Septic lines and tree roots

Septic lines and tree roots


Hello Bill, Can you tell me anything about copper sulfate and roots in septic lines?

Will that product harm a concrete septic tank if it is poured into it?

I have learned that we have several trees planted close to our leach lines and worry that their roots may invade our leach system if they haven't already done so.

The trees are eucalyptus, mulberry, silk oak and dwarf fruit trees. Which ones are the most invasive?

Any help would be appreciated.

Thanks. — BD

Dear BD, Copper sulfate is not used only to kill roots in sewer lines. The chemical is used for disease control on plants because it can kill bacteria and fungi. The formulations are carefully prepared because of copper’s phytotoxicity — it can damage plants, depending on various factors. It might kill the beneficial bacteria in the septic system too. Conversely, in tiny amounts, copper is a necessary micro-element for plant growth.

I did some checking and found some good articles on the subject —one from Oklahoma Cooperative Extension, “Controlling Tree Roots in Sewer Lines with Copper Sulfate,” and another from University of Georgia, College of Family and Consumer Sciences Cooperative Extension Service, “Septic Tank Maintenance and Care.” They state no warnings about the potential effect of the copper on the concrete septic tank itself and they give some very reasonable-sounding guidelines.

Here are some of the main points about copper sulfate covered in the articles.

• Never put copper sulfate into a septic system through a sink or tub because the copper will corrode the metal pipes. Instead, apply it through the toilet. The ceramic surface is not affected by the copper.

• Copper sulfate has been added to septic tanks in tests without harming the bacterial action in the tanks. The recommended amount is two pounds in a 300-gallon tank no more than twice a year. Most of it will settle in the solids in the tank, not the lateral lines, where tree roots may be the worst. For an initial treatment, flush the copper down the toilet 1/2 cup at a time. Much smaller amounts can be flushed two or three times per year for ongoing maintenance against root invasion.

• Some systems have a distribution box where the lateral lines are connected to the septic tank. The copper could be put in there. It is better to apply small amounts each day for a two-week period rather than a large amount all at once.

With respect to trees and roots, the OSU article says,  “This treatment has yet to cause the loss of a tree or a shrub.” The copper is absorbed only a short distance up the roots, so it is like root pruning. It is not a translocated herbicide that would kill the whole plant. But be cautious, since over-use can kill the bacteria needed for the normal functioning of a septic system.

Other articles mention the product RootX as an alternative. Only professional plumbers can apply this foaming root herbicide. The active ingredient is dichlobenil, an aquatic herbicide that kills roots on contact and allows them to decompose in the septic system.

As far as your tree list is concerned,  mulberry (Morus alba) has the most aggressive root system, eucalyptus would be next, then silk oak (Grevillea robusta), and, finally, the dwarf fruit trees.

You could try to prevent roots from entering the leach lines by installing root barriers in vertical trenches between the trees and the leach lines. Plastic panel root barriers are available, in various depths up to 24 inches, at some of the irrigation supply stores. Beware that roots might grow under a barrier and they can grow over the top if it’s installed too low, or covered with mulch.

 Also of interest, copper screening often is used as a root barrier. When roots grow through the openings in the copper screen they get pinched in close contact with the phytotoxic copper, which kills the tip of the root, inducing it to branch behind the dead tip.

To get a better idea of the necessary depth for a root barrier, you might try digging a few test holes to see how deep the offending roots are growing and then buy the panels in the appropriate size.

Trenching and root pruning is a big subject in itself. Tree health and tree stability must both be considered. To be extremely brief and at the risk of over-simplifying, to reduce the risk of harm to the health of the trees I recommend trenching no closer than about five times the trunk diameter of any tree, when digging on only one side of the trunk.

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

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