Hydrangeas need more water than most other plants, especially in their first few years in the garden. In general, you can expect to water your hydrangeas deeply two to three times a week, providing at least an inch of water each time. Thirty minutes with the soaker hose can be a great way to irrigate your hydrangea plants. Remember that potted hydrangeas, or hydrangeas planted in hot climates or direct sun, will need more frequent watering.
A note on hydrangea hydration--hydrangeas prefer well-draining soil, and, even though they require regular watering, don't like to have "wet feet." Choose a location that won't allow your hydrangeas to become waterlogged, and amend the soil to give it a good, loamy texture.