When planting a tree or shrub, dig the hole twice as wide and twice as deep as the root-ball
A planting hole should be twice as wide as the root-ball but no deeper. By applying this planting principle, you will encourage the roots of a plant to grow out, which creates stability and allows the plant to readily find water and nutrients. A good way to make sure that the root-ball is at the right depth is to place the top roots so that they are parallel with the soil surface and then apply 2 inches of mulch over them. Think of it as a "planting area," instead of a "planting hole." Don't add compost or potting soil when backfilling the planting area. Most roots prefer to grow in these amended soils instead of spreading out through the landscape.