Destructive Root Systems
By Tony Tomeo


I hear it all the time; trees blamed for plumbing problems. Sometimes I think it is just an excuse to remove an unwanted tree that is otherwise causing no problems. However, I sometimes think people actually believe that roots actively seek and destroy sewer pipes and main water lines on purpose. In reality, such violation of utilities serving contemporary buildings is very rare.

Roots may be very destructive to surface features, such as concrete or asphalt pavement, irrigation plumbing and occasionally, foundations of buildings. Hence the need for root pruning and the installation of root barriers. It is often falsely assumed that this aggressive behavior occurs deeply below the surface as well.

However, the majority of the root system of most trees within irrigated landscapes is in the upper eighteen inches of soil. This is why trees show little regard for surface features within their limited volume of soil. Roots that extend deeper are mostly support roots that do not expand nearly as rapidly as those closer to the surface. Trees that are not in irrigated areas, particularly those that are native to and consequently adapted to dry climates, extend roots considerably deeper for protection from desiccation that might occur in upper strata. These deeper roots may be somewhat dormant during wet winter weather while the deeper strata is saturated, but may become active as upper roots begin partial dormancy as surface soil dries. Such root systems are only rarely damaging, even to surface structures, because they are dispersed throughout a considerably larger volume of soil.

It is possible for utilities to be crushed by the weight of very large trees, but such instances are rare. Most contemporary plumbing is very resilient and does not crush easily. Another rare possibility is that roots may wrap around or divide at a pipe with one root above and another below. As the roots expand, the pipe can be crushed. Although this is relatively common among irrigation systems that are near the surface, sewer and water pipes are almost always below the strata most densely occupied by roots, as modern building codes require.

There was of course a time when roots commonly damaged utilities, particularly sewer systems. Plumbing of older buildings may not be deep enough to be below aggressive root systems. Sewer pipes were composed of clay segments or iron pipe that were not sealed. Sewage leaking into surrounding soil was not a concern, but roots easily penetrated the pipes through the joints. Once inside, roots expanded, often at an accelerated rate because of regular ‘irrigation’. This problem can be much more severe in septic systems not only because of the volume, but because it is not as immediately apparent as a clogged sewer and may not be discovered until considerable damage has been done.

Another problem associated with clay pipe is that it is porous and typically and regularly moist. This makes it more attractive to roots than surrounding soil in which moisture fluctuates. Contemporary plumbing is typically PVC or a related material. Because it is sealed and not porous, roots perceive it to be inert and are therefore not interested in it.

Many species will not extend their roots into unsealed sewers where they can be burned by detergent, soap or ‘other stuff’; but many others thrive. Riparian species that prefer or are tolerant of swampy conditions are typically more problematic. These include, but are not at all limited to poplar, willow, birch, alder and ash.

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