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

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We have a new septic bed that we need to plant. Is a lawn the only option?

You won’t find many experts sticking up for the mowed lawn as cottage landscaping: fertilizers and weed killers are bad for the lake. Sara Heger is an exception, and even she’s not suggesting a manicured lakeside putting green. Heger teaches and researches onsite sewage treatment—as in septic systems—at the University of Minnesota. 

The health of the septic tank and the drainfield, where liquid effluent seeps out of pipes for aerobic soil bacteria to finish treating it, depends in part on what’s growing on top, she says. “A lot of people won’t like this answer,” Heger says, “but what I recommend is grass.” A mix of native grasses such as fescues is a good choice—it ticks off all the boxes. Just make sure it is shallow-rooted. Deep roots can block the pipes and even grow into the septic tank, though that’s more likely to happen with old concrete tanks. That means no trees within six metres of the drainfield (15 metres for poplar, maple, willow, and others with aggressive roots that seek water reservoirs).

Grass also works because it’s drought-resistant. The liquid effluent in the pipes seeps down, and the porous soil on a properly built septic field means the surface is often dry. Grass also establishes itself quickly and holds a blanket of snow in winter, protecting the soil from erosion and the pipes from freezing. 

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What about a deck over the septic field instead of grass? “I’m glad you asked,” says Heger. “No! No decks, no above ground pools, no structures. And no vehicles on it.” Even a kids’ play structure isn’t advisable. Anything that compacts the soil or hinders erosion-fighting plants could damage the system, she says, and a structure that blocks access will probably have to be torn down anyway, if the septic tank or drainfield needs repairs.

But there are ways to improve a septic system’s appearance. Some installations come with an array of pipes sticking up from the septic field: access ports for cleaning, clog-clearing, and checking for effluent ponding. Septic installers often leave more pipe than necessary poking up, so that construction activities that follow don’t accidentally bury or damage it. “They’re hideous,” says Heger, but you can bury a landscape irrigation box (used for sprinkler systems and available at building centres), at ground level, around the pipe. Lop off the pipe and cap it within the box. Now it’s protected, easy to find, and hidden. 

This was originally published as part of “Back to Beautiful” an article in the Jun/July 2021 issue of Cottage Life.

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