What to plant on a septic leach field?

Various places on the internet suggest that you should never plant on your leach field, or only plant things with short roots, or only plant things you won’t eat. Can anyone share experiences of what they grow on their leach field?

Our leach field is in (NZ) native bush, so lots of tree ferns, shrubs, etc. They seem to be absolutely thriving, although we’re unlikely to eat anything down there. The inhabitants (pheasants, pukekos, quails) seem pretty happy too.

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Friends have had satisfactory results with grass seed mixes formulated specifically for drainfields. I would be inclined to add wildflowers that can thrive in moist soils, but I would be wrong. Although we’re in Central Texas, one of my friends was happy with the general advice from New Moon Nursery, in New Jersey. Even though our climates are hardly the same, they found that many of the specific suggestions worked and that the general information helped them figure out what local plants would also work well. For example, the general information tells why my inclination is wrong.

Still, as you can see from those sources, you aren’t limited to putting a shallow-rooted turfgrass over the area. You can make it provide great value to the ecosystem.

I’ve seen some sources recommend trees that are deemed safe for planting near sewer lines, but to me that’s an apples-and-oranges comparison:

  • A sewer line is sealed when built, but seals can be imperfect or cracks can form over time. The most troublesome species—maples, for example—can work fine roots into those cracks and fill the line with a fibrous mass.
  • A drainfield, though, has large holes by design. The solids settle out in the septic tank, and as water levels rise the wastewater drains out through pipes that are perforated with holes half an inch or so in diameter. Even trees that are less troublesome around sewers will be able to penetrate those holes easily. Roots grow where beneficial microbes lead them to water, oxygen, and nutrients. If your system is working properly, it will be an excellent source of all three.

Of course, at some distance from the drainfield you will be adding trees and shrubs. Having perused some of the online information, I would add only one grace note. They recommend dogwoods for the trees you plant nearer the field. Be careful, though. Roughleaf dogwood (Cornus drummondii) and gray dogwood (Cornus racemosa) don’t have the same type of root systems as other dogwoods. They form colonies by root suckering. I think that would make them poor choices for the plants you put nearer the drainfield.

As for the admonition about planting only things you won’t eat, I wouldn’t be concerned from the standpoint of eating aboveground crops, but I would be concerned about the impact on the drainfield’s function. A dense network of roots above the drainfield will provide an excellent setting for decontaminating the drainage. Crops won’t generally have that dense network of roots, if only because of the space we put between plants for access to them. Even if you cover that ground with mulch, that protects from direct contact with the wet (or damp) soil, but doesn’t provide that decontaminating environment.

One final note, even though I’m sure you already know this: You’ll see various products you can put into sewer pipes to clean out the roots. But even the seemingly less toxic of those—copper sulfate, for example—would be toxic to the microbes that make your septic system work. So your watchword is prevention, not removal.

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