Young trees should be watered deeply on a weekly basis to encourage deep root growth. To do this, set your hose on a slow trickle for several hours at the base of the tree or use a soaker hose around the tree.
Mature trees should be watered deeply beyond the drip line (the edge of the tree’s canopy). Roots extend past this line. This table gives watering needs for mature trees.
Trees in or near lawn areas with frequent, shallow watering may develop surface roots.