Strategies for Survival
Revised January 1998
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 trees 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.
Figure 1A. Root and Top Growth of a
One-Inch Caliper Tree
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.
|First Year. Root
system diameter increases to 6 feet, 9% of original volume. With less than 10% of the
absorbing roots to support a full crown (top), the tree is often under severe water
stress, inhibiting top growth, including bud formation.
||Second Year. Root
system increases to 9 feet, 23% of original volume. The tree remains frequently under
water stress, inhibiting current season growth.
|Third Year. Root
system diameter increases to 12 feet, 41% of original volume. As root/top balance is
gradually restored, the tree is exposed to less water stress and growth improves.
||Fourth Year. Root
system diameter increases to 15 feet, 60% of original volume. The effective rate of root
generation accelerates as the overall diameter of the root system increases.
Fifth Year. Root system diameter
increases to 18 feet, 100% of original volume. Roots and crown are now balanced and the
tree should grow with normal vigor.
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
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
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
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
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
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
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.
Watson, G. 1985. Tree size affects root regeneration and top
growth after transplanting. J. Arboric. 11:37-40.
Watson, G, 1986. Cultural practices can influence root
development for better transplanting success. J. Environ. Hort. 4:32-34.
L. Ham, Extension Forester and Professor
R. Nelson, Extension Forester and Associate
Department of Forest Resources