Feature Article: Rooting around, tree roots

By Dave Hanson, Univ. of MN, Urban & Community Forestry

cottonwood root nesting a garden bur oak perennial wood support roots

Take a good hard look at your favorite tree. What do you see? Nice form, leaves, branches, bark, but, how many of us would respond - "Oh baby! There must be some fine roots holding up this tree." Unfortunately for trees, not too many people respond in this fashion, the world underground does remain a mystery. What is happening down there? What are tree roots and for that matter where are the roots? Why and how do roots get into sewers, cause foundations to crack and sidewalks to lift? The reason that these mysteries remain: we cannot easily see and touch tree roots, we cannot readily access them and typically we do not encounter them without getting out a shovel. This brief article will skim the surface, so to speak, and attempt to provide some insight and answers to these questions.

So, let's start at the beginning and address two basic questions. Question number one, what are tree roots? There are three basic types of tree roots: Structural support, Fluid transport, and nutrient / moisture absorbers. Structural support comes in the form of larger-diameter perennial woody roots that anchor the tree to earth. Near the base of the tree are found the sinker or striker roots that help stabilize the tree and help it exploit deeper soils, but they seldom extend below 1 meter deep (Harris et al.). A second group of perennial support roots grow horizontally radiating away from the tree and also provide transport of nutrients, water and oxygen to the canopy. In return, the canopy delivers energy back to the roots for their use and for storage. The last of the basic root structures to discuss here are referred to as absorbing roots (fine root hairs and the root tips roughly 1/16th inch diameter) that provide nutrient and moisture absorption for the tree.

With that basic understanding, let's move on to question number two. Where are the roots of a maturing or mature tree? Many consider the drip-line of the tree to be the limit of the root spread; therefore, the assumption is made that this is the area to fertilize and water for the benefit of the tree. A better mental picture to form is a wine glass positioned on a dinner plate. In support of this mental picture six species, including green ash, maple, and oak, were studied by Dr. Edward Gilman of the University of Florida. Dr. Gilman and associates performed root growth trials in Florida and New Jersey. The findings clearly debunked the "drip-line" notion of tree roots. On average they found that tree roots extended 3 times the spread of the branches and that more than 50% of the absorbing roots were beyond the drip-line.

Several numbers can be found in the literature concerning where the bulk of fine, absorbing roots are found in the soil profile. The stated depths range from the top 24 inches to the top 18 inches with 50% of the fine roots being in the top 6 inches of the soil profile. Dr. Gilman's research tightens these numbers up considerably. From the same root growth trials in Florida and New Jersey Gilman states: "fine roots are concentrated in the top 12 inches of soil with many in the top 2 inches."

Okay, so some trees have roots near the soil surface to collect nutrients and moisture. "But, I've never watered my tree because its taproot gets all the water it needs from the Saint Peter sandstone aquifer." There are species that as seedlings produce and rely on tap roots, but quite honestly, as "tap-rooted" species mature, tap roots become insignificant in the overall root structure. The bulk of nutrient and moisture uptake is taken over by the absorbing roots near the surface.

Tap roots and other "sinker" roots can penetrate into the soil layers, but research indicates that depth of rooting is dependant on soil-oxygen. Dr. Kim Coder from the University of Georgia states that good root growth requires a soil atmosphere of 15 percent oxygen. He continues by saying that below 5 percent soil-oxygen, root growth will stop and below 2% roots begin to decline and die. Dr. Coder describes the advancing root tips existence as quite precarious and more of a "good news / bad news" scenario. If the root progresses too deeply, oxygen deprivation will be an issue and on the other extreme if a root progresses too shallowly, a dry spell will likely cause its demise. As a matter of fact, this precarious situation translates into a short life span for an absorbing root with the root tip being replaced many times per growing season.

