Erosion Control

Saturday, June 8, 2013

By Bruce Reed, News-Press Correspondent, arborist and horticulturist at the Santa Barbara Botanic Garden

Santa Barbara New-Press

The stark vertical relief of our local mountains show how relatively quickly these forms have been built up.  The forces of wind and water round all edges and wear down every grade.  The dramatic features of the Santa Ynez Mountains show their youth precisely because cliff faces and narrow canyons have had less time to erode.  The mountains are achieving in stages what California has been working toward for the last 200 million years: a dry life, out from under the Pacific Ocean. 

Over the course of a human lifetime, of course, we want to slow down the process of erosion as much as possible.  Our success at gardening and feeding ourselves depends on good soils. 

Vegetative cover helps preserve soils from washing away in two main ways.  First it breaks the impact of rain.  Falling water has a small but definite force (20 mph is possible, depending on many factors). Each direct impact will separate the smallest and lightest particles of soil from the rest.  Such material will often move with the splash of the droplet, becoming mobile in the runoff.  Rain on bare soil with substantial runoff will result in more rocky, sandy soil, as the finest clay particles are washed away.

Brush cover, even very low cover, interrupts and absorbs the energy of falling water.  As it clings to leaves and stems and continues its fall, the kinetic energy of the droplet is a fraction of what it carried in its first fall of several hundred or several thousand feet.

The second main way vegetative cover helps preserve soils is well-established plants provide a rei4nforcing network that helps resist water flowing over the surface of the soil.  Like rebar in concrete, the wandering, dividing roots of the plants grow through and past one another, stabilizing not only the plants themselves but the soil as well.  Living roots are more effective at holding the soil, but dead roots are better than nothing.  Removing dead tree stumps is not helpful if you are trying to keep the soil in place.

The bulk of a plant's roots are in the first one to three feet of soil, with most of it in the first 12 inches.  While some California native shrubs and trees have tap roots, roots that extend downward at 45 degree angles or even straight down, they are the exception to the rule of a great saucer-shaped root mass around the plant's main stem.

In undisturbed soils, the network extends far beyond the plant's roots.  Most perennial plants around the world have evolved relationships with soil fungi.  Many thousands of species of soil fungi exist, each with a network of root-like filaments (called hyphae) which are the bulk of the organism.  Mushrooms that emerge from the soil surface are merely reproductive structures and just a fraction of the fungal body.  Fungi that attach to the roots of plants are symbiotic, taking sugars manufactured by the plant and sharing water and nutrients absorbed through the wide spreading hyphae.  This network of interconnected, interlaced hyphae taken together provide a more solid base to grow in than the aggregated soil and add to the holding power of the plants.

Most of the native vegetation in chaparral and coastal scrub are excellent for erosion control as the harsh conditions (i.e. baking sun and low rainfall) have forced these plants to adapt with an extensive root system.  While tap roots are helpful, a shallower system extending 30-40 feet around the plant is still a major force in stabilizing soils.  It is also an efficient water-saving adaptation because the greater the surface area of the roots the more water the plant can absorb in small rain events. 

 Some of the larger shrubs that will work well for erosion control are manzanita (Arctostaphylos) and wild lilac (Ceanothus).  Both groups of plants have many named, cultivated varieties (cultivars) available even from mainstream nurseries.  Manzanitas bloom in winter with delicate white flowers.  Wild lilacs bloom in shades of blue or white in early spring.  But as large a group of varieties as this represents, it barely scratches the surface of what is possible.  Other great shrubs for erosion control are coffeeberry (Rhamnus and Frangula), which are  evergreen with colorful berries; chamise (Adenostoma fasciculatum), one of the tougher shrubs that occupies rocky, southern-facing outcrops; and quick growing shrubs like California sagebrush (Artemisia californica) and coyote brush (Baccharis pilularis).  Of course, our coast live oak (Quercus agrifolia) is one of the best stabilizers of slopes in our part of the state.

Large woody shrubs are not the only great choice for stabilizing slopes.  Many smaller, more herbaceous plan ts grow in colonies, spreading by underground stems (rhizomes).  These dense, spreading root systems knit soils beautifully.  Such plants include chaparral mallow (Malacothamnus), tall with soft pink flowers, deerweed (Acmispon glaber, formerly Lotus scoparius), a quick fire-follower to two or three feet in height with yellow flowers fading orange, California fuchsia (Epilobium), with late summer blooms of red-orange, mugwort (Artemisia douglasiana) able to deal with dry up-slopes or moist streambeds, and goldenrod (Solidago) with three foot stalks topped by many small golden flowers, just to name a few.

For really impoverished soils, rehabilitation may be necessary.  Such efforts almost always include building a more organic soil layer with mulch.  Landscaping companies, like All Around Landscape Supply or Aqua-Flo, can help homeowners decide what technologies would be workable in their situation.  Additionally, S & S Seeds in Carpinteria provides seed mixes of many native plants that can be sown in the traditional manner or applied as a sticky liquid on difficult slopes.