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Repairing a Hillside : A GUIDE TO POST-MUDSLIDE PLANTING ON VALLEY-AREA SLOPES

A San Fernando Valley homeowner seeds her back-yard slope with grass and plants iceplant, thinking the vegetation will hold the ground in place during heavy rain. She is unpleasantly surprised after a winter storm when the slope soil and foliage slips away.

Appropriate planting is the best way to protect a hillside, and patience is required for roots to reach the most beneficial depth. Deeply rooted native plants are the best solution, says Topanga horticulturist Mary Cohen. “They’re programmed to withstand the rigors of living on slopes.”

Fall is the best time to plant California natives, but after an emergency such as flooding it’s important to stabilize the ground immediately by covering it with foliage. “The earlier you can plant after the rains, the better,” says Holly Wagner of the Theodore Payne Foundation, a native plant nursery and education facility in Sun Valley.

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Preparing the Site

Water should be diverted off the slope immediately after heavy rains. More permanent slope engineering issues, such as surface drainage, need to be considered, along with other erosion-control measures.

Drainage

1. If not already cut into the slope, cement bench drains can be installed to channel water down to the storm drain. Open trenches filled with gravel may also be used. Make sure water is being diverted to drain and not to neighboring properties. Drains should be kept free of debris to prevent blockage.

Other ways to control erosion that slow flow and help redirect water:

Baffles are railroad ties or timber partly buried in the hillside that work best on slight-to-medium slopes, slowing water and giving it more time to soak into ground.

Rip-rap stones or concrete rubble cover slope to slow water runoff. Plants can be placed between rocks.

Ground support

2. Netting or fabric such as burlap is laid on ground after seeding to help hold wind and water damage. Jute is biodegradable and eventually decomposes. Cuts can be made into fabric for plants to grow through.

Planting the Slope

3. Plants of varying sizes should be interspersed with seed. Generally, woody ground cover plants should dominate because their roots extend 3-6 feet. Woody shrubs such as small, drought-resistant landscape trees can be mixed in--their roots range from 6-15 feet.

Surface Saturation

Heavy rains can cause street flooding and minor mudslides, but a longer-term problem occurs when the soil gets oversaturated. Here is a look at what can happen during the rain season:

1. When rainfall is less than six inches, there tend to be few problems. With more than six inches of rain, soil begins to saturate and can absorb less water. Small mudslides with a few feet of soil washing away can occur.

2. With more than 10 inches, real problems begin. These include large mudslides during the storms and, later in the year, the chance of massive landslides, as water undermines bedrock layers of compacted earth.

Long, Strong Roots

Perennials rather than annuals are long-term solutions to erosion problems. It is important to put in self-sufficient slope plants to avoid continual watering, which denudes soil.

Steep slopes generally hold poor soil, but a severe rain can completely wash off any topsoil, leaving mineral soil exposed. Native “pioneer” plants such as morning glory, sagebrush and buckwheat adapt to mineral soil and can help build up humus. After pioneer plants fill in the bare areas, deeper-rooted plants hold the ground in place.

Trees

Once established, their root zone may reach five feet or more.

Shrubs and climbing plants

Roots grow to a depth of 2-4 feet.

Annuals

Their roots typically grow about six inches deep, but may reach as long as 10 inches.

Grass and ground cover

The average depth of grass root and ground cover is 4-8 inches.

Foothill residents have found that short-rooted plants and succulents don’t halt soil erosion on steeper slopes. Experts suggest low-lying, native plants, shrubs and trees, to better anchor soil.

Suggested Slope Plants

Coyote bush (Baccharis pilularis) is a spreading shrub with roots extending to six feet. Low-growing Twin Peaks variety is preferable but can be interspersed with taller type. Best on slopes to 35 degrees.

Toyon (Heteromeles arbutifolia) is a small, flowering native tree that is drought-resistant and is considered by experts to be the most fire-resistant. Its height ranges from six to 25 feet.

Grass is important for initial soil-erosion control, but shouldn’t be considered a permanent solution by itself. Rye grass is not recommended--it out-competes native grass and becomes a fire hazard once it dries.

Carmel creeper (Ceanothus griseus horizontalis) is a low-growing, drought-resistant shrub that spreads to 10 feet wide and has roots 1 - 4 feet long.

Descanso rock rose (Cistus crispus) is a semi-upright flowering shrub 12-24 inches tall with roots 3-4 feet long that does best on slopes of less than 35 degrees.

California buckwheat (Eriogonum fasciculatum) is common in Santa Monica Mountains chaparral and is one of the earliest shrubs to take over naturally.

Native morning glory (Calystegia macrostegia) is true to its Greek generic name, which means “covering cup.” It provides an abundance of foliage to cover the ground and prevent the soil from washing away.

* Succulents are not recommended on slopes, since they become heavy with water and can bring down a slope when pulled out of ground after rain.

Contacts

* Theodore Payne Foundation, Sun Valley, provides information and sells “slope mix” grass seed, along with native plants in one-gallon containers: (818) 768-1802.

* Los Angeles County Fire Department’s Forestry Division provides information on erosion control and fire-safe planting: (213) 881-2481.

* University of California publishes booklets on erosion control: (510) 642-2431.

* Seeds and plant flats can be purchased at local nurseries.

Sources: California Native Plant Society; Los Angeles County Fire Dept.; Los Angeles County Dept. of Public Works; Soil Conservation Service; Theodore Payne Foundation; “California Wildfire Landscaping”; “Sunset Landscaping Illustrated”; “Fix It Yourself Lawn & Garden”; Researched by JULIE SHEER / Los Angeles Times


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