The bottom line, unlike horses, roots do not smell water. Along the same line, roots do not seek water. Instead, roots tend to grow where the growing is good and the growing tends to be good in the top 12 inches of soil where temperature, moisture, nutrients and more importantly soil-oxygen are usually adequate. To underline the fact that roots are opportunistic "absorbers" - there has been a push lately to discourage calling fine absorbing roots, "feeder roots" simply because the term "feeder" implies an aggressive, hunter-gatherer approach to seeking life's necessities. Roots simply follow the moisture gradient of the surrounding soil and continue to grow where the growing is good. Meaning that roots tend to grow where there is a good supply of moisture, nutrients and of course soil-oxygen.

diagram of soil compaction

Now is the time to step back and put this information in perspective. Tree roots require oxygen to remain viable and soil-oxygen decreases the deeper in the soil profile that a root penetrates. Add in soil compaction to your thought process. By compacting a soil the pore space is reduced which in turn reduces the amount of soil-oxygen that can be present. Okay, one more thing - re-landscape the lawn with a bobcat and add 4 inches of soil to bury the roots a little deeper. Hold on, the lawn typically means turf and that means roots from grass and competition for moisture, nutrients, and you guessed it soil-oxygen. The picture should be coming clearer, tree roots, all too often overlooked, spread as far and as wide as they can in compacted soils and at depths below 12 inches trying to survive. Keep in mind that this is biology, and the ultimate root structure depends on many factors below ground. Site conditions, compaction, excess fill, flooding and infrastructure have direct and indirect impacts on soil moisture, soil oxygen, soil texture and structure.

Moving on into the second round of questions: It has happened again! Another basement floods because a sewer line has been clogged by tree roots. Maybe this time it is another foundation or sidewalk being moved or lifted by tree roots. Are trees "perpetrators" of these acts or is it simply guilt by association. One side of the argument states that the tree and its roots are not at fault, but rather there is fault with the design or construction of the infrastructure. Honesty is probably the best policy; trees are not as innocent in the above scenarios as many of us would like to believe. However, why has the tree become the guilty party? Design error is often the only answer to be arrived at whether it is the landscape design or the engineering design. Let's face it, trees require space for rooting and resource exploitation, if a tree root happens upon a nice moist, oxygenated resource-rich environment (a sewer line for example) where it can thrive and perform its duties - it will. In commenting on tree roots in sewers Richard Harris et al. states "the conditions of aeration, moisture, and nutrients are so favorable that it inevitably grows until it clogs the sewer." The question in many minds still remains as to how the root happens upon the sewer line.

Martin Mackenzie of the US Forest Service recently spoke to a conference audience describing the "battering-ram" root that breaks through even the toughest sewer lines and concrete blocks. Of course this statement was "tongue-in-cheek." Roots grow quite slowly and don't have much chance to hurl forward with any great speed. In the case of finding a sewer line there is both speculation and research that explains the encounter. Speculation from Harris et al. is that in some instances sewer line trenches are compacted to a lesser degree than the surrounding urban soil. Thus, homeowners may unwittingly take advantage of this and plant trees in the trench or perhaps the tree roots will encounter this less compacted environment and continue to grow into it unimpeded. Dr. Kim Coder researched the thermal gradients that exist between sewer lines and the surrounding soils which allow two things to happen. One is a moisture condensation layer on the sewer line itself. Secondly, a moisture condensation column develops in the soil layers above the sewer line. Once a tree root encounters this moisture condensation column it grows downward (providing adequate soil-oxygen exists) along the increasing moisture gradient in the soil toward the sewer line. Upon encountering the sewer line, the root tips continue growing towards more favorable conditions, eventually a root tip may find and exploit a crack or fissure in the sewer line. Hence, back to the argument proclaiming faulty infrastructure - "the root tip exploits a crack or fissure in the sewer line."

Once the root tip has entered the line, the real damage to the line can begin. The tree roots tend to form a mass of root tips that slow the flow enough to allow sediments to be trapped, thus clogging the line. Or the root may begin to develop as a woody, perennial root and exert pressures that may crush or burst the sewer line.

Larger, woody, perennial roots exert substantial radial pressure and over time can displace man-made structures. The perennial root increases in diameter by adding a growth layer every year. Commonly, sidewalks are lifted by this type of radial growth. Displacement of structures can be caused by horizontal roots growing under a slab concrete floor or sidewalk or by a vertical "striker or sinker" root growing downward next to a foundation. In any case the root growth is slow and forces can be substantial enough to displace or otherwise damage structures.

