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Erosion Control for Landscape Installation

Article by Ted Pope

Landscape contractors need to consider many factors in planning a new landscape installation. Erosion control is important on all landscape jobs, but of particular concern for landscaping on hills, slopes and any uneven ground.

The action of wind and water on our environment has created works of outstanding natural beauty. Standing on the rim of the Grand Canyon, visiting Zion National Park or marveling at the sculptured bluffs of the California coast can be awe inspiring. However, when this same wind and water brings the newly landscaped backyard hillside down into your swimming pool it can hardly be described as emotionally uplifting.

Erosion is a natural process of our earth's formation. In the Western United States the forces of erosion are particularly noticeable. Back in the days when our country was a frontier, erosion did not represent nearly the impact on our society that it does today. Now, however, as millions of dollars are spent annually on landscape construction and repairing and preventing surface soil erosion, erosion control has become an important aspect of landscape design, installation and construction.

The first step to effective erosion control is recognizing the potential for erosion on your property or landscape project before it has an opportunity to occur. Surface soil erosion takes on many forms, such as hillside rilling or gully formation, silt build-up, slope slippage, and the exposure of barren and unproductive soils. Blowing sand and dust are also a form of surface soil erosion. Obvious danger areas are newly formed or constructed hillsides, changes in surface water drainage patterns, and areas denuded of natural vegetation and topsoil. The installation of irrigation systems can of themselves create a surface erosion problem if not properly designed and installed.

Serious surface soil erosion can also develop from subsurface instability and ground water problems. These are more appropriately problems for a civil engineer and won't be dealt with in this article, however.

Once an erosion potential is identified, the next step is to formulate a plan to deal with it. Needless to say, investing in the proper engineering design and site preparation can materially reduce erosion problems. If the erosion problems already exist and have created damage, often the only approach is to rework the affected areas and start from scratch with an erosion control plan.

A popular approach to surface erosion control, particularly for newly graded hillsides, pads and drainage areas is revegetation. This landscaping usually consists simply of introducing grasses, flowers, ground covers and even shrubs and trees to a erosion vulnerable area. The vegetation creates stability through its root structure and density of growth for a long lasting and attractive erosion control treatment. Some care must be taken in choosing the proper species of plants which will provide the necessary erosion control and be suitable for the climate and terrain in which they will be planted. It can be very beneficial to obtain a soil test for the area so as to properly plan the fertilizer and soil amendment requirements to assure a full, healthy growth of vegetation.

The proper amount of moisture can be critical to a revegetation project. Many landscape projects otherwise properly conceived and implemented, have failed disastrously because they did not receive the proper amount of moisture to insure germination and survival. Though nothing is guaranteed by Mother Nature, planting an area at a time of year to take advantage of the natural rainfall is one way to improve your chances of success. Selecting plant species for the climate in which you are planting makes good sense. You may invest in an irrigation system, temporary or long term, to insure the proper germination and development of your revegetation project. Sometimes there can be too much moisture for a newly planted hillside. The problem then becomes one of holding seed, mulch, fertilizer, ground covers, and loose soils in place on the hillside until the revegetation effort has an opportunity to take hold and develop significant root structure. There are several products available to the landscape professional to help in holding his revegetation effort in place.

Different mulches are often used to provide an element of surface erosion control. This is in addition to the moisture retention and soil surface cooling which they provide to benefit seed germination. Straw has been a popular mulch for years. It is spread either by hand or with special straw blowing equipment. In some cases the straw is "crimped" or disced into the soil. Application rates vary depending on the landscape conditions, but a typical application would be about 2000 pounds per acre. Also available are wood fiber and paper mulches. These products are applied by hydroseeding equipment in a slurry of seed and fertilizer, and are spray applied at rates generally from 1200 to 2500 pounds per acre. The cost of mulch is a fraction of a penny per square foot. Some of the wood fiber and paper mulches on the market are: Weyerhauser Silva-Fiber, Conwed Hydro Mulch, Superior's Turf-Guard, Cellin Fiber Mulch, Pacific Wood Fiber's Spra-Mulch.

