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Plantings and Septic Systems
What can you safely grow over yourseptic?

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

Now, I'm a 'wannabe', but the best place I have for a garden is thefront "lawn", which is over the leach field of our septic system. (Therest 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 ingood 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 ofeating root crops grown there, but I wonder if it would be okay to putraised beds on top? Or would they be detrimental to the septic system?Thanks!
            ---Jo-Ann;Littleton, MA

A. Thank you, Jo-Ann! Now, toanswer 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 financialsupport—are a great way to get us on. (In the meantime, be sure andlisten online at

Now, the standard advice is that you shouldn't plant anything otherthan grasson a drain field—not because of the human waste, but out of concern forthe field itself. Depending on local regulations, your water table,frost line, etc., the pipes in that field can be located fairly closeto the surface. And Rick Stehower, soil scientist at Penn StateUniversity, explains that plants with deep and/or aggressive rootscould clog and/or bust them pipes up—costing you a lot of money inrepairs, and perhaps bringing some things back into your life you wereexpecting not to see again.  

He adds that the liquid being conveyed by those pipes IS a greatnutrient source (unless you put household chemicals, motor oil or otherfoolish stuff down your drains, of course), especially fornitrogen-loving plants. That's why grass is so ideal, he says—it's aheavy feeder that utilizes lots of nutrients that would otherwise getinto the groundwater, where they can damage the ecology of lakes andstreams.

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

Bryan Swistock, an Extension associate in the School of Forestry atPenn State, agrees that anything shallow-rooted and heavy feeding isfine in terms of protecting the pipes and keeping some of the nutrientsout of the ecosystem, but notes that we're now into "graywater" areasin our discussion. Bryan points out that the legalities concerning useof household wastewater for deliberate irrigation are differenteverywhere, and you might want to see if it's even legal as part ofyour decision making. There are many websites devoted to graywater usewhere you can learn more about the issues in general, and specificthings like the best kinds of laundry detergents and such to use foroptimum 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 shadyareas—especially lettuce, spinach and other greens; and potatoes,onions, garlic and other root crops (the classic "leaves and roots forshade"). You can also get away with some herbs; and even tomatoes inspots that get four or five hours of sun a day.

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

Q. Dear Mike: I am a city girltransplanted to a country home, with a well for water and a septicsystem and drain field for…well, you know what. The field is shieldedby a drainage ditch and trees, so I let the weeds take over (they makethe hill less desirable for the neighborhood kids, who ride their threewheelers over top of it). Well, now the weeds have gotten really talland I was told they could mess up the septic system. I have beenadvised to use Roundup but have heard you say its bad stuff. What doyou suggest I do?  
            ----Mary, inMilton, DE

A.    First, Ihope 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'refacing, your most important task is to stop them. You mention a hill,which might mean you have one of those newer 'sand mound' systems. Butyou also say 'drain field'. Whichever it is, those nasty, noisy, heavymachines could destroy it. Plus, the three wheel versions of thesemachines are notoriously unstable and unsafe; and you don't want theliability of somebody breaking their collarbone—or worse—on yourproperty.  So post a lot of "no trespassing/no motorized vehicles"signs, call the police if they're ignored, and fence the area off ifyou have to.

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

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

Dave Gordon, a retired extension agent from Pennsylvania's ClearfieldCounty (who's so busy as a newly-independent wastewater consultant thathe thinks maybe he shouldn't call himself 'retired' anymore) directedme to The Promised Land of all such information. I'll take you there ina 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 organicmatter; and any you add will just compromise its ability to do soefficiently.  AND it needs to evaporate moisture off the top—so nomulch 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 withwarned that you should NEVER water a mound or drain field—they're thereto get RID of moisture and you'd be working against that function.)

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

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

Including a BIG section on septic, from which we reproduce thesegeneral rules:
"Although your septic tank absorption field generally does not requiremaintenance, you should adhere to the following rules to protect andprolong 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 absorptionfield area, because the roots can get into the lines and plug them.
3.    Do not cover the absorption field with hardsurfaces, such as concrete or asphalt. Grass is the best cover, becauseit 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 tothe Summer, 1999 issue of "Pipeline", one of several publications whoseentire full-text contents (going back MANY years!) are availablehere.  This is a special issue on sand mounds that will tell youeverything you ever wanted to know about the things.  And on page7, there's specific information about landscaping a mound, including alist of flowering plants—like peonies and daylilies—you can put upthere: 

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

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

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

Jennifer tells me she's seen a lot of inappropriate use of the landovertop of drain fields and mounds—from vegetable gardens cleverlydesigned so the rows are in between the pipes of a drain field ("wehave to advise against such things," she notes, agreeing that "no oneis going to say it's safe to do") to pine trees and big playgroundequipment erected on top of sand mounds ("very foolish to do with amulti thousand dollar investment"). She warns that compression of thesoil on top of a mound can have a very negative effect. The pipes thattake the water up to the top of the mound, where it then percolatesback down into the earth, can be as close as 6" to 8" from thesurface—and you'll be in a world of trouble if you clog them up.

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

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