Another mechanism by which trees cause structural damage involves the presence of expansive clays. Expansive clays respond to soil moisture changes by expanding and contracting. Man-made structures often rely on the surrounding soils for additional support. If a tree's roots encounter this "support soil" of expansive clay the moisture regime can be dramatically altered by the evapo-transpiration function of the tree. The clay soil type can be dried excessively causing it to contract, thus shrinking away from the structure it is supporting.

The literature identifies several tree species as "culprits" in the "aggressive" root department. The following species are on a "Not Recommended for Planting" list maintained by the Michigan Department of Natural Resources as those having invasive rooting habits: silver maple (Acer saccharinum), white mulberry (Morus alba), white poplar (Populus alba), Eastern cottonwood (Populus deltoides), weeping willow (Salix alba), black willow (Salix nigra), and American elm (Ulmus americana). Is it necessary to "label" these trees as culprits and black list them? A better approach is to pay attention to putting "the right tree in the right place." More and more attention is being paid to tree selection since it is the "key" to a trees long life. Site conditions truly need to be considered prior to starting the species selection process. It isn't always successful to fall in love with a species and then try to fit it in our landscape.

In wrapping all of this together, a number of additional questions come to mind. For instance:

  • What does all of this say for our notion of watering and fertilizing within the drip-line?

Yes, the water and fertilizer helps, but in light of root plates reaching well beyond the crown - consider expanding the applications to a broader area.

  • With a tree's absorbing roots concentrating near the soil surface, how well do the trees and turf co-exist?

Trees and turf are in a competitive battle for resources and let's face it - turf is a tough competitor. From a trees perspective: The more lawn that can be replaced with mulch the better.

  • What does this portray for trees when parking lots, sidewalks, driveways, buildings, roads and other infrastructure are placed in close proximity to trees?

These structures almost always require a compacted base layer to provide additional support for the impervious surface being applied. The needs of the tree and the reach of the root plate are too often ignored leaving trees with reduced stability and with reduced capacity to uptake nutrients and moisture.

There is some good news out there? Recent methods of constructing sewer lines have dramatically reduced the potential for root intrusions. Longer sections of less porous pipe are being used for constructing sewer and water lines. This helps cut down on the conflict between tree roots and sewer lines that often results in a large, mature tree being removed.

Researchers and trials in California are looking at composite sidewalk surfaces that flex, thus giving trees room to grow and more soil to exploit for resources.

Researchers at Cornell University have been testing and working with CU Structural Soils? for some time. These soils are compactable, yet allow tree roots to penetrate. Placing these structural soils in the rooting zone under sidewalks, pavers, parking lots, roads and other structures allows trees to expand root plates farther.

So, next time you look at your favorite tree - give some thought to the root structure. You may not have the, "Oh Baby!" type of reaction, but after reading this article the roots should at least be creeping into the outer recesses of your mind.

For More Information:

Bassuk, Nina. Cornell Structural Soil. Internet. Available 9/25/2001.

Coder, Kim D. Tree Roots and Infrastructure Damage. 9/25/2001.

Coder, Kim D. "Don't Stumble Over Surface Tree Roots" Grounds Maintenance. August 1 1998

Gilman, Edward F. Where are Tree Roots? Extension Service Bulletin ENH 137, Florida Cooperative Extension Service, University of Florida, Institute of Food and Agricultural Sciences. Internet. Available 9/25/2001.

Harris, Richard W., Clark, James R., Matheny, Nelda P., Arboriculture: Integrated Management of Landscape Trees, Shrubs, and Vines. Third Edition. Prentice-Hall, Inc. 1999.

Graphic, ISU Forestry Extension. Tree Roots October 2001. Internet. Available 9/25/2001.

Pool, Bob."With Rubber Sidewalks, Trees Are on the Rebound," LA Times. 14, July 2001. Internet. Available 9/25/2001. http://www.mindfully.org/Plastic/Rubber-Sidewalks.htm.

Randrup, Thomas B, McPherson, E. Gregory, Costello, Laurence R. "Tree Root Intrusion in Sewer Systems: Review of Extent and Costs." Journal of Infrastructure Systems, Vol 7, No. 1, pages 26-31. March 2001.