Your best backfill will be the loosened original soil from the planting hole mixed with 10 to 20 percent compost. Loosen and break up any clods of soil before backfilling. Clods in the backfill create detrimental air pockets around the root ball and could hinder root growth and establishment.
The old adage "never put a ten-dollar tree in a two-dollar hole" applies when planting individual trees and shrubs. Research has shown that a large planting hole – at least twice as wide as the root ball – encourages rapid root growth and plant establishment. Dig the planting hole only as deep as the root ball. If the hole is dug deeper, backfill it with soil as necessary and tamp it firmly to prevent settling. Make certain the top of the root ball is level with the soil surface. Some landscape professionals plant the top of the root ball 1 to 2 inches above grade if they know the soil is likely to settle slightly.
Dig the planting hole two times wider than the root ball. Make certain the top of the root ball is level with the soil surface. Dig the planting hole two times wider than the root ball. Make certain the top of the root ball is level with the soil surface.
Research has also shown that it is not necessary to add organic amendments, such as peat moss, compost or leaf mold, to the planting hole. Organic matter can act like a sponge in the planting hole, absorbing and holding too much moisture and causing the roots to stay too wet. When planting just one plant, it is best to backfill with the same soil removed from the hole. Be sure to break apart any clods and remove stones or other debris before refilling the hole.
Before planting balled-and-burlapped plants, cut any wire or cord from around the trunk and pull back the burlap from the top one-third of the root ball. This will allow newly formed feeder roots to grow into the new environment. When planting on poorly drained soils, remove the burlap completely. When planting trees or shrubs grown in fabric bags, remove the entire bag before planting.
To eliminate air pockets, water the planting site as the backfill soil is placed in the hole. Use your hand, not your foot, to gently firm the soil around the roots. Water thoroughly when finished and water again several hours later.
Slow-release or liquid fertilizers can be added to the planting hole, but granular general-purpose fertilizers, such as 8-8-8 or 10-10-10, can damage tender roots. Wait until the plants are established before applying a granular general-purpose fertilizer. (See section on caring for newly planted trees and shrubs.)
Finally, shape a small ring of soil, 2 to 3 inches high, along the perimeter of the planting hole. This forms a saucer on top of the soil, which directs water to the roots and prevents runoff. Uniformly apply a 3-inch layer of mulch over the soil surface. Mulches promote rapid rooting by maintaining uniform moisture levels and temperatures in the soil and by preventing weed competition. Landscape fabrics can be placed under the mulch to help prevent weeds and to conserve moisture