In winter, water when the soil begins to dry out. Start irrigating in spring at bud break. Increase the amount of water applied as needed to keep the soil moist, but not saturated. The type of soil, the amount of shoot growth, temperature and wind affect how much and when to irrigate. Water requirements can nearly double after flowering, both to produce fruit and to keep up with increased transpiration caused by hot windy days of summer.
Commercial growers watch the condition of the leaves and especially the tendrils for water stress.
If the green succulent tips of the tendrils turn brown or yellow, or if leaves droop or berries shrivel later in the season, you may have waited too long to irrigate. These same indicators may also occur from over watering and when roots are water-logged from poorly drained soils. Flood and drip irrigation are common irrigation methods used with grapes. Wetting the leaves with sprinkler irrigation may increase leaf diseases. The amount of water needed varies among plants, soils, and weather conditions.