There is a family of erosion control products known as tackifiers, or binders, which hold seed, fertilizers and mulches on hillsides and slopes against the effect of either rainfall or excessive irrigation. These tackifiers are divided into two groups. The first group is composed of the organic derived materials (i.e. guar, plantago, flour derivative, etc.), which come in a powdered form. The organic tackifiers are primarily designed to perform with mulches since they act as a sticky adhesive to hold the wood, paper or straw materials in place. They are relatively short lived and will ultimately wash away, but do provide a good, temporary holding for mulch and seed on modestly steep slopes. Organic tackifiers are generally applied at the rate of 50 to 120 pounds per acre and represent an applied cost of less than a penny per square foot. These materials are applied with hydroseeding equipment and are mixed into the slurry of seed, mulch and fertilizer which is then sprayed on the hillsides and other areas. Some names to look for in organic tackifiers are: M-Binder, J-Tac, Az-Tac, Terra-Tack and Sentinel

The heavy duty members of the tackifier family are the copolymer based products. These binders are composed of latex copolymers, sometimes in combination with other materials, and come in liquid form. They are diluted with water in the field. The copolymer binders exhibit a benefit over the organic materials in that they will soak into the soil forming a crust with seed, mulch and fertilizer and the top 1/4 inch of soil. This creates a better holding potential. In addition, the copolymer tackifiers exhibit significantly longer field life than the organic materials. The copolymer materials can also be successfully used on landscape projects without mulch because of their penetrative abilities. This characteristic makes the copolymer products particularly useful in holding soils in place in and around newly planted ground covers. A copolymer application rate will vary depending on job conditions and the type of product used. Generally such an application represents a cost of about a penny per square foot. Application can be made with a hydroseeding unit as in the case of the organic materials, but the copolymers have the added advantage of also being applied with a power spray unit, water truck, agricultural spray unit or even a back pack sprayer. They are generally chosen over the organic materials when the slopes and hillsides are steeper, soils looser and the threat of severe rains or abundant irrigation is greater. Copolymer erosion control products are Soil Seal Corporation's Soil Seal Concentrate and Borden Chemical's Geotech.

Another popular method for slope erosion control in the landscape is the use of jute netting. Jute netting comes in rolls, and is spread over the slope and staked in place after seeding has been accomplished. Jute netting can be applied before ground covers are planted by cutting through the netting to accommodate each plant. As with all netting and erosion control blankets, the netting must be kept in close and continuous contact with the soil surface so as not to be undermined by surface water and thereby fail. A jute netting application will cost approximately twenty to twenty-five cents a square foot installed.

Netting blankets filled with excelsior are used like jute netting and represent approximately the same in place cost. These netting and blankets are designed for the more difficult soil situations and high,intense rainfall and irrigation conditions. American Excelsior and PPS Packaging Co. are two manufacturers of this type of product.

A fairly new product in the erosion control market is a porous matting composed of about one-half inch thickness of twisted, swirled plastic strands. This material will actually withstand direct water flow or fluming, and is suggested as a replacement for riprap and gabions in certain applications. It is also staked in place. An applied cost is approximately $1.40 per square foot, and is generally reserved for the most severe of erosion problems including stream channels and the like. Two manufacturers of this type of product are Mirafi Inc. and American Enka Co.

There are those erosion problems which do not lend themselves to revegetation. An example would be the need for interim erosion control on a landscape construction area which has been recently graded, but is not yet ready for finish construction and landscaping. In this case, a copolymer material, such as that described for use in revegetation, can be used. The liquid copolymer material, after dilution with water in the field, is spray applied to the soil surface to be protected. The solution soaks into the soil and cures to form a crust of material through the cohesive binding of the soil particles. It is this crust, or mass of material, which provides the erosion control on the slopes. Application rates vary depending on job conditions and the copolymer which is chosen. A typical application cost for this type of treatment is two to three cents per square foot. Again, the copolymer solution can be applied with a variety of spray equipment, including a power spray unit, hydroseeding equipment, water truck and back pack spray unit. The materials are non-hazardous and biodegradable. A typical application can last as long as twelve to fourteen months. In addition to a copolymer treatment, silt fencing and sandbagging can be utilized on down stream areas as a backup to controlling water runoff and silt buildup.

Most of the products referred to herein are available through local seed and fertilizer distributors, and irrigation supply houses. Though not always readily available to the weekend gardener, the landscape professional should not have a problem finding them.

There are various publications available on the subject of erosion control published by the Soil Conservation Service. Also, the International Erosion Control Association prints proceedings of their annual conferences. Various educational institutions such as Cal Poly Pomona and the University of California at Davis regularly engage in the testing and evaluation of erosion control techniques and materials, to name but a few areas where additional information can be obtained.

Ted Pope is the Vice President and General Manager of the Soil Seal Corporation, a wholly owned subsidiary of Pacific Architects and Engineers, with main offices in Los Angeles, CA.

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