Watering is necessary for recently transplanted trees or trees suffering from a variety of environmental disturbances, particularly those that reduce available soil water or disrupt or destroy part of the tree's root system. The rule of thumb that is commonly used is to augment natural rainfall to one inch of water per week or two inches every two weeks. If the tree received one-half of an inch of natural rainfall during a week, one would add another one-half inch of irrigation water during that week. Watering can be overdone as well. One landscape contractor noted that his records suggested that it cost him twice as much to replace guaranteed plants where there was an automatic irrigation system installed. One reason for this is that the automatic irrigation system makes it easy to overwater. Hand watering, soaker hoses, and drip irrigation are more efficient than sprinklers and overhead irrigation, but all methods can work.
Another method of monitoring watering needs is to use a biological indicator plant such as impatiens, ajuga, or coleus. These indicator plants, which can be planted in the root zone of the tree of interest, wilt dramatically when water becomes limiting. When the indicator plants stay wilted for more than an hour after they are in the shade, the tree and the indicator plants should be watered. Repeat the watering when the indicator plants again wilt. Indicator plants adjust for climatic conditions such as natural rainfall, temperature, and wind.