Q. How do I get my hydrangea flowers to turn blue?
Some hydrangea species can have blue or pink flowers depending on soil conditions. Hydrangeas that naturally have white or cream flowers cannot be persuaded to provide blue flowers.
To change the color from pink to blue, two conditions are necessary: aluminum in the soil and a low pH (which makes the aluminum available to the plant). For plants that are at least 2-3 years old, add 1/2 oz. of aluminum sulfate per gallon of water, throughout the growing season. That will add the necessary aluminum and also lower the pH somewhat. To further lower the pH, you can add organic matter such as coffee grounds, fruit and vegetable peels, and grass clippings to the soil.
The choice of fertilizer will also affect the color change. It is also helpful to add a fertilizer low in phosphorus and high in potassium. (25/5/30 is good--potassium is the last number). Superphosphates and bone meal should be avoided when trying to produce blue. It may be difficult to keep a flower bed constantly acidic and aluminum-rich. This problem can be avoided by growing the shrubs in containers, where the conditions can be more easily controlled.
For tips on a variety of gardening topics, see our Plant Information Guides.
- Courtesy of NYBG Plant Information Service