EROSION CONTROL WITH VEGETATION FORCALIFORNIA'S MEDITERRANEAN-LIKE CLIMATE^

Theodore E. Adams, Jr. and Burgess L. Kay^

Bare soils on slopes along roadways and around new homes are subject toerosion. Without a dense cover of plants to protect the soil surface theenergy in falling rain and running water can gully surface soil and washmud onto landscaped or paved areas and into storm drains and streams. Wind,too, causes erosion.

One of the best ways to reduce erosion is to grow plants that completelycover the soil surface. There are three steps in reducing erosion: (1)divert water away from the slope, (2) protect the soil with a temporarymulch, and (3) establish a plant cover.

Water must not be allowed to cascade uncontrolled down long or steepslopes that have been disturbed. Diverting the water at the top of theslope with such structures as dikes, ditches and raised berms will helpprevent serious erosion and reduce soil saturation, a condition that oftencauses land and mud slides. Remember, once a slope is badly eroded, plantsare of little help. Damage must be corrected mechanically. Conditionsmust be created for plants to grow and produce effective cover.

A cover of plants grown to protect the soil surface will be effectiveonly if the site is geologically stable and the slope is properly designed.Slopes must not be so steep that gravity alone will cause loose soil totumble down. A good rule of thumb is to construct slopes so that each footof rise or fall takes place over a horizontal distance of tv/o or more feet(a 2:1 slope). A 5:1 slope is preferred. Longer slopes (20 feet or more)should be broken up with small ditches or flat bences on the contour. Some

' situations may require even shorter slope segments.

A complete cover of vegetation planted for erosion and sediment controlhas four major functions: (1) to absorb the impact of raindrops, (2) toallow more rainfall to enter the soil rather than to run down the slope,(3) to bind soil particles together, and (4) to reduce the velocity of run¬off and the amount of soil carried down the slope. Protecting soil fromerosion reduces or eliminates the need for building permanent sedimentbasins and for dredging streams, lakes and reservoirs.

It takes time and care to grow a ground cover, because bare slopes areoften rocky or infertile. Under these conditions, plants are difficult tostart and they grow slowly. But remember, the costs of establishing a plantcover are small compared with the value of improvements the cover is designedto protect.

^Workshop presentation delivered at the SCSA, Calif, Chap., Silver AnniversaryConference: Erosion Control on the Advance, Asilomar, January 25-28, 1983.

^Vlildlands Specialist, Cooperative Extension and Specialist, Department ofAgronomy and Range Science, University of California, Davis.

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What to Plant

Grass is the most rapidly growing type of plant that will produce acomplete ground cover. Anruial grass, adequately fertilized, establishesquickly from seed and can provide complete erosion protection more rapidlythan any other ground cover.

After three to five years, other types of plants--native shrubs, forexample--niay take over an area that was seeded with annual grass. Thisrepresents a natural stage in v/hat is called plant succession, the replace¬ment of one group of plants by another that is better adapted to the site.If desired, erosion control plants later can be replaced by permanent land¬scaping after the soil has been completely staoilized.

Annual grasses^ Annual grasses occur throughout California withoutseeding. They may in two or three years completely occupy disturbed sitesfrom the surrounding reservoir of plants. However, seeding should beconsidered if the grass stand is to develop rapidly. Seed of the follov/-ing species found in California is available and is recommended for use:Blando brome (Bromus mollis), Italian (annual) ryegrass (Lolium multiflorum),and Zorro annual fescue (Vulpia myuros).

Blando brome will reseed on a wide range of sites. Ryegrass is bestadapted to deep, fertile soils with abundant moisture; it does not reseedon shallow, rocky sites. Zorro annual fescue is adapted to the poorest andmost acid soils and lowest rainfall areas. Its seed is the most expensiveof the grasses suggested, but the great number of seeds in one pound meansfewer pounds are needed for a project, compared with the other annualgrasses.

Seed can also be harvested from existing local stands of annual grass.Seed production on natural grasslands is estimated to be 400 pounds or more

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per acre each year. Seed, grass leaves, and stems may be harvested toget^ieras grass hay and spread over the areas to be treated at the rate of 1 to 4tons per acre, depending on the mulching method selected (see Mulching forErosion Control). One acre of annual grasslands will produce about 1 ten ofhay.

