During a recent septic tank pump out, the hauler noticed tree roots in the tank. He suggested adding copper sulphate regularly to the tank in order to prevent tree roots from entering. Is this a recommended method?—B. Helliar, Kingston, Ont.
Not according to any septic expert that we asked. To be clear, your septic hauler’s not wrong. Copper sulphate will absolutely work; it’ll kill the problem tree roots.
But here’s the deal: “Copper sulphate kills roots because it’s toxic to just about everything,” says John Rowse, a consultant and the founder of JRSC, a Vancouver Island wastewater service company. That includes the good bacteria that you want in your tank. Ultimately, copper sulphate “will destroy the biological activity in a septic tank, making it ineffective,” he says. This means that, instead of getting broken down, solids in the tank will be discharged into the distribution laterals, a.k.a. the pipes that run through your leach field. That’s not good.
“Recovering the system’s biological activity after putting copper sulphate into a tank will take a long time,” says Rowse. It could cause your system to fail prematurely—so you might end up trading one bad septic problem for another, potentially worse, septic problem.
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Also, keep in mind, the copper sulphate won’t just sit there in your tank. It’ll get into the environment eventually.
“There’s a pollution component to using it,” says Rob Davis, the owner of EcoEthic in Sunderland, Ont. “The discharge to the groundwater is of concern.” There’s research showing that copper sulphate is toxic to fish and other aquatic creatures, to insects, to birds, to plant life…the list goes on.
Will the amount of copper sulphate exiting your septic tank cause massive environmental destruction? C’mon now. No. But there’s a third strike against the stuff. “It’s just a short-term solution,” says Sara Heger, a researcher and an instructor in the Onsite Sewage Treatment Program in the Water Resources Center at the University of Minnesota. “You’re better off dealing with the problem directly.” This means physically removing the roots from the tank (you’ll likely need to hire experts to do this; it requires specialized equipment), repairing any damage to the tank, and potentially getting rid of the problem tree.
It’s a more complicated, expensive, and time-consuming fix, yes. But permanent solutions usually are.
Got a question for Cottage Q&A? Send it to answers@cottagelife.com.
This article was originally published in the September/October 2025 issue of Cottage Life.
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