By Robert Gillmore

Perhaps the most expensive gardening mistake is retaining walls. They’re usually costly (especially if they’re handmade of brick or stone). They’re frequently unattractive (especially if they’re made with poured concrete). Worst of all, they’re often totally unnecessary.

Even a steep slope usually doesn't need to be “retained” by a wall. If the slope were unstable, it would have collapsed long ago. The fact that it’s holding up is usually a good indicator that it's at what civil engineers call the “angle of repose.” That’s the angle—usually about 45 degrees—that fill, loam or other material will assume when it falls freely and comes to rest, or “repose,” without any retainer or support. The steeper a slope, the more likely some of it will erode, but erosion can be all but eliminated if the slope is well planted.

Think of how civil engineers build a major highway. When they cut the road through a hill, they simply make a big, V-shaped incision in the earth, making sure the newly created slopes on each side of the road are gentle enough to be stable (at repose); then, to prevent erosion, they spread crushed stone or plant grass or other ground cover on top of them. They don’t build walls to “retain” the slope. When they build the road across a low spot, they dump fill (stone, gravel, etc.) in the depression and build the road on top of it. They don’t build walls at the base of the fill to hold it up—they don’t have to. They just plant the slope or cover it with rocks to prevent erosion. (For which, of course, we should be most grateful: If retaining walls actually were needed, the cost of building highways would be astounding.)

The only reason to build a retaining wall is when you need to add fill and you have no room to let the fill spread out at its angle of repose.

Suppose you want a terrace for outdoor barbecues etc. (described in last week’s column) and the only place you have to build it is on a slope. If you spread fill to make a level area for the terrace and let the rest of the fill fall at its angle of repose, the slope created when the fill falls will be at least as wide as the deepest part of the fill. Many, if not most lots will have enough room for the fill to spread, and the new slope can simply be planted with shrubs, ground covers and other plants. Smaller lots, however, may not have enough space to let the fill fall—the space might already be taken up by a building or a vegetable garden or something else. Then—but only then—the fill has to be retained by a wall.

Here’s how two clients of mine saved about $3,000 by building terraces without walls. They couldn’t use the space behind their house for occasional entertaining and other activities because it sloped steeply away from their home. The all-too-common solution to their problem is to create one or more terraces by adding fill to the slope and retaining the fill by one or more walls.

I told the couple that walls would easily be the costliest and least attractive way to support the terraces. Instead of walls, I told them to let the fill fall at its angle of repose at the edge of each terrace, then add topsoil to the slope and plant it with sweeps of colorful shrubs. The slope would be an excellent showcase for the shrubs, and the shrubs would contrast nicely with the grass on the terraces. Instead of $6,000 worth of walls they would need to buy only $2,300 worth of shrubs.

If you already have a retaining wall, and it’s unattractive or needs repair, consider simply taking it down and letting the soil behind it fall to its angle of repose.

Robert Gillmore is a landscape designer. His books include The Woodland Garden and Beauty All Around You: How to Create Large Private Low-Maintenance Gardens, Even on Small Lots and Small Budgets. Evergreen, his one-acre woodland garden in Goffstown, is regularly opened to the public without charge. For more information e-mail evergreenfoundationnh@gmail.com or call 603-497-8020.