In the Garden:
Northern & Central Midwest
October, 2007
Regional Report

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2597

This berm is a lovely landscape element, even in the mist.

A Berm is Not Just a Pile of Dirt!

How often have you seen a berm that just doesn't work? Can you figure out why? It all has to do with proportion and size. Berms, those lovely mounds of soil, usually topped with plants, are great for screening, shutting out noise, giving instant height to a garden. They can also add interest to a flat landscape, direct foot traffic and drainage, provide some wind protection, and be a focal point in the landscape.

Before you start to build a berm, plan ahead for the amount of soil required. A properly proportioned berm will require a lot of soil. If you skimp on soil or size, you will end up with what looks like a leftover pile of dirt, not a focal point.

Soil
Depending on what you intend to plant, you can build a berm with lower-quality soil in the interior and only the top foot or so of high-quality topsoil. Make sure that the berm construction is done in such a way as to keep the berm in place -- to prevent soil from washing away or the whole berm from sliding. Some berms are built on a firm base of clay, but you should use whatever you can access easily and inexpensively.

Slope
As you begin to determine the slope of your berm, always go with as wide a base as you possibly can. The slope is determined by finding the vertical distance and the horizontal distance. The ratio of these two distances is the slope. The recommended minimum ratio of vertical to horizontal is 5:1. In other words, for every 1 foot of height, you will need 5 feet of width. Berms with a steeper slope have inherent problems.

Berms that will be mowed must be particularly well-planned because a too-steep slope is not only dangerous, it also leads to grass scalping. When in doubt, make the berm wider and use mulch instead of mown grass.

Design Elements
Berms should appear to be natural elements, not just piles of dirt stuck in the midst of a landscape. Varying the slope of a berm will help in the natural design, as will making the berm curved instead of straight. Follow the natural contours of the land, and vary your heights to add interest. Using plants of different heights will also help accentuate the natural look.

Plants
As you begin to choose plants for your berm, there are a few necessary characteristics to keep in mind. First of all, putting plants up into the air on top of a hill may subject them to more wind than usual. Select plants with small leaves that will resist tattering, and make sure the branching structure is sturdy to avoid wind breakage.

Berms naturally have terrific drainage, but they also tend to dry out fairly quickly. Select plants that will tolerate dry conditions and that have naturally deep roots to make use of water that may collect at the base of the berm. In fact, when designing a berm, you may even make a swale at the base to catch water.

Mulch
Beds with plants in them need to be mulched, and the best type of mulch for a slope is elongated and flat. That way, it will not roll down the slope easily. The finer and more fibrous the mulch, the better it will stay in place.


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