Tree roots and septic lines

2008-10-18T00:00:00Z Tree roots and septic linesBy Bill Pramuk
Trees & People
Napa Valley Register
October 18, 2008 12:00 am  • 

For reasons I do not fully understand, sewer lines are most likely to get clogged when one is having a big gathering at the house. Sometimes it’s just because a well-meaning volunteer in the kitchen put two pounds of potato peels down the garbage disposal instead of putting them in the compost bucket. But it may also be because a faulty sewer line is filling with tree roots that are just doing their job: growing wherever conditions are suitable.

 Since I’ve just received  a couple of questions on the subject, and the season for entertaining house guests is coming soon, let’s take a look at tree roots and septic lines.

 Here are some examples:

 “… we have two septic lines in our back yard. I’d love to plant a tree for some shade near the grass,  but the septic guys said it’s not a good idea to get too close, for future root issues. Will any trees work better than others,  maybe 15 feet away from septic lines?” J.K.

 “One of our (valley) oak trees has sent roots into and around the sewer pipe, completely encircled the wax ring and loosened the toilet! The research I’ve done so far says to use copper sulfate or herbicides to poison the soil without killing the tree as a long term solution. We plan on getting Roto Rooter out to use a camera in the sewer line, use a cutter for the roots in the line and hopefully have the trenchless reline process done. Naturally, my preference is not to poison anything, much less kill the tree. Do you have a recommended approach?”    E.B.

 One of the great rules of thumb about tree roots is that they require equal amounts of water, soil and air. That’s why they are so often found thriving near the soil surface, under sidewalks, in septic leach fields, and in sewer lines. They follow a path of least resistance and if they find those three components, they just keep growing. They don’t “seek” those conditions. They simply respond. The path can be the relatively porous backfill soil in a sewer line trench, or the interior of a sewer line if it is breached, even a tiny bit.

 When roots following a trench grow alongside a sewer pipe they exert a tremendous force as they expand in diameter. That force can displace pipes and rupture pipe joints. Root tips finding the tiniest opening may enter and rapidly proliferate.

 The old “Orangeburg” asphalt impregnated fiber pipes are most susceptible to this sort of damage. Its use was discontinued in the 1970s, when PVC pipe replaced it. So, 40 year old homes may still have the the old-style pipe, approaching its 50-year life expectancy. 

 In contrast to the expansive force of roots, it has been shown that growing root tips are easily deflected by obstacles. One study showed that a single 4 ml. thick (4/1000th of an inch) can deflect a root from its direction of growth. 

 If they are in good condition, the pipes in and under your house and leading to the city sewer system are tightly sealed, so they easily deflect root tips. In contrast, the perforated lines in a septic leach field are an open invitation to plant roots. The same goes for perforated flex drain pipe that is sometimes used in backyard drainage systems. Not long ago I saw an example of this sort of pipe completely filled with fine roots from a coastal redwood tree growing nearby.

 Regarding tree selection for planting near a leech field, it’s easier to find “don’ts” than “do’s.” Typical worst offenders include willows, poplars, silver maple, elms, birches beeches and fruitless mulberry trees.

 Recommendations I have turned up include: Dwarf fruit trees (with their smaller root systems) and other small trees such as dogwoods. Medium size trees should be planted 10 or more feet away from leach lines. A few larger trees mentioned in one reference as less troublesome around leach lines include red, scarlet and white oaks.

 Various barriers are available, such as the interlocking root barrier panels and Biobarrier, which is a geotextile fabric impregnated with a time released herbicide. These might be used in some situations to exclude tree roots from the vicinity of a leach field. 

 An intact PVC sewer line is a good barrier in itself.

 As I think about it,  the case, above, of the valley oak root loosening the toilet is the only one in memory where that species caused a problem with a sewer line. It sounds like a classic example of roots entering a breached line and following it to a dead end. If you think about it, you’ll notice that the oak roots didn’t grow up through the water into the bowl. They need air! It’s also a reminder that roots don’t grow straight down, nor stop at the edge of the canopy. They grow wherever they can.

Bill Pramuk is a registered consulting arborist. Visit his Web site,, e-mail questions to or call him at 226-2884.

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