Many people like to
form a little dam around the circumference of the
hole where a tree or shrub has been planted. The
ridge of water is meant to hold the water in close
proximity to the trunk of the plant so that it will
seep into the ground and into the root ball.
Generally these basins are fill by hand watering
with a hose and extension tube.
In recent years, this
method has been superseded somewhat by plastic bags
that can be attached around the trunk to
automatically release water into the root zone.