Best Plants and Erosion Controls for Slopes and Hillsides
Rocks add visual appeal and function to anchor the soil. A variety of sedums offer textural variations.
One of the greatest challenges to both the home gardener and the professional landscaper is a steep slope. Wind and water wreak havoc on the topsoil, uniform water penetration is poor with additional run-off, and the thought of clearing weeds makes us want to go back to bed and pull the covers over our heads! Where does one start?
There are a few different options such as building retaining walls, terracing with horizontal paths, laying down erosion control grids, constructing shelf-like basins around trees and shrubs, and selecting deep-rooted groundcovers. They can all work effectively, but each has its own considerations.
First consider the size of the area and the pitch of the slope. You'll also want to take note of any existing trees and shrubs and the boundaries of neighboring properties. These will all figure in to your final decision.
If the area is large and your slope is at least 30%, creating terraces would be good plan for making more level planting, recreation, and seating areas. Another option for a steeply sloped area is a retaining wall, but these work best in a smaller area where the run isn't too long. A wall can be both costly and subject to damage due to shifting earth. If you do put in a retaining wall, it is important to think about how it will affect any existing trees. You'll have to be careful not to damage roots and to not allow the run-off sediment to collect around the trunks. Terraces should be slightly sloped perpendicularly to the hillside to allow for run-off. In areas of heavy rains or poor soil, pipes with drainage holes can be laid along the terraces in gravel runs then stabilized with rock edging. Soaker hoses and drip irrigation allow easy adjustments to the water delivery for each plant. Remember that deeper, infrequent watering is the best approach.
Good choices for sunny slopes:
Planting well-rooted groundcover is the easiest solution. Three great evergreen choices for a sunny area are Myoporum parvifolium, Rosmarinus officinalis "prostratus" , and Baccharis pilularis. All of these prefer well-draining soils and are tolerant of drier conditions once established.
Myoporum parvifolium, a small leathery-leafed member of the myrtle family, blooms in Spring with pink or white flowers. It is deeply rooted and has dense overlapping finger-like stems which block out weed growth. It grows to 3"-4" with a 6 ft. spread. This Australian native is also fire retardant and deer resistant, especially the "Putah Creek" variety. It does attract bee activity in April-June. Myoporum typically lasts for 5-6 years.
Rosmarinus officinalis "prostratus" is common rosemary in a low-growing and spreading form. It is a hardy Mediterranean native which is well suited to sunny and water-wise gardens and can grow to 1' with a 6-8 ft. spread. It is wonderfully fragrant, can be used for cooking, produces lovely bee-attracting lavender-blue flowers from winter to late spring, and is resistant to both insects and deer. "Huntington's Carpet" is less woody and works especially well for cascading over retaining walls.
Baccharis pilularis, commonly referred to as dwarf coyote bush, is a California native which grows 1'-2' high with a 6'-8' spread. It is evergreen and can get along with little water once established. It is long-lived, low maintanence, and deer resistant. The "Pigeon Point" variety is a good one and looks beautiful combined with flowering accents like California poppies and salvias.
For shady slopes, this member of the boxwood family from Asia is a great option.
Groundcovers for shade:
For areas with more shade, three good choices for evergreen groundcovers are Pachysandra terminalis , Fragaria chiloensis, and Arctostaphylos "Emerald Carpet.
Pachysandra terminalis, commonly referred to as "green carpet" or Japanese spurge, can be slow to start but will be vigorously growing by the 3rd year. It grows to 6"- 10" high and spreads by rhizomes. This Asian native is good for erosion control and will grow under trees, including pines. It has insignificant white flowers in Spring, and is not bee attracting.
Fragaria chiloensis, beach strawberry, is a California native well-suited to areas of part-sun to shade and well-drained soil. It bears white flowers from spring-summer, produces edible fruit in fall, and has touches of red foliage for year-round interest. It is a must for bird gardens and dappled woodlands. It stays under 1' with a 12"-18" spread and needs moderate water. A similar alternative is the woodland strawberry, Fragaria californica, whose leaves are brighter green and less leathery.
