Newly planted trees need to be watered in the summer – about 5 gallons per week for every inch of trunk diameter. Water should always be applied slowly and don’t overwater. Even mature trees may need one or two good soakings of water a month during the hot, dry season. Water slowly and throughout the root zone, not at the trunk of the tree.
Watering newly planted trees
Watering Newly Planted Trees - Seattle Dept. of Transportation
How to water your tree - Vacaville Tree Foundation