Watering requirements for deciduous trees is dramatically different in autumn than at other seasons throughout the year. In the autumn there is less wind, less evaporation, the temperatures are lower and so there is less of a problem with dehydrating the plant. When you plant the tree. Dig a hole and put the tree in the hole. Fill the hole half way full of soil and flood the rest of the hole with water. Let is soak in and then fill it again. This will soak the roots and also the ground around the tree. Then you can water the tree about once per week until the ground freezes. Roots continue to absorb water until the temperature falls below 40 degrees. That will prepare the tree root system for the winter. If you get an inch of rain during the week you can skip the watering for that week. If you underwater the tree it stresses the tree and results in undersized leaves and fewer flowers. Over watering will result in the tree sinking into the ground. Clay soil requires less water because it holds water much better. Fall is a great time to plant a new tree.