Root Misunderstanding

"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 a root?

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 first.

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 branches.

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 ornamentals.

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 6 inches.
2. Never add fill within 30 inches of trunk, preferably 40+ inches.
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 trunk.
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.