Newly planted & existing trees in Minnesota require watering, especially in the warmest days of the year. When a tree is transplanted, some root structure is cut off. The first 2 years after transplanting, trees require care weekly. The object is to help the tree reestablish the root that was cut off. Most balled & burlapped trees require approximately 20 gallons weekly. By running a garden hose at a trickle amounts to about 5 gallons in 5 minutes. So 20-30 minutes a week will provide the water the trees need.
This is just a general rule. Excessive temperatures, extreme heavy soil , type of tree will affect how much to water.
In the warm summer weather, if a tree is dropping leaves, it is definately telling you it is not getting enough water. The first response of a tree to a dry condition is to drop leaves. This is a survival instinct.
For more detailed or specific questions on any tree, please contact me by email or phone.
Just a little care early after transplanting will allow the tree to thrive as well as survive.