GARDENING : If You're Inclined to Plant on Slopes

By KAREN DARDICK

April 10, 1993 12 AM PT

SPECIAL TO THE TIMES

Now that the worst of the winter rains have finished assaultingOrange County, it’s time to replant hillsides and slopes that didn’t meet the test of the torrential rains.

After six years of drought, many homeowners were surprised to discover that their supposedly solid hillsides eroded or were pocked with rain-driven gullies. Some were left with nude or near-nude ground that was supposed to be secured by plants.

Much of the problem occurred where the soil mainly consists of heavy clay that retains water and leads to soil slippage.

“The key to successfully securing hillsides and sloping property is to landscape with a variety of plants for a layering of their root systems,” said Bob Stone, a landscape architect with Cardoza, Dilallo, Harrington in Costa Mesa.

Stone added that the most effective way to ensure a strong root system is by including trees and shrubs as well as ground cover in the landscape design.

For years, ice plant has been the ground cover strongly favored for soil control on hillsides. But as some property owners have recently discovered, ice plant can give way with the soil in which it’s planted.

“The problem with ice plant is that as the planting matures, it can get so heavy that it tends to pull a slope down,” said Deborah Visbal of the Cathcart, Begin landscape architectural firm in Orange. “This is especially a problem with steep slopes of heavy clay.”

Visbal said that many homes in Orange County are constructed on cut-and-fill slopes where the soil can be clay or decomposed granite. She recommends that homeowners have their soil tested to determine its composition and drainage capabilities.

Before planting, amend the soil where possible with organic material such as compost or planting mix. This helps loosen and aerate clay soil to promote better drainage. In the case of decomposed granite, which won’t slip but contains few nutrients, amending will help the plants grow better.

If there are bare spots in existing plantings, these can be filled in with one-gallon-size shrubs or five-gallon-size trees. New plantings can include a combination of the same size shrubs and trees with ground cover that can be obtained in flats.

When choosing trees, Visbal recommends conical-shaped trees that are deep-rooted.

She advises forgoing magnolias or other flowering trees as inappropriate visually for hillsides.

“The angular slopes don’t lend themselves to flowering trees,” she said.

Stone favors deciduous trees because hillsides often are subjected to winds; deciduous trees present less mass and therefore less of a problem in heavy winds.

Visbal suggests a combination of planting trees and shrubs with hydroseeding to provide quick coverage.

Hydroseed mixes contain a combination of annual and perennial flowers for quick cover and color. The seed mixes often include California poppies, alyssum, O’Connor’s legume and gazania.

Stone reports that low and quick-growing shrubs such as Acacia redolens and Myoporium pacifica, an evergreen, are proving more effective than hydroseeding.

“Some of the landscapes we designed for housing developments in hillside areas which were hydroseeded when they were built one or two decades ago are now being replanted with acacia and myoporium,” he said.

He also noted that yarrow, a drought-tolerant perennial used on hillsides, may not please everyone’s eye.

“After yarrows flower, they retain their brown seed pods,” Stone said. “This can be a maintenance problem if the homeowner wants to deadhead the yarrow or an eyesore if the homeowner doesn’t like the brown tone.”

Stone also cautions against using ivy, another traditional favorite, on hillsides because rats and other rodents like to hide in it.

But what to plant isn’t as crucial as how that planting is maintained.

“Hillside plantings must be irrigated to establish them and to prevent the plant materials from turning into fire hazards in the hot weather,” Visbal said.

Many homeowners over-water their landscapes, which contributes to the slippage problem when saturated soil is subjected to heavy rainfall.

Problems also arise from having one valve control the entire hillside planting.

“Plants at the base and middle of the slope don’t need as much water as do the plants at the top since the water will course down the slope,” Stone said.

“We recommend separating the hillside landscape into three zones, each with its own control valve.”

Stone recommends that when possible drip irrigation be installed with emitters to the individual shrubs and trees. This assures that the water goes to the plant roots instead of the surrounding soil, thus lessening the soil saturation problem.

Homeowners with existing sprinkler systems may be able to retrofit them with the newer, low-volume sprinkler heads and reduce the amount of water broadcast over the slope.

The specific amount of water required by the landscape varies according to the soil condition, air temperature and time of year.

A general guideline for established plantings is to irrigate once or twice a week in hot months and every two weeks or so in cooler or overcast weather.

Hillside Plant Recommendations

TREES

California pepper

California sycamore

Eucalyptus (can be flammable)

London plane

Pines

Toyon

SHRUBS

Acacia redolens

Cistus (rockrose)

Cotoneaster