The hay should be harvested after seed ripens, but before it shattersand falls to the ground. The time of harvest is generally May, but hiswill vary with the species and with site conditions (elevation, directionof slope, soil moisture).

If hay harvest is impractical, small amounts of seed can be collectedafter it has ripened and fallen to the ground. Such seed harvest can bedone with a yard vacuum or a heavy-duty shop vacuum cleaner connected to aportable generator.

Perennial grasses. Several commercially available introduced drylandperennial grasses can be used where onnual rainfall is 20 inches or more.They develop more slowly than do annuals and do not provide as good pro¬tection the first season as annual grasses. When established, these grassesbegin grov/th earlier in the fall than do resident annuals and they remaingreen longer.

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Perennial grasses should be considered for use only where the reservoirof seed of resident annuals is low, because annual seedlings compete stronglywith more slowly-developing perennials.

A good perennial grass choice is a sumiier dormant orchardgrass (Dactylisglomerata). The preferred variety is Berber, which develops fairly rapidlyand is not likely to produce an objectionable amount of top growth. Perennialgrasses intended for irrigated lawns will not survive under dryland condi¬tions.

If initial stands of perennial grass are acceptable, replacement byannuals is unlikely. As the plants develop in subsequent seasons, it becomesincreasingly difficult for annuals to grow. This has been demonstrated inCalifornia where perennial grasses have been seeded for range improvement.

Two species of introduced annual clovers and California poppies(Eschscholtzia californica) can be seeded individually or in a mix withperennial grass" Wilton rose clover (Trifolium hirtum) and crimson clover(T. incarnatum) in northern California and Hykon rose clover in the southadd color. The nitrogen they produce will enhance the vigor of grass seededwith them and will reduce or eliminate the need for supplemental nitrogenfertilizer after the first year.

Good growth of clovers requires two factors: inoculation of the seedbefore planting with commercially available bacteria specific for the cloverbeing used and fertilization with single superphosphate after planting.Directions for inoculation are described in University of California,Division of Agricultural Sciences, Leaflet 1842, Range-Legume Inoculationand Nitrogen Fixation by Root-Hodule Bacteria. An alernative is a commercialprocedure such as the Pellnoc system developed by the Nitragin Company,Milwaukee, Wisconsin. Some seed dealers provide a pellet-inoculationservice.

Rocky Sites

Rocky sites present a special problem. Zorro annual fescue or a mixof clovers and flowers can be used. Clovers, especially rose clover, arewell adapted to rocky sites where fertility and soil depth limit growth ofgrasses.

Flowers. California poppy and African daisy (Dimorphotheca sinuata)can be seeded alone or with clovers. Like the clovers, California poppydoes well on sites that restrict grov/th of grass.

Because poppies grow slowly, establishment success and rate of growthare affected by grass competition. Where dense stands of grass are expected,as many occur when nitrogen fertilizer is applied, or where grass is seededat high rates for erosion control, seeding of poppies is not suggested.

Fertilization

Where fertile top soil has been stripped away or v/here soils are shallow.

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fertilization will be necessary when seeding to produce adequate grass growthfor erosion control. In these situations apply 80 pounds per acre of nitro¬gen and 100 pounds per acre of phosphorus pentoxide, P2O5. Five hundredpounds of ammonium-phosphate-sulfate (16-20-0) per acre will provide thenecessary nutrients including sulfur, which also is deficient in mostCalifornia soils. This is about 11 pounds per 1000 square feet. Fertiliz¬ing in later years generally is not necessary.

Clovers and flowers need mostly phosphorus and sulfur for good growth.Five hundred pounds of single superphosphate (0-20-0) per acre (11 poundsper 1000 square feet) will provide enough of each element.

The amount of growth that results from fertilizing grass presents agreater fire hazard than unfertilized stands. This must be weighed againstpotential soil loss and erosion damage on critical areas. Do not over-fertilize.

Do not use nitrogen fertilizer with flowers or clover if annual grassand other weeds are present. Nitrogen encourages vigorous growth ofvolunteer grass that will restrict growth of flowers and clovers.

Seeding Conditions and Techniques

Scattering seed on the surface of the soil (broadcast seeding) isusually necessary because of site conditions. Depending on the size ofthe job, this can be done commercially or by hand. Broadcasting using ahand spreader or seeder makes the job easier.

