Making the Grade - Part 2

We discussed basic soil work in a recent article - see http://clearwaterlandscapes.com/Nsltr3late.htm

If you're creating planting beds and lawns on a sloped site, or if you want to create more vertical interest with raised or mounded beds, careful planning and choice of materials is vital.

Understand that soil is extremely heavy, especially when wet. The forces of gravity will work to tear down any structures installed to retain soil, so these structures must be built with great care and careful planning. This may seem elementary, but examples of inadequate soil retention can be found in almost all neighborhoods.

For simple mounded beds, slope the soil gently toward the middle of the bed. Making the grade too steep will result in unnatural "bumps" in your landscape. For example, if you have a planting bed that is 25 feet across, you might have an elevation change of plus 1 1/2 feet at the middle of the bed...if you use taller plant forms near the center of the bed, the vertical change will be even more significant.

Raised beds for ornamental plants or vegetables should be bordered with massive, rot resistant materials. "Landscape timbers" of cedar, redwood or cypress have become common at some building suppliers. These can vary from a minimum of four inches thick to ten inches, and can be anchored to the native soil by boring 5/8" or larger holes completely through and then pounding rebar of equal size through the timber and into the soil at least 1 1/2 feet. In general, two courses (vertical rows) should be maximum for the do it yourself landscaper. "Tie walls" can be built several feet high, but the engineering and construction of these should be left to professionals.

Other materials that can be used for raised beds include concrete block and native stone. Beautiful results can be obtained with stone, but considerable skill, patience and hard work is required. Use of pressure treated wood (CCA process) is not recommended...the copper arsenate breaks down to release toxic arsenic. Similarly, railroad ties are objectionable because of the creosote.

For very stable and long lasting retaining walls, consider modular concrete wall blocks. These are engineered to fit precisely with connecting pins - and to "step back" into the retained soil. The well engineered system allows the do it yourself landscaper to construct retaining walls of four feet or more in height. Of utmost importance, though, is following manufacturer's specifications and instructions to the letter...this includes using proper backfill materials.

The modular block system has improved in looks considerably in recent years. New manufacturing techniques have produced block with more natural color and pattern - certainly not like a stone wall, but much improved over previous styles.

For a brief overview of the construction of retaining walls, see: http://clearwaterlandscapes.com/retaining_wall.htm

Again, remember that any change of soil grade should be undertaken with great care...the weight of the soil and the force of gravity will be working against you. Any mistakes made in soil retention will result in expensive, discouraging and potentially dangerous results. Do it right the first time!

Happy Trails,

Dan Eskelson
Clearwater Landscapes, Inc.