A common question we get from gardeners each year is: "Why are my blue Hydrangeas pink, or, Why are my pink Hydrangeas blue?" The answer is really quite simple (and no, it's not magic). The color of your Hydrangea blooms are directly linked to the PH levels of your soil.
Knowing this important information means you can easily (well, somewhat easily) change the color of your Hydrangea blooms! Alkaline soil produces pink blooms and acidic soil produces blue blooms. The first step in determing the PH of your soil is to do a soil PH test to determine the acidity of your soil.
Blue to Pink: A test result below 7 means that your soil is acidic (blue blooms). To raise the PH of your soil (to turn blooms pink), try adding limestone – most packages will tell you how much to add to increase your PH to the correct levels.You can also try using a fertilizer with high levels of phosoporus in it. This helps keep the aluminum in your soil from entering the root system of your plant. If you can't seem to lower the acidity in your garden beds, consider planting in pots.
Pink to Blue: A test result above 7 means that your soil is alkaline (pink blooms). To lower the pH of your soil (to turn blooms blue), you can add things such as sulfur, compost, pine needles, or pine bark. This will help to add some acidity to your soil. You can also try an organic fertilizer that is low in Phosphurus and high in Potassium.
The best success rate comes from gardeners who grow their Hydrangeas in containers, which allows for them to completely control the acidity of the soil with no outside factors.
Also note that it is easier to change a pink Hydrangea to blue than blue to pink. Other colors of Hydrangea cannot change colors with soil acidity.