Design & DIY

Cottage Q&A: Will these plants destroy my septic tank?

A wild raspberry bush By Vics Photos/Shutterstock

Our sewage tank is made of fibreglass and is more than 30 years old. To this point, there have been no leaks, and we get it pumped out year-round. There’s a growth of wild roses and raspberry plants over the tank. I pulled them out last fall; there were very large roots. Is there a danger of the roots penetrating the 1,000-gallon tank? Pretty scary as we’d have to replace an otherwise functioning tank.—Larry Lee, Flanders Lake, Man.

This would depend on how deep the tank is buried and whether the roots of the wild roses and raspberries could actually reach it. The first variable is knowable. But the second one? 

“There is a lot of variation in terms of how different root systems behave,” says Lorraine Johnson, the co-author of A Garden for the Rusty-Patched Bumblebee and other books on native plants. It can depend on factors such as the type of soil that they’re growing in, for example. Still, “I’m not sure that I’d call the root systems of wild rose or wild raspberry particularly large—though it varies with the species,” she says. “But I’d be surprised if the roots of a shrub could actually break through the fibreglass walls of an intact tank.”

What’s the best thing to plant on a septic bed?

Roots are typically more of a concern with a leaching bed than with a septic tank, says Bill Goodale, a septic system inspector with Ontario’s Township of Tiny. (Roots can interfere with the pipes in the leaching field.) But over time, it’s possible that the roots could get through the lid or the seams of a tank, he says. Or, they could take advantage of weakened spots or breaches, adds Johnson. 

If you want to err on the side of caution—and, since you’re asking the question, you probably do—it’s best to remove them, says Goodale.

Cottage Q&A: Are ant nests a problem for the septic bed?

A super-safe alternative groundcover would be a shallow-rooting, native grass mix, says Sara Heger, a researcher and instructor in the Onsite Sewage Treatment Program at the University of Minnesota. “We don’t recommend anything with a woody stem or root be planted over the septic tank or drainfield.”

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This article was originally published in the May 2023 issue of Cottage Life magazine.

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