The Coastal Gardener: In heavy rain, it’s a slippery slope

Last week my little rain gauge mounted on my fence reached its capacity and overflowed. Next time I checked it, water was over the rim again. I have to buy a bigger rain gauge. But not far away a neighbor, an amateur meteorologist, has recorded almost 16 inches of rainfall so far this season. Because our yearly rainfall averages about 13 inches, and we’re only a third of the way through the rainy season, we can safely say it is going to be a wet winter.

During the early morning following the wettest night of the storm, I managed to weave my way through flooded, rain-soaked streets. I carefully navigated alternate routes, avoided at least two road closures from flooding, and finally got to work.

During my drive, as raindrops pelted at me, gutters raged and wipers worked hurriedly, I noticed several residential hillsides that had slipped, causing serious and expensive damage.

Wherever heavy erosion or slope failure is a possibility there are several considerations. Is the soil on the slope naturally formed or is it the result of artificial grading and construction? Is there a history of erosion on the slope? Does water move across the slope or is it moving down the slope? Are there erosion channels on the slope and are they small or large? Where does the water go that hits the slope? Where does the water go in the areas above and below the slope?

On landscaped slopes, soil is often artificially in place. In my experience, the placement of one soil type or texture on to the top of another soil type of texture is the most likely cause of slope failures. It’s like trying to balance a cherry on the side of a mound of melting ice cream. We shouldn’t be surprised to find the cherry in the bottom of the bowl, nor should we be surprised when these hillsides fail.

Slopes and hillsides with layered soils like these need trees and shrubs and other plants to stabilize them and to bind the different soil layers together.

Most hillsides can be made relatively stable with the right plants. A proper planting can stop almost all erosion and slippage on an incline. The most important rule when using plants on slopes is to use a mixture of species and sizes: groundcovers, shrubs, trees and perennials. Never plant a monoculture of one groundcover, shrub or other plant on a slope, no matter what you might have heard or read regarding its soil binding qualities. Unfortunately, many people still mistakenly think they are doing good work by installing a large expanse of ice plant, ivy, honeysuckle, myoporum or rosemary. These single plantings are a big mistake and can actually make the likelihood of a slip even more likely.

Generally, the bigger a plant grows, the deeper it roots. The object on a slope is to bind the soil at various depths, not all at the same depth. Trees will bind soil deeply, shrubs to a medium depth, groundcovers and perennials more shallowly. On a slope, a variety of plants, thoroughly interspersed, growing to different sizes and different root depths is essential. If these homeowners, such as the one in the photo, would have heeded this lesson their slope would still be where it is supposed to be, instead of 30 feet away.

Additionally, when you have a mixture of plants on a slope, you also have layers of vegetation that the rainfall will hit. When the water finally reaches the ground its force is greatly reduced, creating far less erosion.

Areas of a slope that are not covered with plants should be occupied with a mulch of shredded redwood or cedar. Don’t bother with common bark nuggets or ground products, they’ll just wash off the slope during the first good storm; shredded mulches will stay in place.

When planting a slope be sure to use water-thrifty plants. Retaining water on a slope during the warm season is a difficult chore and too much irrigation encourages slippage and erosion. When planting the slope never till the soil and don’t dig planting holes larger than necessary.

With a little planning and knowledge, slopes and hillsides can not only be attractive parts of a landscape, they can be safe and stable, even in the heaviest of rains.

RON VANDERHOFF is the nursery manager at Roger’s Gardens, Corona del Mar.

Ask Ron

Question: I cannot remember the name of the large evergreen shrub with the attractive red berries that I am seeing around town. I think it is a native plant.

George, Newport Beach

Answer: I’m sure it is toyon, also sometimes called Christmas berry (Heteromeles arbutifolia). This common local native plant is one of our best for low maintenance, low-water and wildlife-friendly plantings. The small red berries are prolific at this time of year and the plant somewhat resembles a big bushy holly, but without any sharp spines. In fact, the city of Hollywood was named in honor of this plant, which grew abundantly in the nearby hills.