Landscaping berms are mounded hills of soil that can serve multiple functional and aesthetic purposes in the garden. A berm can be placed to block or change the flow of water across the property. It can prevent water from draining back toward a house foundation or prevent water from puddling in the wrong place. In an area of heavy soil, a berm can improve drainage to accommodate plants that require faster-draining soil.  A berm can also improve an area of poor soil. Berms can be used to form “islands” of landscaped areas within larger and mostly un-landscaped areas.

An important aesthetic purpose for a berm is to add height and interest to an otherwise flat landscape. Other aesthetic purposes include filling an empty spot, camouflaging an eyesore, creating privacy, and framing a view. 

Building a berm in your yard is a good do-it-yourself project, as long as you keep the berm to a manageable size.  First, decide where you want to place the berm, taking into account how it might affect the movement of water across the space. Then determine the size and shape you want for the berm. An asymmetrical shape is generally more pleasing than a circular one. A good rule of thumb is to make the berm about four to six times longer than it is wide. A kidney shape works well for a berm at a corner of the yard. The berm should not be more than 18 to 24 inches tall, as a higher berm might be subject to erosion. Use a garden hose to lay out the size and shape of the berm. Make larger curves rather than tight turns.

Once you are satisfied with the overall size and shape of your design, remove the plants or grass within the berm site.  Dig up the lawn along the edge of the berm, turn the pieces over so they are grass-side down, and use them to start building up the berm.  Use fill dirt or inorganic fill for the base and topsoil for the top of the berm. Inorganic fill can include gravel, rocks, or concrete rubble. (You can certainly use topsoil for the entire berm, but doing so would be more expensive and not necessary). If you are using fill for the base layer, keep the fill 18 inches away from the outer edge. This outer area will be part of the topsoil layer. Use dirt to fill in spaces between pieces of rubble and to mound the berm in the desired shape. Be sure to tamp down the surface frequently. Firm tamping will help prevent erosion and reduce the chance of the surface slumping. Keep adding fill until the berm is about half the height you desire.  At this point, switch to using topsoil, mounding, shaping, and tamping as you go. If you want to add large stones or boulders to your berm, they should be buried about one-third of their height in the soil, as they would be found in nature. Having one or two peaks at the top of the berm can also add visual interest.

Even a small berm can require quite a bit of topsoil. There are online soil calculators that will estimate the amount of soil required based on the length, width, and depth of the berm.  The calculator can determine the amount of topsoil needed in cubic yards or by the number of bags of soil. Topsoil is sold by the bag at home improvement centers, but for larger amounts it's more practical to go to a store specializing in landscaping materials.

If the berm isn't very tall and the slope is gradual, edging usually isn't necessary. If the berm is taller or has a steep slope, you may need a retaining wall to help keep the berm in place. A short wall of flagstones, dry stacked three to four deep, is all that's required.  Or you may want to use stones or bricks to separate the berm from the surrounding area, even if they aren't necessary. On the other hand, you might prefer a more natural transition from ground level to the berm.

Once you have constructed the berm, it's time to add plants. Put taller plants at the back, especially if you are trying to create privacy or screen an undesirable view. Shorter plants work best on top and down the sides. Let trailing plants spill over rocks or trail downhill. Ornamental grasses work very well in a berm as their long roots can resist erosion and help hold soil in place. Select plants that look natural together: for example, cactus would look out of place planted next to a rose bush. Research the size of plants at maturity so you leave enough space between plants for them to grow without crowding each other.

The last step is to cover the berm areas surrounding the plants with either inorganic or organic mulch. Inorganic mulch includes gravel or small stones. Organic mulch is generally some kind of bark (nuggets or shredded) or compost. Mulch will help control erosion, reduce weed growth, and keep the soil cooler and moister in the summer.  It will also give the berm a more finished look.

Get inspiration for your berm garden design from the two berm gardens at the Master Gardeners Demonstration Gardens at Patrick Ranch (10381 Midway, Durham). The Berm Garden is just east of the Visitor Center. It has a retaining wall about three feet tall surrounding most of the garden, with the soil sloping to ground level on the open side. The Native Plant Garden is mulched with red lava fines (particles), against which the colors of the native plants “pop” when they are in bloom. The Demonstrations Gardens are free and open to the public on Saturdays and Sundays from 11am to 3pm.

UC Master Gardeners of Butte County are part of the University of California Cooperative Extension (UCCE) system. To learn more about us and our upcoming events, and for help with gardening in our area, visit our website. If you have a gardening question or problem, email the Hotline at mgbutte@ucanr.edu or leave a phone message on our Hotline at (530) 552-5812. To speak to a Master Gardener about a gardening issue, or to drop by the MG office during Hotline hours, see the most current information on our Ask Us section of our website.