This gives the roots extra energy to expend on growing and getting established. Some researchers have even suggested that roots continue to grow until the ground freezes. In Utah this can be as late as December or even into early January some years. When planting a shrub or tree, be sure to dig a hole that is as least twice as wide as the root ball, he explains. This loosens the soil around the roots and gives them a place to grow.
Most roots grow laterally rather than downward.
The hole should be dug no deeper than the root ball or pot the plant came in, he says. In fact, I have had better luck with planting the root ball a little high, leaving it an inch or more out of the ground. This allows for some settling and makes sure that the plant is not placed in the hole too deep. Back fill the hole with the native soil. It can be useful to mix a little organic matter into that soil before filling it in around the roots.