image by Muffet: flickr.com
Hydrangeas are somewhat of an oddity in the gardening world, as they are one of the few plants known to be able to change bloom color. Color changes from pink to blue, or blue to pink, are the result of soil pH and the presence of aluminum. It's much easier to change to blue because, to do so, aluminum sulfate needs to be added to the soil instead of subtracted from it. If the existing soil has a low pH, or is acidic, and plenty of aluminum sulfate is already present, hydrangeas will naturally produce blue or purple-blue flowers.
Use a pH test kit to determine your adult hydrangea's soil level of acidity. For blue flowers, the soil pH should be low, somewhere between 5.2 and 5.5. Anything above that will begin to cause blooms to turn pink. It's best not to tamper with the natural bloom color in developing plants younger than 2 years old.
Water your hydrangeas very well prior to adding aluminum sulfate to the soil to avoid the possibility of burning the roots.
Dissolve 1 tablespoon of aluminum sulfate in a gallon of water. Apply this solution to your hydrangea throughout the growing season according to the packaging instructions.
Fertilize with a good quality, low potassium 25/5/30 plant food according to the manufacturer's directions all season. This helps to encourage color change. Don't use bone meal or super phosphates when working for blue blooms.
Mulch the hydrangea with organic matter to further reduce the soil's pH level naturally. You can use grass clippings, fruit and vegetable peels, and even coffee grinds for this.
Choose a planting location far removed from concrete walkways, sidewalks and foundations when putting in hew hydrangeas. Lime leaches out of concrete directly into the soil, considerably raising soil pH levels.