How to Build a Berm
A berm may look like a simple, long mound of soil, but careful planning and building are required to make a berm that is stable and has a natural, attractive effect. Berms separate garden areas, screen wind and noise, prevent or direct water runoff and provide privacy. They also provide raised, well-drained growing areas for plants. Because plant roots don't grow deeply into a berm, usually only a berm's surface layer is topsoil, and its lower layers are less expensive materials. Local regulations sometimes apply to berms. So contact your city's and county's government offices before you start building a berm.
Preparing the Site
A berm's slope must be gentle enough to allow plants to grow or for it to be mowed if it is covered in grass. If the berm won't be mowed, then prepare an area of ground as long as the intended berm and four times as wide as its intended height, which provides a 4-to-1 ratio for the slope. If, however, the berm will be planted with grass that will be mowed, then the slope ratio should be 5-to-1, which allows the grass to be mowed easily. For example, to build a 3-foot-high berm that won't be mowed, create a base 12 feet wide. Remove large and woody plants from the area, and dig into the soil surface to break it up. Doing so allows the water to drain freely from the substrate, or lowest layer, into the ground beneath it.
Laying the Substrate
The berm's substrate makes up about two-thirds or more of the berm's depth, and it is laid in roughly the final shape of the berm. Regulations in some areas state that the entire substrate must be clay to provide stability for the berm; in other areas, though, it is legal to use gravel, rubble, asphalt and other inexpensive filler materials for the substrate. Pile the substrate along the center line of the berm, and shape it into the intended final form of the berm. Create an asymmetrical design to give the berm a pleasing, natural effect, such as using slopes that differ in steepness, placing the berm's highest point on one side rather than in the center, and varying heights and curves.
Finishing the Construction
Clay and topsoil form the upper layers of a berm. Prevent the topsoil from washing into the substrate by spreading a 1-foot-thick layer of clay over the substrate, and firm it in place. Spread good-quality topsoil over the clay, making the topsoil layer the depth needed for the plants you intend for the site. A topsoil layer 5 or 6 inches deep supports grass and small perennial plants, and 9 inches is sufficiently deep for larger perennials and many woody shrubs. If you want to grow large shrubs or trees, spread a topsoil layer 12 to 18 inches deep. Smooth the topsoil to the desired form for the berm.
Maintaining the Structure
A berm must be covered in grass, other plants or mulch to prevent heavy rain from washing its topsoil down its slopes. Sow grass seeds or lay sod as soon as possible after creating a berm, or plant the desired plants. Cover bare soil between plants with a mulch that won't run or wash down the slopes; such mulch includes shredded bark and other fine mulches made of long fibers. Berms are very well-drained. So grow plants that thrive in good drainage, and water them regularly so the soil stays moist until the plants are established and growing strongly.