Site Preparation and Tree Planting Detail

Dr. Peter May

May Horticulture Services

Successful tree planting depends on the ability of the tree to rapidly initiate root growth, into the backfill of the planting hole, and then into the soil beyond the planting hole.

Recommendations

  • Check physical and chemical restrictions of the site before planting; improve if possible or select a compatible tree.
  • Improve sites where access is readily available by improving water penetration and oxygenation.
  • For inaccessible sites, ameliorate drainage and compaction by planting technique.
  • In planter boxes, use a high mineral content mix to lessen compaction promoted by organic matter decomposition.
  • Plant trees only as deep as the rootball; make the hole 2x as wide as the rootball; do not “fluff-up” soil under the rootball; do not amend backfill; mulch after planting.

Background: physical soil properties and tree growth

The main soil properties influencing planting success are physical, primarily being the availability of water and the levels of oxygen to the soil. The other important physical soil property is soil strength: how difficult or easy it is for roots to physically penetrate the soil.

Water availability is essential for rapid root growth and tree survival. Even where the tree has been selected to match the patterns of natural water availability of the site, most often trees will perform much better if supplemented with water in the year following transplanting; the late spring, summer and autumn periods are the most critical. The practices for ensuring adequate water are dealt with in the next article. Pre-planting treatment of the site and planting practice should ensure that rain or irrigation can penetrate the root ball and backfill soil.

Oxygen availability is primarily determined by the water content of the soil. Soils which are too wet contain little oxygen and this will reduce root growth or kill root tissues (and even lead to tree death). Site preparation that ensures that the backfill and site soil have open structure (porosity) and allows reasonable drainage of the planting hole will result in better tree establishment.

In many urban sites, development activities can lead to serious soil compaction. Compacted soil has poor water movement rates/aeration and is difficult for roots to penetrate. These effects can result in root growth restricted to the planting hole.

Background: chemical soil properties and plant growth

Soil pH should be checked before selecting trees. Extremes of pH can occur and should be remedied before planting. In some circumstances (for example highly alkaline soils) it may be better to select for tolerance of the property, rather than trying to amend the soil.

Woody plants are not highly responsive to plant nutrition and unless soil is very infertile, the use of fertilizers may not be an essential part of tree establishment. Research indicates that fertilizer responses may only occur where growing conditions are very good.

High levels of salt in soil will affect tree performance and if a problem is suspected, a soil test should be done before species selection is made. Use of tolerant species is the best approach to this problem.

Site Preparation: readily-accessible sites

Assess the site to establish whether any conditions will limit growth. If the soil is compacted, cultivate the surface soil to alleviate compaction. Where dense subsoils exist, these should be ripped to provide drainage channels in the subsoil. Stabilize the soil with gypsum if soil tests indicate it is needed. Incorporating composted organic matter into the surface 100mm of a poor soil will improve plant growth. Suggested rates of organic matter incorporation are 10% by volume.

The planting hole should be wider than, but not deeper than, the rootball. Backfill the planting hole carefully with the soil dug from the planting hole, ensuring that it is not compacted in the process. Water in well.

Where machine access is more difficult and compaction cannot be readily alleviated, research has shown that a planting hole with sloping sides gives better establishment.

Limited sites: e.g., street trees, tubs or planters.

Provide as large a root volume as possible. In sites with limited root volumes trees are often badly stressed through water shortage. Ensure that the planting hole or planter has drainage. It may be necessary to install a drain at the base of the planting hole. For street trees the site soil is the preferred backfill. For planters or tubs, use a planting mix with a moderate drainage rate. These mixes are preferably mostly soil rather than highly organic planting mixes which will lose volume and porosity through decomposition.