"Out of sight, out of mind" is a saying that I believe
is the basis for most of the plant problems that have occurred
during the severe drought of August and September.
People are neglecting the portion of the plant that they
never see and are subsequently causing the plant's death.
The mysterious portion of the plant that is seldom seen is
the root. People have no idea about the dimensions of a root
system. How large is it? How deep is it? Are there different
kinds of roots in a root system? What are the functions of
People are not really interested in plant roots. Roots are
not pretty and are usually dirty. I have never seen a plant
root arrangement for sale. Folks don't give plant roots as
a gift. Yet the roots constitute one?half of the plant and,
maybe, the most important half. Now, if you worry over little
things, such as "which came first the chicken or the
egg?" you won't have to fret about plants-the root comes
I have come to realize that people do not understand the
plant system, especially the root system. I receive lots of
questions from gardeners worried about the root confinement
of plants caused by the peat pots in which many transplants
are sold. These peat pots, which are less than two inches
square, persist on plants for months after transplanting occurs.
When gardens are being renovated, old plants that are being
pulled out of the ground may still have these peat pots on
the root system. Some people then think that the peat pots
have inhibited root growth because only a few large roots
have seemingly penetrated the persistent pots. Such deductions
are evidence of the misunderstanding that exists about the
root systems of plants. These misunderstandings mainly involve
under-estimating the massiveness of a plant's root system.
When a plant is pulled out of the soil, more than 98% of the
root system remains in the soil. Most of us should quickly
realize that a plant that is 6-feet high, 4?feet wide and
that has produced an abundance of fruit during a 4?month period,
could not have done so with only four large roots and/or the
roots which occur in a 2?inch square pot! Even a bonsai (the
art of developing and growing miniature trees or shrubs) expert
couldn't make it happen! Most of us have never actually seen
the "working" root of our plants. They are referred
to as root hairs because they are about the size of a hair.
These are the structures that absorb water and minerals from
the soil. So forget about the peat pot-it does no harm!
Where are the roots of a plant? At the base of the main
stem or trunk, right? Wrong!
Obviously many people think that the working roots of a plant
are right around the area where the stem or trunk enters the
soil, because this is the area that is watered heavily and
the area on which fertilizer is dumped. As the old saying
goes: "You done good but you done wrong!" Actually
the magnitude of a plant's root system is greatly underestimated
by most people. When I tell people to water their trees they
place a sprinkler in one spot under the canopy of the tree,
allow it to operate for an hour or so, and think that a thorough
job has been done. These plant caretakers do not understand
or appreciate the complexity of a tree.
Trees are huge-larger than the biggest whale. Individual
leaves and roots are extremely small in relation to the whole
tree. Very few human beings have had the privilege of actually
seeing even a comprehensible fraction of the root system of
an entire tree. Illustrations in textbooks, natural history
books, and in manuals of landscape architecture are usually
creations of artistic imagination and are usually terribly
wrong. It is impossible to accurately portray or represent
an entire tree and its roots on a single page of a typical
textbook. The problems of scale are overwhelming. A healthy,
open?grown oak tree, 40 years old, has a trunk 70 feet tall
and 2 feet in diameter. The spread of the branches of an open?grown
tree is rarely less than two?thirds of the height of the tree
and is often equal to or greater than the height. The leaf-bearing
surface of the tree extends to within 10 to 20 feet of the
ground, and a typical branch forks four to five times from
its origin to the smallest twigs. A root spread of a typical,
open?grown tree can commonly extend in a circle with a diameter
two or more times the height of the tree or an area equal
to four to seven times the area beneath the tree canopy.
A large tree has hundreds of miles of roots to anchor it
in the soil. But most of that length is dead, woody matter.
At the very tips of the roots, there are living, growing cells
that push a protective cap of dead cells through the soil.
Just behind the tip are the root hairs-tiny, single?cell projections
that absorb water and dissolved minerals from the soil, and
start it on its way up to the leaves.