Commercial seeding is often done by hydroseeding—sprayed on in awater slurry. Inoculated clover seed should not be hydroseeded becausethe inoculant may wash off the seeds.

Preparation of a loose seedbed is desirable--some place for the seedsto lodge and be covered with soil. Construction activities or tree andbrush removal generally leave sites sufficiently disturbed to permit theseed to reach mineral soil. Graded slopes should be left rough. Rainfallwill help cover much of the seed, but raking or dragging the seeded areaswith tire chains, sections of cyclone fence, heavy boards, etc., covers theseed much better.

Mulching for Erosion Control

Straw mulch. If soil is heavily disturbed or has little surface organicmaterial (branches, roots, leaves, etc.) to retard water runoff, protectionwith grass hay or grain straw is necessary. This mulch provides initialerosion control while the plants are getting started. It may take as longas three or four months after the first fall rains for ground cover tobecome effective.

Spread 1 to 2 tons per acre (about 50 to ICQ pounds per 1000 squarefeet) of full-length (not chopped) straw on the surface after seeding andfertilizing. Cover it with a fabric net of plastic or fiber. Straw can

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also be kept from blowing away by punching it into the soil with a long,narrow bladed-shovel, sometimes called a transplanting or drain spade.The results should look like the tufts of a tooth brush. With eithermethod, mineral soil should be visible when you look straight down throughthe mulch, and the loose straw should be about 2 inches deep. More thanthat may produce a mat too dense for seedlings to penetrate.

If the areas to be protected are large, full-length straw can be blownon using commercial equipment designed for the purpose. Special strawcrimpers and punchers are generally used to incorporate straw on theselarge areas.

Use of commercial equipment allows an application rate as high as 4tons per acre. At this rate, one-half the straw is usually spread andincorporated at one time followed by a second application, also incorpor¬ated. One-half the seed and fertilizer are broadcast after each applica¬tion of straw has been crimped or punched. The International Erosion Con¬trol Association, P.O. Box 807, Freedom, CA 95019, can provide informationon contractors who do this work.

Tackifiers. Materials called "tackifiers" are sometimes sprayed onstraw to keep it from blowing away, particularly in the eastern UnitedStates. The effectiveness of many of these has been tested by the Universityof California. They are most often used on-steep slopes where crimping orpunching straw is not possible because of limited access or soil too hardfor the equipment to be effective.

Asphalt emulsion is the tackifier most commonly used. It is appliedat the rate of 200 to 500 gallons per acre, either during the straw-blowingoperation or sprayed over the straw mulch after it is in place.

A material called Terratack I®, a natural plant gum extracted fromguar (Cyamopsis tetrogonoloba) seed, has been shown to be effective in hold¬ing straw in pface. It is mixed wich wood fiber and water and sprayed on |after straw is in place. A mixture of 45 pounds of the chemical, 150 poundsof wood fiber, and 750 gallons of water is enough for one acre.

Ecology Control M-Binder® is a more effective plant gum. It is produced |by an annual plantain, Plantago insularis. One hundred pounds of this materialmixed with 150 pounds of wood fiber and 700 gallons of water will treat oneacre of straw mulch.

A copolymer emulsion of styrene butadiene, synthethic butyl rubber (SBR),also is an effective tackifier. The recipe for one acre includes 60 gallonsof the material, 75 pounds of wood fiber, and 400 gallons of water.

Wood fiber alone can be sprayed on in a slurry to hold dry straw in place.The application rate is 750 to 1000 pounds per acre. However, its effectsare short-term, because it dries out.

Increasing the rate per acre of any of the above materials will increasetheir effectiveness, but none are needed after it starts to rain. Wet strawstays in place.

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Fabric mulches. Erosion control fabrics made from jute, excelsior,and woven from paper or plastic fibers are available in rolls. They aresometimes used as a weed-free substitute for straw mulch. When unrolledover seeded and fertilized areas, they are held in place with heavy wirestaples.

Use of these substitutes is limited by their cost and their effective¬ness. The basic price of these materials is high compared with straw, andthey require a great deal of labor to install. They do not lie close tothe soil surface in rough or rocky areas, and as a result, erosion beneaththe mats is common. Unless carefully anchored in place, wind may getbeneath the mats and roll them up, uncovering the soil. Jute matting isprobably the most effective of these artificial mulches, but none are aseffective as straw.

