In the previous blogs, I covered the merits of planting up slopes and the visual delight of terraced slopes. But which method works best for your site? How do we decide? According toUniversity of Nevada Coop Extension, the slope angle decides the method!
Terrace or plant slopes?
To plant a slope you need a gentle slope. At a 20 percent slope (5:1), the probability of establishing vegetation is very good. However, when you reach a 33 percent slope (3:1) the chances of vegetation success drops to fair, and with a 50 percent slope (2:1) vegetation success drops to poor. You’ll definitely need to terrace if vegetation isn’t going to succeed.
Ripraps tend to be used with moderate slopes (33 percent slope, 3:1) to prevent erosion. A riprap is the name given to a rubble of concrete, brick and stone, packed in chunks into the soil. The riprap is then planted with fibrous rooted shrubs, which binds the soil. Slopes steeper than 50 percent generally require structures (i.e. retaining walls) combined with vegetation to stabilize the slope.
If you’ve a backyard steeper that 50 percent slope (2:1), chances are that it’s better to use terraces. Sure, it’s more expensive, but if you’ve a sloped backyard you wouldn’t want guests of water, soil, stones and debris dropping by after a heavy rainfall.
What about costs? If the retaining walls of your terrace exceed 4 feet in height, you’ll need to factor in the costs of hiring an engineer to design the wall. There’s also the construction cost of excavation and building retaining walls and staircases. Terraces are more expensive than planting up a slope!
But there’re costs to planting a slope. You’ll need to think about the added expense of a graded path or staircase. Walks usually are less expensive than staircases, but you’ll need to adhere to ADA (Americans with Disabilities Act) standards if the path slope is between 5 to 8.33 percent. Paths this steep are considered ramps.
Norman Booth and James Hiss’ 5th edition of Residential Landscape Architecture suggest several methods to keep the paths under the 5 percent slope:
“To maintain this standard, walks should be spread out over a greater distance in order to reduce the walk gradient. In extreme situations, walks or paths may need to “switch back” to avoid being too steep.”
And to keep soil in place while plants take root, you’ll need to factor in buying erosion control blankets such as jute netting.
With either terraces or planting a slope, there’s the cost of plants. The difference is a wider selection of plants for terraces than slope planting. On slopes you’re limited to slope-stabilizing fibrous-rooted tough plants.
So, this is the end of the three part series on gardening on slopes. I hope you have enjoyed them!
Terrace or plant slopes?
Booth, K. and James E. Hiss. Residential Landscape Architecture. Design process for the private residence. Fifth Edition. Pearson Prentice Hall. 2008
GeoEngineer: Structures for the stabilization of slopes
New Mexico State Extension YouTube: How to build a unique terraced garden
University of Nevada Cooperative Extension. Stabilize Steep Slopes with Plants and Erosion Control Structures