The finest roots of a tree are connected to the leaves by
an elaborate plumbing system of larger transport roots, trunk,
branches, and twigs. The composition of a tree is 5% fine
or feeder roots, 15 % larger or transport roots, 60% of trunk
or main stem, 15% branches and twigs, and 5% leaves. The quantity
of roots decreases rapidly with increasing depth in normal
soils, so that 99% of the roots are usually included in the
top 36 inches of soil. In oaks, a given root is directly connected
to a particular set of branches usually on the same side of
the tree as the root. Death, damage or drought to the roots
of trees with such restricted, one?sided plumbing systems
usually results in death or leaf?fruit drop of the corresponding
In typical clay?loam soils, roots are usually located less
than 8 to 12 inches below the surface and grow outward far
beyond the branch tips of the tree. The system of framework
roots, often called "transport" roots, extends frequently
to encompass a roughly circular area 4 to 7 times the area
delineated by an imaginary downward projection of the branch
tips. A complex system of smaller roots grows outward and
predominately upward from the system of framework roots. These
smaller roots branch four or more times to form fans or mats
of thousands of fine, short, non?woody tips. Many of these
smaller roots and their multiple tips are 0.2 to 1 mm or less
in diameter, and less than 1 to 2 mm long. These fine, non?woody
roots constitute the major fraction of the surface of the
root system of a tree. Their multiple tips are the primary
sites of absorption of water and minerals. Hence, they are
often called "feeder roots."
The size of a typical tree's root system begs a question.
So if it covers an area 4 to 7 times larger than the projected
crown area, or has diameter of twice the height, why do folks
think they can adequately water a tree by setting a sprinkler
in one spot under the canopy of the tree and letting it operate
for an hour? The tree may be getting watered regularly, but
obviously, not adequately!
Once you understand the massiveness of a root system you
will be able to better care for the entire plant and decide
what can be done to a root system without endangering the
entire tree. One activity that will endanger a tree is changing
the original soil level around the tree. Depending on the
species and amount of soil placed over the root system, a
tree may slowly decline or die over a period of 1 to 10 years
from root suffocation or root fungi. Fortunately, live oaks
withstand human abuse better than most species, and to achieve
death requires 12 or more inches of fill, particularly fill
with a high percentage of clay. The two most frequent causes
of death from changing the original soil level are fill soil
around the trunk and a change of drainage. Both the fill soil
and lack of drainage encourages fungal growth or will kill
the tree directly by suffocation. The good news-if the trunk
is spared and a significant portion of the original root system,
which can be huge, is unaffected, the tree recovers.
The same is true for the opposite of increasing the soil
level-which is removing several inches of soil and root system-unless
the damage is over the entire root system, the tree may thin
foliage and slow growth, but it will recover with care. I
have seen trees that are 30 feet high have three-fourths of
their entire root system removed and recover after 10 years.
I have seen 15 feet of fill rock piled around three-fourths
of a tree and the tree's canopy did not ever thin. The key
to both of these tree survivors was that the one-fourth of
the root system that was not covered or removed was nourished
from watering and fertilizing newly established lawns and
So with all of this information about how much abuse a tree
can endure, what are some guidelines that homeowners can follow
to protect the precious trees and avoid a "near-death"
experience? Mark Peterson, Urban Forester for Texas Forest
Service gives these guidelines:
1. Limit fill over the entire root system. Depending on soil
consistency, the fill layer can be 3 to 6 inches, e.g., clay
loam no more than 3 inches and sand/sandy loam no more than
2. Never add fill within 30 inches of trunk, preferably 40+
3. Never water anytime within 30 inches of the trunk.
4. Never create a flowerbed around the trunk. OK, this one
is controversial (even though I've never seen a flower bed
around a tree in the woods). If you must have one, use plants
that do not require daily watering and do not ever water the
5. If you have excess fill, excavate immediately down to the
original grade and create a tree well. Ensure that water does
not flow into the well.
6. Always follow basic arboricultural practices: Fertilize
in the fall; prune every 5 years; water only when necessary;
aerate soil as often as possible.