One of the most effective actions you can take to mitigate the problem of an eroding slope is to break up the rate of water decent by constructing terraces or retaining walls. It’s also what allows someone to have a garden on an otherwise challenging if not impossible location.
Use a terrace or walls to slow the flow
I get a lot of questions from gardeners of every level. One of the most common is how to plant a garden on a slope. I’m always happy to answer this frequent inquiry because it gives me the opportunity to talk about the more important issue first. Knowing how to plant on a slope is important but of even greater significance is why to plant on a slope.
Soil erosion and surface runoff occurs as water moves across the ground. The more exposed the soil and the faster the rate of flow, the greater the damage and the bigger the problem. The fact that I’m being asked this question tells me it’s not too late for those asking…I think. But it is imperative to make sure a slope is covered or planted so that erosion is minimized.
One of the simplest and significant actions you can take to mitigate the problem of an eroding slope is to break up the rate of water decent by constructing terraces. It’s also what allows someone to have a garden on an otherwise challenging if not impossible location. Terraces give you the opportunity to create a series of mini-gardens. Erosion is prevented by shortening a potentially long slope into a series of more level steps. This allows heavy rains to soak in rather than run off, taking soil with it.
Think of terraces like steps in an embankment. Soil is cut out of the hill to create the level tread or landing area. As with garden steps, the level area is not exactly level. Sloped terraces should be graded by about 2% perpendicular to the incline in order to gently direct drainage towards one side or the other.
Proper spacing between slopes depends on the slope itself. But in all cases, the shorter the slope length, the less chance there is for runoff. Terracing is most effective when the slope is divided into discrete segments.
For extra water management, you can capture and redirect excessive runoff by installing perforated drainage pipe just below the surface. Run the pipe across the direction of the slope. Position the drainage pipe in a gravel bed, with the perforated side down. Again, position the drain pipe at a 2% slope in the gravel bed.
When constructing terraces, it is important to retain the exposed side. A number of building materials can be used. Some of the most common include: landscape timbers, railroad ties, interlocking landscaping blocks, stones, bricks and treated and weather resistant wood. Whatever you use, be sure the material is firmly anchored against the soil. If you are stacking material, angle it back slightly towards the terraced level with each course. The force of water is powerful. It is always flowing downhill and the pressure can easily push out against the wall; especially in freezing and thawing conditions.
The height of a terraced wall depends on the steepness of the slope. However, because of the force of a wall under pressure, it is advisable that to seek the help of a professional for heights greater than 24 inches (61cm). Also check local building codes for constructing walls and terraces.
Retaining walls are another way to slow runoff and erosion but their primary function is to support and retain an embankment. Unlike a terrace that is designed to have a level surface area, hence the name, the area behind a retaining wall can be level or sloped.
Materials used for constructing retaining walls are generally more decorative. In addition to the materials listed above, native stone or stack stone is often used. However, whatever the weight, it must be strong enough to hold back the pressure of a great amount of soil weight, yet porous enough to allow for adequate drainage. Pipes for drainage are often installed every 24 inches (61cm.) and six inches (15cm.) from the ground.
Retaining walls can be stacked without the use of mortar or with a bonding agent such as cement, concrete or mortar. Dry walls should be sloped back against the soil to give it greater strength. The general rule is to slope the wall back 4-6 inches per foot of rise. Another rule is the width of its base should be about one third of its height.
So, now that you know why it’s so important to retain that sloped area of your yard for runoff and erosion control, I suppose I can go ahead and tell you how to plant that garden on the slope. It’s easy. Simply make sure the soil behind the terraced support or retaining wall is well amended and plant away. The hardest part was getting the soil level!