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Hundreds of thousands of trees are planted along city and community streets and on other public property throughout the United States each year. Unfortunately, many of these trees, perhaps 50 percent or more, do not survive beyond one or two years. Why?
Most newly planted trees are subject to stress-related problems due to tremendous root loss when dug at the nursery. This condition, commonly called transplant shock, results in increased vulnerability to drought, insects, diseases and other problems. To a greater or lesser degree, transplant shock lasts until the natural balance between the root system and the top or crown of the transplanted tree is restored. Of those newly planted trees that do not survive, most die during this root-establishment period. A tree’s chance of survival can be drastically improved through practices that favor establishment of the root system. This involves regular care during the first three years following transplanting.
Tree Roots and Transplanting
For an undisturbed, healthy tree, the root system is normally very shallow. Even the major structural roots grow almost horizontally. The root system normally extends far beyond the branch spread, and fine roots that absorb water and nutrients are located very near the soil surface, usually in the top four to ten inches. A natural balance exists between the roots (where water is absorbed) and the top of the tree (where water is utilized and transpired to the atmosphere).
When a tree is dug for transplanting, more than ninety-five percent of the absorbing roots are severed. With less than five percent of its root system remaining, the newly transplanted tree suffers from water stress. The crown is capable of losing water faster than it can be absorbed by the limited root mass. Water stress, in turn, can reduce the ability of leaves to produce carbohydrates (energy), diminish the growth of all parts of the tree, and subject the tree to many other environmental and pest-related problems. Combined, these problems all contribute to "transplant shock" that can kill the tree.
Generating Root Systems of Newly Planted Trees
Successfully establishing a transplanted tree depends primarily on rapid root generation. However, keeping the top of the tree alive and healthy until the natural balance between the roots and top is restored is essential. Initial root development of a newly planted tree is supported by energy (carbohydrates) stored within the trunk, branch, and root tissues. Continued root growth during the establishment period depends on the leaves of the tree producing high levels of carbohydrates during the growing season, especially during the first year following transplanting.
For this reason, pruning transplanted trees to compensate for root loss is not recommended. Leave the entire top intact to favor rapid development of a supporting root system. Top pruning should be restricted to removing broken and damaged branches and developing a good tree structure. Supplemental watering is critical to avoiding moisture stress.
The length of the tree-establishment period depends on both the size of the root system prior to digging and on having conditions that are favorable for good root growth. Larger trees lose a larger mass of roots than smaller trees and, if all other conditions are the same, require a longer root-establishment period.
The time required for establishing a transplanted tree can be better understood by comparing root restoration of a one-inch tree with that of a four-inch tree. For the examples in Figures 1A and 1B, the original root system diameters are given and a root growth rate of 18 inches per year under good soil and moisture conditions are assumed (1).
The diagram below illustrates the relative time required to restore the root systems to their pre-transplant size. As stated, larger trees require a longer recovery period than smaller trees. They must grow a proportionally larger volume of roots in order to support a much larger crown.
A 1" caliper tree would have a root diameter of 4.5 feet.
By nursery standards a root ball of about 1.5 feet would be moved.
Less than 5% of the root system is transplanted.
First Year. The tree is under severe water stress soon after transplanting. With good care the stress diminishes, and the root system diameter should increase to 4.5 feet, 100% of the original, by the end of the first year. Roots and top are now balanced and tree should grow with normal vigor.
Figure 1B. Root and Top Growth of a Four-Inch Caliper Tree
A 4" caliper tree would have a root diameter of 18 feet. A root ball of 3.5 feet would be moved. Less than 5% of the root system is transplanted in the root ball.
Both root generation and shoot growth are drastically reduced following transplanting. Except for the year following transplanting, when twig growth is influenced by buds formed the year before, the amount of twig growth is closely related to the amount of the root system that is present. As long as the roots and the top of the tree are out of balance, the roots cannot supply sufficient water and nutrients to the upper portions of the tree for vigorous growth. Vigorous top growth does not return until the root system is replaced, after the fifth season with the four-inch example tree. Increasingly larger trees would require even more time. For example, a ten-inch tree would require at least 13 seasons following transplanting to restore the root system to its pre-transplant size.
Proper Planting and Regular Follow-up Care
Proper tree and site selection are the first critical steps to successfully planting trees. Trees planted on inappropriate sites or in poor soils may never regain their proper root-to-top balance. However, if the tree and the site are properly matched, successful transplanting can be achieved with good planting procedures and regular maintenance following transplanting. Both are essential to minimize the establishment period and the stresses that make trees susceptible to attack by insects and disease pests.
Urban planting sites typically have dense, compacted subsoils with little or no top soil. Water cannot easily infiltrate compacted soils and, with heavy rains or overwatering, may remain for long periods in the loose soil in the planting hole. In excessively wet soil, oxygen is unavailable in sufficient amounts to support root growth.
Trees planted in compacted or wet soils must develop fine root systems near the soil surface where oxygen is most available. Root generation and tree survival can be improved by using a modified planting hole similar to those illustrated in Figure 2. Enlarging the top of the hole two to three times that of the root ball diameter increases the amount of loose, backfill soil near the surface where conditions are favorable for root growth. On wet sites, a tree can be planted so that one-third of the root ball is above the original grade (Figures 2B and 2C). This method will keep most of the root system above the saturated soil even during very wet periods (2).
Generally, soil from the planting hole should be used to backfill around the root ball. If organic matter is to be used to amend the soil, it should be incorporated in an area large enough to accommodate root growth for several years.
Soil moisture is especially important during the first three years following transplanting. Studies have shown that the carbohydrate levels critical for root generation are not lowered if transplanted trees are adequately watered. One inch of water each week for the first season is a good rule of thumb, but monitoring soil moisture and applying water as needed is preferable. Remember, overwatering can reduce soil air space, lowering oxygen availability, and that can be just as stressful as drought.
Mulching a large area around newly planted trees with 3 to 4 inches of wood chips or bark also conserves soil moisture and moderates soil temperatures. Mulch also inhibits the growth of grass. Grass roots can present serious competition to the fine absorbing root system of trees, since they all grow in the top four to six inches of soil.
It is easier to successfully establish smaller trees than larger trees. Regardless of size, newly planted trees experience a period of transplant shock during which they are highly vulnerable to stress. Through proper planting and regular early care directed at rapid root development, the period of transplant shock can be shortened and the probability of survival greatly increased.
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