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Plantings and Septic Systems
What can you safely grow over your septic?

Q. Mike McGrath! Organic Gardening magazine has been a mere shadow of its former self since you left. How great to find you again! Unfortunately, WGBH, my local public radio station, does not carry You Bet Your Garden (sob!).

Now, I'm a 'wannabe', but the best place I have for a garden is the front "lawn", which is over the leach field of our septic system. (The rest of the yard is shaded much of the day, so not a good candidate fora vegetable garden.) It is important to us to keep the septic system in good order, so we are very careful about what we put down the drains, and we have the tank pumped every few years. I don't fancy the idea of eating root crops grown there, but I wonder if it would be okay to put raised beds on top? Or would they be detrimental to the septic system? Thanks!
            ---Jo-Ann Littleton, MA

A. Thank you, Jo-Ann! Now, to answer the most important part of your question first:
Call or write your local station and tell them to pick up our show! Such requests—especially from members who make pledges of financial support—are a great way to get us on. (In the meantime, be sure and listen online at

Now, the standard advice is that you shouldn't plant anything other than grass on a drain field—not because of the human waste, but out of concern for the field itself. Depending on local regulations, your water table,frost line, etc., the pipes in that field can be located fairly close to the surface. And Rick Stehower, soil scientist at Penn State University, explains that plants with deep and/or aggressive roots could clog and/or bust them pipes up—costing you a lot of money in repairs, and perhaps bringing some things back into your life you were expecting not to see again.  

He adds that the liquid being conveyed by those pipes IS a great nutrient source (unless you put household chemicals, motor oil or other foolish stuff down your drains, of course), especially for nitrogen-loving plants. That's why grass is so ideal, he says—it's a heavy feeder that utilizes lots of nutrients that would otherwise get into the groundwater, where they can damage the ecology of lakes and streams.

I suggested that other shallow-rooted nitrogen-loving crops—like say, lettuce—would also take up lots of those nutrients. His response echoed that of every other expert I spoke with: No one in their right mind is going to risk the liability of telling you to grow food crops over aseptic system. AND he adds that it's also just plain impractical—people should stay off that area as much as possible to avoid damaging the pipes.

Bryan Swistock, an Extension associate in the School of Forestry at Penn State, agrees that anything shallow-rooted and heavy feeding is fine in terms of protecting the pipes and keeping some of the nutrients out of the ecosystem, but notes that we're now into "gray water" areas in our discussion. Bryan points out that the legalities concerning use of household waste water for deliberate irrigation are different everywhere, and you might want to see if it's even legal as part of your decision making. There are many websites devoted to gray water use where you can learn more about the issues in general, and specific things like the best kinds of laundry detergents and such to use for optimum plant-growing results.

Now the good news: I am personally 'blessed' with a great lack of sun, and have learned that you can grow quite a few foods in somewhat shady areas—especially lettuce, spinach and other greens; and potatoes, onions, garlic and other root crops (the classic "leaves and roots for shade"). You can also get away with some herbs; and even tomatoes in spots that get four or five hours of sun a day.

And maybe you can remove and/or prune a fading tree or three to create more sun. Or join a nearby community garden; they're a great alternative for people with challenged landscapes—and a fabulous way to spend time with other folks who share our…Passion? Addiction? Self-Delusion?

Q. Dear Mike: I am a city girl transplanted to a country home, with a well for water and a septic system and drain field for…well, you know what. The field is shielded by a drainage ditch and trees, so I let the weeds take over (they make the hill less desirable for the neighborhood kids, who ride their three wheelers over top of it). Well, now the weeds have gotten really tall and I was told they could mess up the septic system. I have been advised to use Roundup but have heard you say its bad stuff. What do you suggest I do?  
            ----Mary, in Milton, DE

A.    First, I hope you're talking about little kids riding those "big wheel" toys. Because when someone says "three wheeler", I think of those giant, ill-advised gasoline-powered habitat destroyers. If that IS what you're facing, your most important task is to stop them. You mention a hill, which might mean you have one of those newer 'sand mound' systems. But you also say 'drain field'. Whichever it is, those nasty, noisy, heavy machines could destroy it. Plus, the three wheel versions of these machines are notoriously unstable and unsafe; and you don't want the liability of somebody breaking their collarbone—or worse—on your property.  So post a lot of "no trespassing/no motorized vehicles" signs, call the police if they're ignored, and fence the area off if you have to.

