Cactus spines flame up with backlighting as the sun drops slowly behind the roofs of your neighborhood. Alpine cushions sit prim and unruffled under a raucous wind that forces taller plants to double over. Miniature blue violets bloom in the sporadic warmth of February, drawing you outside in appreciation. All are garden berm residents; they reside happily where native conditions may not be quite to their liking. A berm allows you to create microclimates, vary your planting soil, and catch or divert water as it flows through your property.
What’s the Point of Building a Berm?
Berms vary the topography of a flat space and add visual interest to a landscape, be it large or very small. In nature, plant communities are varied where topography changes. Species which tolerate strong winds and long hours of harsh sunlight may grow well on a western or southern exposure in dry soil with quick drainage. Other species may prefer low spots or ditches, where the collection of organic matter breaks down into richer soil, and where water drains slowly.
In the garden, a berm can raise plants and provide them with a soil medium that is faster draining than that which naturally exists on the site. Especially where heavy clay is present, a berm can offer a different mix, one which you design for plants which demand it.
Water Efficiency
Berm placement is important to achieve either maximum drainage or water collection. Your purpose may be to utilize the rainwater that falls on your property, rather than allowing it to run off. Berms serve as dams and can be constructed to divert or gather water as it runs downhill. Rainwater gardening is an idea gaining popularity in drier regions of the world. The process includes building berms to prevent runoff, slowing down and spreading rainwater, and even collecting it in basins which support plants. Arizonan Brad Lancaster uses these methods widely to help municipalities and homeowners utilize water that otherwise would run down sidewalks and streets to storm drains.
Berms can be advantageous in sometimes surprising ways. In her book Cutting Edge Gardening in the Intermountain West, Marcia Tatroe noticed that plants of the same species growing on the ridge and at the base of a berm were inconsistent in height. Surprisingly, those at the top, where one would think soil would be drier, grew taller. She realized that the berm was placed below a drainspout where it both gathered and diverted roof water. It gathered enough to keep the plants on its top consistently moist, but ran past those growing near the bottom.
Consider the natural flow of water across your property before you build your berm. Water needs to run away from the foundation, but may be utilized in several ways by the addition of berm structures.
Increased Planting Area and Extra Protection
Adding two slopes to replace flat ground gives you more surface area to plant. If your primary viewing area faces one of them, the slope arranges plants so that you can appreciate layers as they proceed back and upwards. Larger grasses or shrubs can be placed on the side away from your view, helping to create the illusion of infinite space. A large berm in a small space can be dramatic and give you more garden to work with. Or, a small berm can become a manageable project for the gardener who wants an intimate and accessible plant collection.
On a more exposed site, the windward side of a berm can be planted with tougher specimens, while more delicate individuals will happily grow on the protected, opposite slope. Boulders can act as wind or shade screens, as well. Individuals which may burn on top of an exposed berm will rest quite happily on the leeward side or in a pocket between rocks.
How Do You Build One?
A berm is, simply, a raised mound of soil. Directions on how to build berms can be straightforward or complex, depending on the source you choose. Boulders may be worked into the construction and can help to hold sloping soil in place, especially before plant roots take hold. Draw the outline shape on the ground with spray paint or use a hose to loosely define the edges of your planned berm. If you’re working directly on turf, you can kill it beforehand or strip the sod and place it upside down to form the bottom layer of your construction. Move the pieces in from the edge, as your new soil won’t be deep enough there to completely cover them.
Next, if you are using boulders, dig a depression to hold about one-quarter to one-third of the boulder and slide it into place. If your boulders are very large, a small tractor will facilitate this process. In a berm, larger boulders work best. They may look impossibly big and bulky at the beginning of construction, but with plants growing behind or cascading over them, they’ll soften and become visually smaller. Boulders act as a neutral backdrop for plants and show them off to advantage while slowly releasing mineral matter and holding moisture on their buried surfaces.
Preparing the Base
Prepare the berm’s base by digging your new soil into that which already exists, especially if you are adding a well draining mix on top of tighter clay. After spreading a few inches over the area, dig the new mix into the original soil, so that both begin to interplay. Otherwise, you may create an interface where water can roll over the tighter surface too quickly, not staying long enough to infiltrate the soil above.
When your soil is dumped inside the outlined shape, it will tumble down as it pleases. In other words, don’t be too tightly attached to your original plan, let the falling soil “tell” you what the general shape of the berm will be. Be careful to slope the soil reasonably; a slope of about 4:1 is generally recommended. Even with plants and mulch, a very steep slope will tend to wash away. Look for your soil mix’s “angle of repose,” the slope at which the soil stops falling and holds its place.
Sculpting the Form
You can also use your shovel to create depressions, backfill around boulders, and shape the top. An irregular overall shape and top have a more natural look, less like a grave mound and more like a sculptured earth form. Play with it. Have fun. Building a berm is the ultimate sand box experience. Use a shovel; use a bucket; walk up and down it; pull the soil over and around boulders as though you’re dressing them.
If you’re using rock, continue to dig back into the berm along its natural slope to add layers of boulders upward. Rather than placing rocks helter skelter everywhere, think of grouping them, much as you might group plants, then place their edges somewhat logically, so that they seem to tie into each other. Plants will bridge the gaps, so that two or three boulders will appear to belong together as the plants among them gain maturity.
To prepare for your second layer in height, dig back into the berm behind the lower boulder and possibly off center a bit. Sink the second rock slightly, so that it is well anchored behind the first. Bury it so that, again, about one-quarter of the boulder is covered with soil after backfilling. Consider tilting boulders back somewhat, so that they have a face which is level. Then, you'll be able to use them as solid steps as you move around your berm.
Finishing Up
Your berm is ready for compaction after you shape it. Walk on it, tamp it, insert the hose to release air pockets. Any method will help to prepare for planting. But, most of all, build it about one-third to one-half taller at the outset in anticipation of its settling, then wait to plant. It’s disheartening to have to lift all your plants a year after construction because your berm has settled to a mere bump. During its first year, a planting of annuals might make sense. You’ll still have root systems to help hold the soil mix and you can still add mulch. Then, when the annuals finish, you can add soil as you need it to reclaim your intended height before you commit the berm to a more permanent planting.
The network of root systems will hold a berm’s soil in place to a great extent, while surface soil and its moisture is held with a fairly deep topping of mulch. Mulch adheres to itself from the bottom up on a berm; it, too, has an angle of repose. It almost tells you when you have applied enough. Pea gravel stops sliding and shredded wood grabs onto neighboring pieces, creating some stability and preventing the soil's surface from quick drying.
Your choice of mulch depends on what you are planting as well as on your climate. Succulents don’t grow in forests, so gravel makes more sense as a mulch for them. Look at the ground surrounding the plants you wish to grow. You’ll see gravel around alpines, desert, and plains plants and organic material around woodland specimens.
Soil Mixes
Soil mixes vary widely, too. The mix you use should be one which accommodates the plants you plan to grow. Cactus like good drainage, as do many alpine rock garden plants. Shade plants like more decaying organic matter and prefer more consistent soil moisture; some prefer a more acidic soil. Research the plant group that will inhabit your berm and prepare your soil, or order it from a reputable landscape yard, accordingly.
Whether you’re creating a visual effect, efficiently using water, or preparing a special planting area for a prized collection, a berm may be a logical choice for your garden. A bermed planting can be a manageable problems solver, while it adds the dramatic and unusual to your landscape.
Kay Galvan - Kay Galvan has created residential and commercial gardens for twenty-one years on Colorado's high plains.
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