Mound septic systems are a popular option in areas with
thin soils. Mounds provide an effective, ecologically sound way to handle effluent
from septic tanks.Many persons are interested in planting vegetable gardens, flowers and flower beds, trees or shrubs on top of their mounds, instead of just grass. At first this seems like a reasonable approach, but consideration of how septic mounds work shows otherwise.
A typical septic tank mound for a house might be 10 ft. wide (3 meters) and 30 to 60 ft. long (10 to 20 meters). This is roughly 25% of the area of the house served.
It is important to minimize the amount of rainfall (and other water) that enters a mound, so its full capacity can be used for disposal of septic tank effluent. Therefore, a mound needs to have a rounded top to shed rainwater. A healthy covering of grass helps to enhance runoff, compared to bare soil.
In order to maximize the amount of evapo-transpiration, a mound needs to be covered with dense healthy plants. Turf grass best fulfills this requirement. A vegetable garden would not, since there would be a lot of bare soil between the garden plants.
In very cold climates, freezing of a mound could be a concern. A layer of grass will serve as an insulator and provide freeze protection for the mound.
Trees and shrubs, if planted on top of the septic mound, will send down roots that can damage the structure (including the geotextile layer) of the mound.
From a wastewater disposal perspective, the best plantings for a septic tank mound is grass on the mound itself and trees or shrubs adjacent to the mound to absorb any excess effluent.