The best use for fabric mats is to hold soil in place around homes untillandscaping provides effective soil protection. They can also be used torepair small areas where another treatment has failed. In these situations,costs are usually of secondary importance.

Mulch combinations. Jute is sometimes used on top of straw to increasethe effectiveness of straw mulch. On steep slopes (2:1) under artificialrainfall, jute laid over straw spread at 1.5 tons per acre and tacked- inplace with asphalt emulsion was much more effective in controlling erosionthan straw alone spread at 4 tons per acre and punched into the soil.

Plastic mulches. Water soluble plastic emulsions sprayed on the soilsurface and allowed to dry have been used as effective but very expensivemethods to control erosion. However, these materials--many are similar towhite glues used as adhesives around the house--have several undesirablecharacteristics: (1) they delay seedling emergence and reduce the number ofplants that develop; (2) temperatures below 40 to 55°F, depending on thematerial, and high humidity delay curing; (3) freezing temperatures destroyuncured emulsions; (4) rain falling on an uncured crust reduces or destroysits effectiveness; and (5) crusts are not self-healing, they need protectionfrom traffic.

Hydraulic Seeding

Hydraulic seeding or hydroseeding is the application of seed andfertilizer in a water slurry, using a hydroseeder. A wood-fiber mulch isusually included. This method is much more expensive than hand seedingand is more appropriate on steep areas where straw cannot be used.

Wood-fiber mulch holds seed and fertilizer in place. It also providessome mechanical stabilization and protection from rainfall until a plantcover becomes effective. Increasing the rate of wood fiber from the 1000pounds per acre commonly used to 3000 pounds per acre will improve annualgrass establishment and provide greater erosion protection.

Mulches applied hydraulically are less weedy than straw mulches butmuch more expensive. Also, they are much less effective in controllingerosion and aiding plant establishment compared with the straw mulches.

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Time of Seeding

Seed in September or early October before the first rains. Laterseedings often germinate and develop slowly because of cold temperatures,and they may not produce a protective soil cover.

In studies at U.C. Davis, emergence of annual grasses used in erosioncontrol programs is directly related to planting date. If water is notlimiting, seedings in the middle of October are up within one week or less.Later seedings require much longer to emerge, two to three weeks when plantedin the middle of November and up to four weeks if planting is delayed untilDecember 15.

Although not usually needed and very expensive, irrigation can be usedto ensure germination and adequate growth before heavy winter rains.However, if not carefully applied, irrigation water can cause erosion andwash seed away. Once started, irrigation should continue without interruptionuntil grass is 2 inches tall. Proper irrigation can produce effective cover,eliminate gambling on fall rain to initiate germination and growth, andeliminate the need for supplementary protection on disturbed sites whenvegetation establishes too slowly.

If erosion control is not a major consideration, seeding can be as lateas Christmas. Perennial plants from later seedings may not survive.Annuals may not produce enough seed for stand survival if seeded much laterthan December.

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SUMMARY OF SEEDING RECOMMENDATIONS

Annual Grass Seeding Recommendations

Areas above 10" rainfall Areas below 10" rainfall

Blando brome withannual ryegrass

0 300 20

1b/acre1b/acre

Zorro fescue 0 20 1b/acre

Total 50 lb/acre

Perennial (jrass Seeding Recommendations

Northern California Southern California

Berber orchardgrassalone or with any or

0 20 1b/acre Berber orchardgrassalone or with any or

0 20 lb/acre

all of the following: all of the following:

Crimson cloverWilton rose cloverCalifornia poppy

310

1

lb/acrelb/acrelb/acre

Hykon rose cloverCalifornia poppy

101

lb/acrelb/acre

Seeding Recommendations for Rocky Sites with Little Soil

Northern California Southern California

GrassZorro annual fescuealone

0 20 lb/acreGrass

Zorro annual fescuealone

0 20 lb/acre

or or

Clover and Flower MixCrimson cloverWilton rose cloverCalifornia poppy

320

2

1b/acre1b/acrelb/acre

Clover and Flower MixHykon rose cloverAfrican daisy

20 lb/acre5 lb/acre