December 16, 2017
Winter is the season for holidays, football games, Dungeness crab and — rain! In Marin’s Mediterranean climate, 90 percent of the annual rainfall normally occurs between November and April. After our dry summers, we depend on the rainfall for the deep watering our native and garden plants need. Ideally our winter rains fall gently on rich and stable soil, nourishing our thirsty plants. But when the rain washes away topsoil and floods flowerbeds, it can become too much of a good thing.
While we can’t control the rain itself we can employ some landscaping strategies to direct the path of the water and reduce garden damage. Since erosion is a common problem in our hilly residential areas, a first defense against the destructive effects of rain runoff is to stabilize slopes. The most effective techniques for erosion control are determined by the steepness of the terrain. In its “Homeowner’s Guide to Erosion Control,” the California Natural Resources Conservation Service provides guidelines for slope stabilization based on the extent of the grade. Moderate slopes, with less than a 33 percent grade, can be protected with plants and mulch. Plants selected for erosion control should have deep spreading root systems. A mix of ground covers, perennials, shrubs, and trees is ideal. Some native species that are good for slopes include eriogonum (California buckwheat), arctostaphylos (manzanita), rhamnus (coffeeberry), ceanothus (California lilac), and baccharis pilularis (coyote brush). Bare soil between the plants should be covered with a layer of organic mulch such as small bark or wood chips that are not likely to wash away easily. Avoid the finely shredded bark that is sometimes sold to cover slopes. It is a fire hazard.
The steeper the slope the more likely additional structural support will be needed for stabilization, Slopes with grades between 33 and 50 percent can also be planted but erosion controls such as jute netting and straw wattles (long fiber-encased straw tubes) may be need to be installed to retain the slope until the plants can take over.
Slopes over 50 percent will require retaining walls or terracing for stabilization. Terraces can be an attractive option and provide more usable garden space allowing for a greater variety of plant selection. Rocks, boulders, concrete blocks and railroad ties can be used to create simple low retaining walls for hillside terracing. Taller walls may require help from a professional contractor to insure the results are stable and properly reinforced. The Marin County Building and Safety Division requires a permit for retaining walls over 4 feet in height.
If last year’s rainfall produced a mossy bog in your garden, then consider installing a drainage system to redirect the flow of water to areas where it will not collect and cause problems. Much of the soil in Marin is fine-textured clay that drains slowly and increases the potential for surface runoff. A dry creek bed is a popular type of surface drain. Constructed with boulders and river rock to appear as a natural stream, a dry creek bed can be designed to channel water down a slope or simply to provide a trough to capture the rainfall and allow it to soak into the ground.
Where runoff is heavier, a French drain can help. A French drain is a subsurface drain consisting of a gravel-filled trench with a perforated pipe in the bottom. The perforated pipe directs the excess water collected in the trench to a disposal site. French drains can be used to divert rainwater around houses and patios and to provide additional drainage on terraced hillsides behind retaining walls.
You can design your landscape to make rainwater a welcome asset and not a problem in your garden. Methods for controlling runoff and preventing erosion can also add interest and beauty to your site. It’s time to welcome the winter rains.