperative extension office to discover the pH value of the soil where your hydrangea will be planted. For blue hydrangea, your soil needs to be acidic, with a pH reading of 5.0 to 5.5 for best results.
Plant your hydrangea in soil that is rich in organic matter and that will drain well. Add organic matter, such as coffee grounds, leaves, pine needles and grass clippings, to supplement and decrease the pH value of the soil.
Apply a solution of one tbsp., or one oz., of aluminum sulfate in one gallon of water to the soil around the base of your hydrangea. Before applying this, water the plant so the root system does not receive too much of the aluminum sulfate solution too fast. Continue to apply this solution every one to two weeks throughout the growing season.
Fertilize your hydrangea with a fertilizer low in phosphorus and high in potassium, such as a 25-5-30 formula. A low phosphorus blend allows better absorption of aluminum in the soil, which is what creates the blue of a hydrangea.
Test your soil pH level with a soil pH tester or take a sample to your local cooperative extension for testing. Ideally, you need to have the pH level of your soil between 6.0 and 6.5 to create pink blossoms.
Add lime to the soil around your hydrangea. Generally, adding four oz. of dolomitic lime to the soil should increase the pH level by one point. After adding the first four oz., test the soil pH before adding more lime. The addition of lime will need to be done several times a year to maintain the higher pH level.
Use a fertilizer high in phosphorus, such as a 25-10-10 mix. Phosphorus prevents hydrangeas from absorbing the aluminum in the soil. Aluminum is the element that causes hydrangea blossoms to turn blue.