Perfect plants to place over septic systems
by Jacqueline Soule
American humorist Erma Bombeck wrote "The grass is always greener over the septic tank." Like so many things, she was right about this, too.
Here in the Southwest, not only is the grass greener, but so are any plants near the septic tank, leach field or city sewer drain pipes — all places where extra soil moisture and nutrients provide a real boost to plant growth. Since Southwestern plants are especially good at seeking water, this makes them especially problematic around septic systems. But with some planning, you can have both plants and a trouble-free sewer system.
Wildflowers, smaller ornamental grasses and non-woody perennials are fine to plant over a septic system. These are plants with shallow roots, which will not invade system piping. In fact, such plantings are helpful because they stabilize the soil surface and don't interfere with soil transpiration.
Soil transpiration around a septic system is the movement of oxygen into the soil to aid in the process of breaking down matter and the evaporation of leachate or extra septic moisture. But planting should not be so dense or with a species that will interfere with soil transpiration or evaporation of leachate. Avoid large ornamental grasses like pampas grass or bamboo.
Vegetables fit the bill for shallow rooted plants, but are they safe to eat after growing near a septic system? Debate rages over this topic. The most-often cited study against the practice looked at raw sewage used on vegetable fields in a Third World nation. Our American septic systems are underground and are designed to eliminate biohazards. If "healthy" pathogen-free sewage enters your system, then your soil and the products of it should be healthy as well.
Trees are a problem. Trees on top of or even near septic tanks, sewer lines or in leach fields are to be avoided. Actual tree to septic system distance depends on the tree variety and its normal range of root growth. The rule of thumb is to keep a distance equal to the anticipated height of the tree at its maturity plus 20 percent. Thus, a tree 30 feet tall at maturity should be kept 36 feet away from your system.
In the Southwest, there are some trees that are considered safe near septic systems. Select from fibrous-rooted or deep-rooted trees, such as American acacias, cherry, crabapple, dogwood, hemlock, ironwood (Olneya), oak, olive, palms or pine. Avoid trees with shallow invasive roots, such as Australian acacias, ash, beech, birch, cypress, elm, eucalyptus, maple, mesquite, mulberry, pepper tree, poplar, sycamore and willow, including desert willow (Chilopsis).
If roots have already invaded your drain pipes or septic system, you have a problem. Avoid using chemical treatments. Many are toxic, contaminate the environment and are illegal in some jurisdictions. Even if you kill the plant producing the roots, the roots are still in the system, clogging it. Often pipes are not only clogged, they're also broken or dislocated. After costly chemicals, you may still end up digging up the clogged line to replace it.
One final reminder: No matter how well you care for your system, maintenance will be required. Avoid placing valuable plants too close to the drain pipe clean out or the septic tank cover. Plants may be damaged or destroyed if you have to excavate to open the system's cover.
Jacqueline Soule is a garden writer based in Tucson, Ariz. She has lived and gardened in almost every U.S. Department of Agriculture zone from 2A to 9B. Everywhere she's lived, she's striven to make her yard a haven of serenity.