Arctostaphylos "Emerald Carpet" is another California native from the manzanita family. This variety requires more water than the "kinnikinnick" spreading manzanita and is more tolerant of part-shade, but it can handle the inland heat. It reaches 1' in height and can spread 6-8' Small white bell-shaped flowers appear in winter-spring, followed by red berries. The bright green foliage and reddish stems are very attractive. Hummingbirds love the flowers!
Take design cues from natural areas:
It's always best to take cues from nature, so I think the best approach to a slope or hillside is a combinaton of trees, shrubs, and groundcovers. Not only are they more visually appealling, they will attract more birds and wildlife and break the rainfall which reduces erosion.
Mulches are a great way to conserve water during hot summers. Bark and shredded wood are great choices, but the breakdown of carbon can rob the soil of valuable nitrogen. Fertilize accordingly.
When clearing an area for planting, it's best to leave a bit of organic debris. Sticks, stones, and bark help fortify the soil against run-off. Shredded bark is a good mulch to add. I prefer shredded redwood which has a more fur-like quality. It stays matted and is less apt to be disturbed by winds and blowers. Create shelf-like basins around trees and shrubs that are planted into the slope and use varying sizes of rocks for stability and attractiveness. Arrange plants for a layered look, using largest in back then balance the visual layout by randomly placing medium shrubs. Fill in with perennial flowers and clusters of bulbs and groundcover. Jute netting can be used to anchor the hillside until the plants have grown in. It will gradually disentigrate. Sometimes birds or squirrels will swipe it for nesting!
Accessibility is often a problem with sloped areas, so I recommend choosing drought tolerant varieties and being careful to not overfertilize. You don't want to encourage weeds. Newly planted trees and shrubs, even those for xeriscape, will still need regular deep water until established. The addition of a michorrizae-based organic fertilizer at the time of planting wil help the plant grow more vigorously since it helps with the uptake of nutrients. California native plants, however, do best without soil additives and fertilizers.
Whatever you choose as your solution, that difficult slope holds the promise of many things. It doesn't have to be a constant source of work. If you tackle it with thought and care, you can eventually sit back, relax and enjoy watching as nature takes over!
This jute netting and ground staple kit is a great way to prevent erosion until new plantings are established.
It's easy to to install a French drain for diverting run-off:
Good water-wise plants for erosion control and wildlife:
There are so many choices! Cacti and succulents are also water-wise and visually diverse; they, however, are not helpful for erosion control on their own because of shallow roots. This is especially true for iceplant which can slide away like a skin during heavy rains. They do work well when combined with other well-rooted specimens. The overall considerations should be low maintenence and deep rooting unless terracing allows easy access for pruning and clean-up. Weed cloth also helps to reduce frequent clean-up in these hard-to-work areas.
Grevellia- many varieties
Phormium- New Zealand Flax
Westringia- Coast rosemary
Callistemon "Little John"- dwarf bottlebrush
Quercus chrysolepis- Canyon live oak
Aesculus californica- California buckeye
Ceanothus -many varieties
Trichostema lanatum- Wooly blue curls
Ribes sanquineum- Red-flowering currant
Baccharis pilularis- Dwarf coyote bush
Artemesia californica- California sagebrush
Arctostaphylos - manzanita. There are many varieties from small trees to shrubs and groundcover. They have beautiful sculptural shapes,red bark, and white Spring flowers which provide food for hummingbirds.
Salvia- many varieties
Zauschneria- California fuchsia
Miscellaneous (remember to check for your specific areas)
Salix hookerina- Hookers willow
Cornus stolonifea- Red-twig dogwood
Heteromeles arbutifolia- Toyon
Mahonia- Oregon Grape
Berberis- Barberry (deciduous shrub)
Heuchera- Coral Bells
© 2011 Catherine Tally