06986 Revised 2008
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This publication is meant to be used in conjunction with the Septic System Owners Guide, Item # 06583 which further explains the site requirements for placement and location of a mound system. See back page for ordering information.
Landscaping near and around septic systems is of concern to many homeowners. Whether drainfields or mounds, Minnesota septic systems must have a minimum of three feet of unsaturated soil between the drainfield or point of infiltration and limiting soil condition such as hardpan, bedrock or saturated soil in order to properly treat sewage. A mound system is required if three feet of separation cannot be achieved with an inground trench system. Mound systems are designed to maximize the absorption capacity of existing soil (see Figure 1). Mound location, size, shape, construction procedures and maintenance of the mound all interact in determining how well the system will function. Placement of plants on and near the mound must be done with care to ensure a properly functioning septic system. Plants enhance the system by removing moisture and nutrients from the soil and providing cover to prevent erosion.
Figure 1. Cross section of a mound system.
A rectangular mound rising 18 inches to 5 feet above the surface of a relatively f at home landscape may be quite noticeable and possibly very unsightly. Careful design planning before the mound system is installed can help to create a sustainable landscape with lower maintenance costs, greater environmental benefi ts, and much higher aesthetic value. Mounds can be placed to suit individual landscaping and lot size needs (see Figures 2 and 3). Properly landscaped areas around the mound can serve as privacy barriers, windbreaks for homes and a screen from unsightly views.
Figure 2. A and B represent possible placement of mound septic systems.
Figure 3. C and D show additional mound system locations.
As a homeowner considering the placement of a septic mound, know your options. The earlier in the process that you become involved in expressing your preferences, the more options you will have. For instance, before the house is designed or built, potential septic locations, along with soil borings and percolation tests, can give some control over the placement and fi nal outcome. State and local laws set distances from wells and lakes, based on well casing depth and lake classification. Property lines (PL) or rights-of-way set backs are usually 10 feet (see Figure 4). Additionally, on even a slight slope, it is paramount that the mound rockbed be on the contour for proper operation of the system. Soil must not be moved to create a different slope, as this reduces the ability of the soil to accept the effluent. Once the drainfield or mound is determined, protect it from compaction and disturbance to ensure proper sewage treatment.
When planning a landscape, there should be an invisible or concept line the eye follows around the yard (see Figures 2, 3 and 4). Where space allows, this line is a graceful curve which represents the division between two different types of plant material (turf or ground cover versus shrubs) or between plants and hard goods (turf, patio or deck). The concept line becomes a real bedline where the lawn ends and the shrubs begin. If your property is large enough to plan an unbroken turf area as the fi rst priority, then place the septic system beyond the bedline disguising it as part of a shrub or perennial area or screening it with trees or large shrubs. If your property is smaller, blending the mound into the overall landscape is still the key. Try moving the focal point from the mound to other plants or features. Extending the actual berm with additional topsoil at the same height and curving the perimeter, then sloping down and away can conceal the existence of the system (see Figure 4). If the drainfi eld must be located near a drive, patio, or walkway, retaining walls may be used to save space, but only on the uphill side of the berm. Connect the extended berm to another large feature in the yard, such as a pool enclosure or other fenced area, using retaining walls for transition. ALWAYS design with the entire yard or viewing area in mind—connect it to the whole picture.
Figure 4. Model landscape plan for mound septic system.
It is very important that the integrity of the mound be kept intact and that soil does not wash away. A permanent vegetation cover is required to minimize topsoil loss. Open sites are more susceptible to frost, heaving and erosion. Plants trap snow which acts as a mulch and prevents erosion.
Figure 5. Pasqueflower is a native wildflower that tolerates dry soil conditions.
The following native prairie plants grow well on dry soils and would be good choices for a mound septic system:
These plants are propagated by seed or plants. A combination of both will make a faster cover. Use a mulch of clean straw or a cover crop of annual ryegrass or oats to prevent erosion while the plants become established.
Low maintenance lawn grasses, such as fine fescues, can make a dense cover and only need to be mowed once or twice a year. Mow in October and late June to reduce weeds. Fescues are traditional lawn grasses that tolerate dry soils and shady sites. A mixture of fi ne-textured fescues, such as creeping red, hard and sheep’s fescues (Festuca rubrum, Festuca longifolia, Festuca ovina), in equal proportions can be seeded at the rate of 3.5 pounds per 1000 square feet. Traditional lawn grasses, such as common Kentucky bluegrass and perennial ryegrass, can be planted on a mound and regularly mowed. Mowing will increase evaporation from the mound and aid in rodent control.
Perennial flowers, such as daylilies and peonies, can be grown; however, extra care must be taken to mulch or plant close together so soil will not be exposed and erode. Low maintenance plants that do not need tending and care (remember minimal traffic on the mound) are best. Enjoy from afar, and do not walk on the mound.
Proper selection and placement of plants best suited to the site means that those plants will be healthier, more attractive, and have fewer pest problems.
For landscape design publications, refer to SULIS.
For more information about septic systems, refer to UMN Onsite Sewage Treatment Program website.
In accordance with the Americans with Disabilities Act, this material is available in alternative formats upon request. Please contact your University of Minnesota Extension office or the Extension Store at (800) 876-8636.