Hydrangeas are tall, bushy shrubs with coarse leaves that produce big, round clusters of flowers. The most common variety grown in the United States is Hydrangea macrophylla, which grows to between four and eight feet tall and blooms pink, red, blue or white. The acidity of the soil in which it's planted determines what color the flowers will be. For hydrangeas to flower blue, they need acid soil.
Choose a site for your hydrangeas where they'll get some shade and where the soil is moist but drains well. Dig up the spot where you'll plant the hydrangeas, deep enough to hold the plants upright. If drainage is a problem, add compost to the dirt till the mixture is light and loamy.
Put the hydrangea plants into their holes and cover their roots well.
Water your hydrangeas well and regularly, especially if you live in a hot climate.
Prune hydrangeas in the winter time, cutting off as much as one-third of the old stems to encourage new growth. To control height, prune the top of the plant.
In the spring, prune out any dead growth. If, after the plant greens out, it is putting out stems from its base while the top stems remain bare, cut off all the stems coming up from the base.
To keep your hydrangea flowers blue, make sure your soil is acidic, with a pH of 5.5 or lower. Have your soil tested by sending a sample to a laboratory. Most state agriculture departments offer these tests to residents free of charge (see Resources section).
If you find that your soil is too alkaline to keep your hydrangeas flowering blue, ask the agent who helped you with the soil test how you should amend the soil to correct the problem.