Water a newly planted tree with 5 gallons of water, letting it soak in slowly. An easy way to do this is to make a very small hole in a 5 gallon pail and then set the pail beside the tree.
If planting a dormant bare root tree or plant, do not water it again until leafing begins. Too much water in the dormant stage can reduce root development. Once the tree is leafed out, water it with 3-5 gallons twice a week for the first season depending on how well drained your soil is and how much rain falls throughout the first season.