In simplest terms, a berm is nothing more than a mound of dirt. Sometimes it answers to hold something back--water for example--but more often it serves to add variety to a flat landscape by creating raised planting space. If you have rocky soil, berms are great for growing root vegetables that need fine soil and depth. If your ground is overly wet, it provides a high, dry space for growing.
Pond and Berm
If you need a raised bed in your garden, and have always wanted to incorporate a garden pond or other water feature requiring excavation, combine the two. Use the soil removed from the space for the pool to build a berm. To ensure that the best soil for plants remains on the top, make a pile as it is removed from the pool area. When building the berm, simply reverse the order--putting the soil that is now on top, at the bottom of the berm and finishing the bed with the topsoil from the bottom of the pile.
Flower "Stadium" Berm
Just as seats in sports stadiums are staggered to give unobstructed views of the game, a steeply sloped berm can give you unobstructed views of flowers in a couple of different ways. Make a one-sided sloped berm against a short wall and flowers planted in the top portion will appear taller, making a handy screen, while remaining short on the bermed side. Same-sized flowers grown on a slope will appear to be of graduated sizes, so you can get a variety of heights for your bed with the same plant species.
Build a berm in layers, placing long terra cotta drainage tiles between each level of soil so that they are completely covered except for the openings on the ends. (Lay plastic sheeting or thin rocks on top of the tiles if they are of the perforated variety to reduce the amount of soil and water that may fall into them.) When the mound is completed, plant vines or a trailing ground cover over the entire bed. The tile openings will be partly obscured by vegetation and make cool, cozy homes that will lure beneficial insect-devouring amphibians, lizards and small snakes to your garden.
Rain Garden and Berm
If you have an abundance of rain in your area or need a way to drain excess water from a low garden, consider building a rain garden on the lowest side. Dig out a large free-form area and plant it with grasses, cattails, iris or any other marginal, moisture-loving plants. Use the excess soil to raise the height of your garden in one large berm. A rain garden differs from a pond by being a temporary storage area for excess water. It has the added benefit of providing a place for flowers with high-water needs, that you may have been unable to grow previously.