A garden berm is an artificial hill that can perform several functions in your landscaping. This isolated, often-rounded slope can provide an interesting focal point in a completely flat yard. A berm can capture the sun, providing room for flowering trees and plants that would do less well in shade. Building a berm is also an excellent way to improve or redirect water drainage in your yard, providing a small, artificial watershed to keep rainwater from damaging your foundation or creating stagnant, muddy pools in your yard.
Design a Berm
Take some photographs of your yard, both the area where you plan to put a berm and the yard as a whole, including your house. This will help you see the effects of changing the land forms in your yard, and may help determine the location of a strictly-decorative berm.
Examine your yard from a height--a second story windows, the top of a flat garage roof or even the second story of the house next door. This will help you gain a view of your yard as a whole.
Measure your yard, including the parts of the property occupied by your house, driveway, garage and other structures. Draw a map of your property, including these dimensions. (It's easy to lose a sense of perspective when you're up close to your project on the ground.)
Stake out your proposed berm. Note where it lies in relation to structures; distance your berm at least 25 feet from your house's foundation to prevent excess water from being directed toward the house. Note the position of the berm in relation to utility lines; do not build a berm directly over underground lines.
Watch the effects of weather on your planned berm area. In some cases, a low-lying, chronically-damp area will determine the location of the berm. In other cases, you may find out that your proposed location captures the sun, but also takes severe wind during a storm.
Creating the Berm
Remove turf from the area. Save it to replace on the berm slope, or use turf chunks to patch other parts of your lawn. Store turf in a plastic bag--damp, if it must wait several days for replanting.
Shovel in new topsoil to create your berm's shape and slope. In general, berms attain a height of 4 to 6 inches. Tamp down layers of topsoil as you build; walk on a board to tamp quickly and efficiently. Allow 4 to 5 feet of width/radius for each 1 to 2 inches of center height to keep the berm from being obtrusive, and to limit washdown of soil over time. Bury one end of a measuring stick at the highest point of your berm to measure your progress.
Mulch the top 2 inches of your berm with hardwood mulch (nuggets) or gravel; shredded bark and peat moss blow or wash away easily. Water the berm area thoroughly, add plants and water again.
Use a Berm as a Rain Garden
Observe the drainage in your yard before installing a rain-garden berm. The berm's exposed area will soak up rain water faster than the lawn's surface, and provides a slope designed to redirect the flow of excess runoff.
Consider using a berm to augment or interrupt the runoff properties of an existing slope on your property. Many decorative berms create texture on a flat yard. A rain-garden berm, however, usually coordinates with existing topography. This kind of berm is suitable for placement closer than 25 feet to your home's foundation, although it must be sloping away from the structure. Placing a berm at the bottom of a rocky slope or rock garden can often forestall the runoff that leaves puddles at the bottom of the yard.
Select plants that tolerate lots of water. Woody, native perennials are ideal candidates for a rain-garden berm. Annuals have limited use in a rain-garden berm; reserve them for quick color at the edges of your plantings.