Groundcovers suitable for a septic mound

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Q. We purchased a home last fall that has a sand mound septic system. The previous owner had planted it with grass, but it is very steep and scary to mow. Can you recommend a groundcover that would not interfere with the sand mound's function and that we would not have to mow? If it helps, the mound is in full sun.

A. It is desirable to plant the sand mound because plants absorb excess nutrients and moisture from the soil in addition to preventing erosion. Grass is generally recommended as an ideal cover for sand mound septic systems because it has a relatively shallow, fibrous root system that protects the mound from erosion without having deep roots that could cause problems. Regular mowing keeps the growth low, which allows sun to help evaporate the moisture from effluents in your septic system.

However, there are options in a situation like this.

It is best to avoid trees and shrubs that generally have more extensive root systems that could impact the function of your sand mound, including clogging the pipes. These may be planted around the mound; large, thirsty trees such as a silver maple, willow, elm and poplar should be at least 50 feet away. Most shrubs and small ornamental trees such as dogwood, flowering cherry or crabapple can be planted within 20 feet. Sand mounds are also not the place for your vegetable garden. Besides the obvious risk of contaminating the produce with bacteria, common management practices such as rototilling can damage the mound.

Low-maintenance perennial plants that minimize the need to walk on the mound are ideal. Walking compacts the soil and may interfere with the evaporation of effluents. Drought-tolerant plants that do not require long-term irrigation are preferred because irrigation also reduces the evaporation of effluents and can lead to contaminated runoff. Fortunately, short-term irrigation to establish new plantings is not a problem. Do as little digging as possible when planting to avoid disturbing the mound and be sure to wear gloves to minimize your physical contact with the soil.

Prior to planting, kill the existing vegetation with a non-selective herbicide containing glyphosate (the active ingredient in Round Up) or herbicidal vinegar (20 percent, not what you use in the kitchen) according to label directions. Herbicidal vinegar will require several applications because it is a contact herbicide that is not translocated to the roots as glyphosate is.

Ordinarily, for those who prefer not to use herbicides, I would recommend smothering the area with a tarp, clear plastic or a thick layer of organic matter such as straw, shredded leaves or compost, but because this is a sand mound septic system, those methods would interfere with evaporation that is critical to its function. The dead turfgrass will stabilize the soil and act as a mulch for the new planting.

One planting option that would work well is a combination of the low-growing stonecrops (Sedum spp.) that are recommended for green roofs, including white stonecrop (Sedum album), pink Mongolian stonecrop (Hylotelephium ewersii), Russian stonecrop (Sedum kamschaticum), and Caucasian stonecrop (Sedum spurium). Stonecrop is a tough, durable plant with fleshy, succulent leaves that help make it drought-tolerant. Its foliage ranges from green to blue-green to red, and some varieties have variegated foliage. It also has a wide range of flower colors and bloom times. You may be able to purchase sedum as plugs or in 4-inch pots that require less soil disturbance than larger containers when planting.

Another option is to plant a mixture of warm season, native grasses and sun-loving wildflowers that will turn the sand mound into a small meadow garden. Some of the grasses best suited include prairie dropseed (Sporobolus heterolepis) and little bluestem (Schizachyrium scoparium). These are bunch-type grasses, and they should be planted relatively close together to completely cover the soil.

Buy the smallest plants you can find, again to reduce disturbance to the soil. You may want to cut the "meadow" with a lawn mower or weed whacker and rake off the clippings in late fall. Other than that, no other maintenance is required for either planting option.

Tough, drought-tolerant wildflowers suited for this use include:

'Blue Ice' bluestar (Amsonia)

Pussytoes (Antennaria neglecta)

Butterfly weed (Asclepias tuberosa)

Sand coreopsis (Coreopsis lanceolata)

Purple coneflower (Echinacea purpurea)

Blazing star (Liatris spicata)

Wild bergamot (Monarda fistulosa)

Black-eyed Susan (Rudbeckia spp.)

'Fireworks' rough goldenrod (Solidago rugosa)


Send questions to Sandy Feather by e-mail at slf9@psu.edu or by regular mail c/o Penn State Extension, 400 N. Lexington Ave., Pittsburgh 15208.

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