The word "hydrangea" means "water vessel" in Greek, so it's not surprising that hydrangeas (Hydrangea spp.) are water-loving plants. Most species thrive on moist, well drained soil in an area with some shade. Your irrigation duties depend, in part, on the weather in your region. Hydrangeas live in U.S. Department of Agriculture plant hardiness zones 3 through 9, depending on species.
Hydrangeas are undemanding shrubs, grown for their large, beautiful blossoms. Each inflorescence is composed of showy male flowers and tiny but fertile female flowers. The plants can keep your garden ablaze with blossoms from early summer though autumn. Each of the five species widely available in the United States has its own specific requirements, but generally hydrangeas like a dappled-sun location that gives them shade in the hot afternoons, and moist, well draining soil.
Enough Water to Prevent Drooping
Don't look for a specific rule for how many gallons of water to provide your hydrangeas per month. You'd have to factor in many variables, like species of hydrangea, daytime temperatures, type of soil, time of year, amount of shade and level of precipitation. Horticulturists at North Dakota State University offer the following guideline regarding frequency of irrigation: water enough to prevent wilting. Generally, this means you'll need to water hydrangeas once or twice a week during the summer season. Water in the morning or evening rather than in the heat of the day, and avoid overhead irrigation. Hydrangeas are dormant in winter, so no irrigation is required.
Hydrangeas grow best in a location with partial sun rather than direct, all-day sun. Choosing a too-sunny spot causes the plant to wilt. However, if you irrigate the shrub frequently and generously, it may tolerate the sunny placement. The relationship between a shrub's sun exposure and its water requirements is not difficult to understand. Extra sun dries out the soil, causing drought stress, while extra irrigation prevents or at least limits that stress. It's sometimes said that the amount of sun a hydrangea can tolerate depends on how much water it receives.
Too Much Water
If you deep-water hydrangeas every day, you are giving them too much water. However, most excess-water issues occur when the shrubs are planted in soil that doesn't drain well. If the hydrangea is planted in heavy soil like clay, water doesn't drain away from the roots and root-rot may develop. If hydrangea leaves droop and do not perk up 30 minutes after you irrigate them, you likely have a drainage issue. You might also notice browning leaf edges. Either replace the soil with something lighter and more porous, or move your shrubs to well-draining pots.