Much has been written and discussed regarding landscape planting methods for trees and shrubs. Many techniques are successful but traditions are often contradictory. Research during the past several years suggests several key considerations in the planting of woody landscape plants.
Proper plant selection for the sites, soil conditions, exposure and spacial requirements will reduce future maintenance and problems caused by crowding and obstacles. The International Society of Arboriculture (2) points out that
Generally the spread of a tree's crown is equal to that of its height. When planting close to a building, a tree should be placed about half its ultimate height away from a building to allow the crown to develop freely.
The urban soil environment suffers from compaction. These densely compacted soils, covered by a thin layer of top soil, are low in oxygen and poorly drained. Plants suffering from poor drainage exhibit symptoms similar to those of drought conditions: slow root generation and growth, sparse and chlorotic foliage.
To check the drainage of a landscape site, place a coffee can with both ends removed on top of the soil surface. Fill the can with water and observe the time it takes for the water to drain. The infiltration rate, the rate at which the water drains away, should be at least 1 inch (2.5 cm) per hour. An alternate method would be to dig a hole dug to the required depth and fill it with water and allowing that water to drain away. Fill the hole a second time and note the infiltration rate.
The planting hole must provide optimum conditions that would encourage the development of the fine absorbing roots close to the surface of the soil. These roots are generally concentrated within the top 20-30 cm of the soil. Most holes that are dug in the compacted urban soils do not allow or encourage root development into the native soil. Rather, they act as a sink for water, becoming much like a bathtub.
Watson (5) illustrates several approaches in preparing the planting hole to encourage root growth and alleviate water problems. By increasing the diameter of the planting hole to two or three times the diameter of the root ball and sloping the sides of the hole, rather than leaving them vertical walls, Watson notes that the volume of backfill placed in and around the root ball increases 150 to 400 percent. The sloped sides of the hole assist in directing root tips up to the surface and discourages circling of roots within the hole. It also creates a larger surface area between the backfill soil and the compacted native soil.
On very wet sites, the root ball can be planted a little higher with about the top third above grade. This helps in keeping the ball out of the saturated water layer in the bottom of the hole. Some suggest placing the ball on a compacted pedestal to avoid settling and to keep the ball above the saturated layer.
Backfilling around the ball should be carried out using the native soil. If new soil is going to be added, match it as closely as possible to the native soil. The backfill should be lightly packed around the ball. No packing will leave large air space in through which the roots cannot grow, while packing the soil heavily will reduce soil oxygen and increase water logging potential. Watering the soil around the ball will help it to settle without excessive compaction.
Before filling the hole, make sure that all ropes and burlap secured around the base of the trunk are removed. Wire baskets need not be removed. Basket loops should be bent back or removed and the top horizontal wire should be at least several inches below the top of the root ball (3).
The root ball should be placed into the planting hole so that the original soil level is even with the soil surface at the landscape site. The root collar must remain at the soil level. The root ball should be examined closely. If additional soil may have been added at the time of digging it should be removed prior to planting. Deep planting results in an oxygen poor environment. Planting too shallow encourages the development of surface roots which can dry out or be injured through cold winter temperatures.
Tree trunks are often wrapped prior to digging to protect the bark from any potential injury that may result during digging or transplanting. Wraps are left in place at the landscape site in order to reduce damage from insects, disease, herbicides, maintenance, vandalism and other physiological problems.
A recent review notes that these expensive wraps provide little benefit and may indeed cause future problems. Wraps have been noted to retain excessive moisture against the trunk, predisposing it to injury, cankers, fungus and encouraging borer problems. It has also been observed that cloth wraps do not reflect heat when applied for winter protection and once wet they absorb and retain heat. Tree wraps should not be used indiscriminately but rather evaluated on site with plant considerations.
Mulches, applied in the landscape, improve weed control, help retain soil moisture and minimize root zone temperature fluctuations. When employing a mulch, there are a few points to consider to keep them effective and avoid future plant problems. Mulches should not be applied over black plastic. The plastic layer, placed over the soil will restrict the movement of air and water into the soil. McClure (4) also points out that a hardwood bark mulch, applied more than 5 cm thick, intercepts large amounts of rainfall and irrigation. If a thick mulch layer is applied over a heavy, moisture retaining soil both layers would absorb moisture until saturated before water would infiltrate into the soil. These wet layers then create an impermeable barrier preventing oxygen to filter through to the soil.
Optimum mulch depths vary with particle size and absorption capacity or water permeability of a mulch (4). Some suggestions are:
Mulch should be kept away from the base of the trunk and, if organic, should consist of composted materials.
Trees should only be staked when necessary. If the tree cannot stand alone or would be injured in heavy rains or winds, it should be staked. Remove stakes after one year. When staking a tree, use broad, belt-like ties that will not injure the stem of the plant. Stakes should be placed on either side of the tree and kept low, about 12 to 18 inches (30 to 35 cm) above the soil (6).
When preparing a site for a landscape planting, the long term survival of woody plants depends on careful preparation and consideration of the plant's needs. Being aware of the affects on plant growth that a site has prior, during and after planting, will help to ensure survival.
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