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Preparing a Hillside

September 21, 1997

The threat of El Nino storms has underscored the need to protect hillsides from erosion and appropriate planting is the best way to prevent mudslides, experts say. Although deep rooting trees and large shrubs are ideal for holding hillsides together, the fall is too late in the year for those varieties to take hold. September and early October are the time to plant fast-growing grasses and native chaparral for at least some anchoring effect before the start of winter rains.

"What you're looking at is vegetation that prevents erosion," said Stephen Jewett, district conservationist for the United States Department of Agriculture's Natural Resources Conservation Service in Somis. For a hillside "that gets any kind of concentrated flow, you want something that can come up fast to protect when the heavy rains do come."

Start with low-growing shrubs such as Dwarf Coyote Brush and California Lilacs and mix in wildflower and grass seeds. Fast-growing grasses such as Blue Grama and Western Fescue and wildflowers such as California Poppy and Yarrow are what will be most likely to grow by December if planted now, said Dustin Alcala, nursery manager of the Theodore Payne Foundation, a native plant nursery and education center in Sun Valley.

Larger shrubs may be planted as a long-term solution to hillside erosion, but the fast-growing grasses form a basic topsoil barrier that prevents gulleys and rivulets from forming during heavy rains, Alcala said.

But property owners should avoid clearing their hillsides before planting because removing existing roots may destabilize the soil. When it comes to anti-erosion tactics, the more roots, the better, said Herb Spritzer, assistant chief for the Los Angeles County Fire Department.

Surface Saturation

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

1. When rainfall is less than 6 inches, there tend to be few problems. With more than 6 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, more serious problems begin. These include large mudslides during he storms and, later in the year, the chance of massive landslides, as water undermines bedrock layers of compacted earth.

LONG, STRONG ROOTS FOR THE FUTURE

Perennials rather than annuals are long-term solutions to erosion problems. Though a few deep-rooting trees can be planted now, homeowners should focus on grasses, wildflowers and low-growing shrubbery to hold the hillsides in winter rains.

Steep slopes generally hold poor soil, but a sever rain can completely wash off any topsoil, leaving mineral soil exposed. Native "pioneer" plants fill in the bare areas, deeper-rooted plants hold the ground in place.

Trees: Once established, their root zone may reach 5 feet or more.

Shrubs and climbing plants: Roots grow to a depth of 2 to 4 feet.

Annuals: Their roots typically grow about 6 inches deep, but may reach as deep as 10 inches.

Grass and ground cover: The average depth of grass root and ground cover is 4 to 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.

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, concrete 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 1650813294 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 the ground

Riprap 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 ground in place and protect mulch from wind and water damage. Jute is biodegradable and eventually decomposes. Cuts can be made into fabric for plants to flow through.

Planting the Slop

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

SUGGESTED SLOPE PLANTS TO PLANT NOW

California Buckwheat (Eriogonum fasciculatum)

This low-growing plant is common in the Santa Monica Mountains chapparal and is one of the earliest shrubs to take over naturally. The Bruce Dickenson and Dana Point varieties are the most readily available.

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