Changing Flower Color for Hydrangeas
To encourage flowering, we recommend a fertilizer low in nitrogen and high in phosphorous, with a number over 30. For instance an N-P-K ratio of 10-40-10 would be ideal.
Big leaf hydrangeas are unique in that their flowers can change color. The color of hydrangea blossoms depends on the soil’s pH and its ability to absorb aluminum at different levels. An alkaline soil produces pink colors. An acid soil produces blue flowers. You can buy a soil pH testing kit to test your soil.
It is possible to manipulate the color of hydrangeas, but one word of caution: many people have killed their plants by applying too much aluminum sulfate. More is not better.
Changing to Pink:
To change from blue to pink you need to change from an acid soil to an alkaline one.
To help raise your pH you can use dolomite lime several times a year. You will have to retest your soil and aim for a pH of about 6.0 to 6.2. If it goes above 6.4, your hydrangea may experience an iron deficiency.
Use fertilizers with high levels of phosphorus. Phosphorus helps to prevent aluminum from being taken up in the plant’s system.
Consider growing your hydrangea in a large pot where it is easier to control the pH.
Changing to Blue:
Lowering the pH of your soil will produce blue flowers with a pH level of about 5.2-5.5. Using a soil acidifier will lower your soil pH for beautiful blue blooms. Tips for success:
Ask what’s recommended to acidify the soil in your area.
Follow the directions carefully on the product you purchase.
Thoroughly water the plant before adding any acidifier.
Adding large amounts of organic matter, such as peat moss and composted leaves, will acidify the soil as they break down.
Remember to check the pH of your water. If you are trying to turn your flowers blue and you have hard water, it will be difficult to achieve. Also remember that concrete foundations and walkways tend to leach lime, raising the pH in that area.