Now—like we said, you want to keep nasty roots out of drain field pipes—and sand mounds. If it's just grassy weeds and shallow-rooted plants growing there, I'd leave it alone—as you say, the tall brush is a good deterrent. But if you see anything like a real tree, or kudzu, bamboo, thistle or other member of "America's least wanted weeds", wipe them out and plant a nice lawn in their place. You'll find a lengthy list of organic options to frog-killing, human reproduction-harming Roundup in this recent Question of the Week.

Now, if you DO have one of those newer sand mound systems instead of an old style drain field, you have some really interesting landscape options beyond simple grass.

Dave Gordon, a retired extension agent from Pennsylvania's Clear field County (who's so busy as a newly-independent waste water consultant that he thinks maybe he shouldn't call himself 'retired' anymore) directed me to The Promised Land of all such information. I'll take you there in a moment—but first, his thoughts on the matter…

"Don't ever work organic matter into a sand mound to help grow things, "he explains, "because the purpose of the mound is to get RID of organic matter; and any you add will just compromise its ability to do so efficiently.  AND it needs to evaporate moisture off the top—so no mulch either." (Which makes weed control of crop plants a real problem.And it can get REAL dry up there, AND all the experts I spoke with warned that you should NEVER water a mound or drain field—they're there to get RID of moisture and you'd be working against that function.)

"Grass is perfect for a sand mound, because it removes some of the excess nutrients, while preventing any deep-rooted weeds from establishing themselves. And a well constructed mound can be mowed and the clippings removed," he adds. And as all good little organic gardeners know, those clippings make for great compost pile ingredients, and nothing beats dried clippings as a weed-suppressing mulch (AND when used as mulch, they add lots of nutrients to the soil as they slowly decompose).  So 'harvested' grass from your mound would be far from a gardening waste of space!   

And alternative non-edible planting ideas abound at that "Promised Land" I hinted at earlier—The National Small Flows Clearinghouse at West Virginia University in Morgantown. (I know—it could be information about prostate problems instead, couldn't it?)  Dave directed me to this wonderful resource, which he calls "an EPA Clearinghouse for all things septic".  There's a toll free number (1-800-624-8301) you can call to speak to experts about any waste water management topic big or small, and they have a huge amount of information on-line at:

Including a BIG section on septic, from which we reproduce these general rules:
"Although your septic tank absorption field generally does not require maintenance, you should adhere to the following rules to protect and prolong its functional life:
1.    Do not drive over the absorption field with cars, trucks, or heavy equipment.
2.    Do not plant trees or shrubbery in the absorption field area, because the roots can get into the lines and plug them.
3.    Do not cover the absorption field with hard surfaces, such as concrete or asphalt. Grass is the best cover, because it will help prevent erosion and help remove excess water.
4.    Do divert surface runoff water from roofs, patios, driveways, and other areas away from the absorption field."
And now, the payoff you've been waiting for! Here's a direct link to the Summer, 1999 issue of "Pipeline", one of several publications whose entire full-text contents (going back MANY years!) are available here.  This is a special issue on sand mounds that will tell you everything you ever wanted to know about the things.  And on page 7, there's specific information about landscaping a mound, including a list of flowering plants—like peonies and daylilies—you can put up there: 

Just be aware that this list comes from the University of (chilly!) Minnesota and that the plants recommended may not be appropriate for your growing zone. But if you show it to someone in your local county extension, they can use it to make suggestions for your climate.

I also decided to test their 800#, and spoke with Jennifer Hause, who reaffirms the Pipeline info that the best plantings are various types of grasses and low-care perennial flowers with relatively shallow root systems. But this is not a 'one size fits all' list or answer by far, she warns, explaining that every system is different, and regulations governing them vary state-to-state (and often within states).

And she adds that everyone should try and find out more about their specific system—especially how much room there is between the pipes and the surface.  A professional can figure it out quickly with a probe (so can you—just carefully insert a metal probe down in the area till you tap a pipe and see how deep it is; in a typical system, the pipes will be three feet apart).

Jennifer tells me she's seen a lot of inappropriate use of the land overtop of drain fields and mounds—from vegetable gardens cleverly designed so the rows are in between the pipes of a drain field ("we have to advise against such things," she notes, agreeing that "no one is going to say it's safe to do") to pine trees and big playground equipment erected on top of sand mounds ("very foolish to do with a multi thousand dollar investment"). She warns that compression of the soil on top of a mound can have a very negative effect. The pipes that take the water up to the top of the mound, where it then percolates back down into the earth, can be as close as 6" to 8" from the surface—and you'll be in a world of trouble if you clog them up.

You Bet Your Garden   Question of the Week  ©2005 Mike McGrath