Dig the hole only deep enough to hold the root ball. When planting in loam soil, the root flare on the tree or shrub trunk is planted level with the surrounding soil. If you can’t see the root flare, remove soil or burlap until you do. When planting in clay soil, the root flare is planted 1 to 2 inches higher than the surrounding soil (see diagram). Alternatively, you can form a one-inch pedestal at the bottom of the planting hole for the root ball.
The planting hole diameter should be two times the diameter of the root ball. The minimum planting hole diameter can be 12 inches wider than the root system and the maximum can be up to five times the root system diameter. The sides of the planting hole should be vertical. If the sides of the planting hole appear shiny or glazed, rough up the edges with a shovel to loosen the soil before planting.
Before adding soil back into the planting hole, make sure that roots are not kinked or circling. Start adding soil in three to four inch layers lightly firming the soil between layers by lightly stepping on the soil. Backfill soil may be mixed with organic matter at a rate of 3 parts soil to 1 part organic matter to help improve soil texture of medium or fine textured soil. Adding organic matter to the backfill soil is not always recommended, especially in a heavy clay, since sometimes roots will not leave a richly amended hole to grow into the adjacent inferior soil, and circling of roots may occur. Avoid covering the top of the root ball with backfill soil. Make a 2 to 3 inch soil berm around the edge of the planting hole to form a shallow basin. Water the plant in well by filling the basin with water. In heavy clay soils, watch that this berm drains within a couple of hours. Fertilization is not recommended at time of planting. Once the plant is well watered in, apply a layer of mulch 2 to 3 inches thick to the basin, avoiding placing mulch against the base of the trunk.