Let's cut to the chase: Our natural slopes seldom slip. Their rocky soil and deep-rooted mantle of chaparral and coastal sage scrub have evolved in union and held securely -- despite cyclical fire and flood -- for eons.
But local hillsides are weakened and give way when people, by design or in ignorance, destroy the natural topography, remove native vegetation or water their gardens unwisely. Once the land has been amputated, raped and scraped -- and thus stripped of its natural defenses -- the soil must yield to gravity when weighted down with water.
So what can hillside dwellers do to keep their sloping land stable? Start sooner than later, the experts say, and let nature be your guide.
If native plants still dot the property, by all means, let them be.
"California natives are often the first plants removed from an at-risk landscape. This could be a costly mistake," says Melanie Baer-Keeley, a restoration horticulturist at Sequoia & Kings Canyon National Parks.
"These plants perform many essential functions," she adds, "including erosion control, watershed protection and food and cover for wildlife -- while retaining the unique character and beauty of the land."
In most situations, fire is a parallel concern that should also influence design and maintenance.
If a site has been stripped, the best way to ensure slope stability is to plant or seed a broad range of native annuals, perennials, shrubs and trees for various depths of erosion control, with emphasis on deep-rooted shrubby species that naturally dominate plant communities.
Ground covers and other low growers produce wide-spreading roots and minimal fuel for fire. In addition, they break the fall and lessen the force of water. Taller natives develop massive root systems that bore to enormous depths (some to 15 or 20 feet). For fire safety, large plants should be spaced at least 15 feet apart.
Greg Rubin of Escondido-based California's Own Native Landscape Design (www.calown.com) likes California cultivars -- plants selected for garden use -- because they can tolerate a little more water. During fire season, they get extra irrigation to fortify and plump up their foliage.
Although nonnative plants, including deep-rooted ones from climates and soils like our own, can be used for erosion control, many of them (lavender and rockrose) require more water to thrive than plants from California.
And because a dry slope is less likely to slide, it makes sense to use plants that need minimal irrigation.
"Think of the ecology of every square foot as you put your landscape together," says Bert Wilson of Las Pilitas Nursery (www.laspilitas.com). Choose the right natives, and once they're in, he says, "Water them once and walk away." Though it sounds radical, he's had 90% success using this technique.
The drier the better, it seems.
"Automatic irrigation systems are the No. 1 cause of slope failure," says Klaus Radtke, a wildland resource scientist and author of "A Homeowner's Guide to Fire and Watershed Management at the Chaparral/Urban Interface" (downloadable free at sandiego.gov/water/pdf/firewatershed.pdf). If you garden on a slope, he says, set the controller on "manual" and water only as needed to get plants established and keep them going.
But let's rewind and consider those storm- and fire-ravaged slopes in need of immediate care. In every case, the most important job is to stop further erosion.
However, the once-common practice of reseeding denuded slopes with quick-sprouting, shallow-rooted exotic grass is no longer considered smart. It muscles out natives whose seeds emerge eagerly -- and sometimes only -- after a fire. Grass also lies flat when wet, the water sheets off, and eventually the whole soggy mess can slide down the slope.
Certain ecologists and members of the California Native Plant Society (www.cnps.org) believe exotic grass and nonnatives, in general, may increase the intensity and frequency of fire. As a stopgap where needed, both Radtke and Rubin suggest seeding bare ground with lupines, poppies and other native wildflowers.
Jute netting is another quick fix, with wide windows that make planting easy. Though effective, it rots quickly in place. Some professionals prefer the endurance and appearance of straw wattles -- rolled hay-like bumpers that are laid across the slope to decelerate water flow. Others swear by a mulch of shredded redwood bark, especially following a fire. "It is fibrous and stays put like a carpet," says Rubin. "It also encourages mycorrhizal fungi that attach to plant roots and unite and nourish each plant community.
"I try to emulate the natural ecology," he adds, "a chaotic system that's so poorly understood."
He doesn't add soil amendments before planting, and he never fertilizes the plants. "Natives like their soil clean, mean and lean. I show up with 400 plants and a jackhammer."