By Deanna Babcock and Rich McLaughlin

Erosion prevention is a major consideration on virtually every development site. The regulatory and enforcement implications can be substantial. While many are aware that erosion control techniques must be used near the final phase of a project to ensure that seeds and/or other plantings are not washed away or displaced, they are also used as temporary covers during different phases of grading.

Like most segments of our industry, innovations and new products have resulted in numerous options in the area of erosion control. A variety of mulches are now used as a protective barrier that covers at least 75 percent of the soil surface, helps prevent seeds from washing away, provides a suitable environment for germination, and slows runoff on slopes. Mulches used for groundcover vary in cost, application procedures, and appropriate conditions of use. Three basic classes of mulch include loose mulch, erosion control blankets (ECBs), and hydraulically applied mulches (hydromulches).

Loose Mulch
Straw is usually the least expensive mulch available, with a cost as low as $0.09 per square yard. Straw mulch with seed, lime, and fertilizer ranges from $0.31 to $1.00 per square yard. Large areas can be covered with straw using commercial blowers. Applying straw by hand is also effective but is more expensive due to labor costs. Straw mulch is applied at a rate of about one to two tons per acre on slopes of less than 2:1 and lasts about three months, enough time to establish permanent vegetation in most areas of the country.  

Because straw is lightweight, it is easily blown away. Tackifiers or “crimping” help reduce this problem. Common tackifiers applied on the straw include:

• Emulsified asphalt, once a common tackifier because of its excellent adhesive properties, is toxic to both plants and animals. Prohibited in some states and all national parks, emulsified asphalt is usually applied at a rate of 12 ounces per square yard.

• Wood and paper fiber hydromulches are natural tackifiers that are used at a rate of 125 to 750 pounds per acre. They are less toxic and more visually appealing than asphalt.   
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• Guar- and starch-based tackifiers are two natural products usually applied at 100 to 200 pounds per acre. Starch-based tackifiers, unlike guar, contain seven to eight percent natural nitrogen and have a high-moisture retention capability, both of which help establish permanent vegetation.

Although the use of tackifiers in conjunction with straw mulch is the most common practice to prevent displacement, using heavy equipment or hand tools to “crimp” or “punch” straw into the soil may be sufficient when the soil is not too sandy. Ideally, some straw is partially buried during crimping, which holds it in place and may increase infiltration rates.

Straw has a relatively high rate of failure compared to other erosion control options. Main causes of failure include insufficient coverage during application, use on slopes that are excessively steep or long, or insufficient application of tackifier. However, when properly applied on acceptable slopes, straw can be an inexpensive and effective mulch for erosion control. The growth of vegetation in strawed areas is similar to that of other mulch types.

Other loose materials used for groundcovers include compost and wood chips. Compost is made from a wide variety of organic materials, including animal manure, crop residue, municipal waste, biosolids, and yard waste. As a result, it can be highly variable in its properties. For example, the carbon to nitrogen ratio (C:N) of manure ranges from 3 to 56, while the C:N ratio of some crop residues may be more than 1,000. Materials with high C:N ratios can stunt or kill seedlings through microbially-mediated depletion of nitrogen and other nutrients from the root zone.

Compost is generally applied as a one to two-inch layer on top of prepared soil with seed, fertilizer, and lime previously applied. On steep slopes (2:1 or steeper), up to four inches of compost may be applied. Some professional compost applicators include seed in the compost, which is recommended if more than two inches of compost is applied. Studies have shown that compost can be an appropriate groundcover for establishing grass on low to moderate slopes, but compost is often more expensive than other erosion control options.

Wood chips, or shredded woody materials, are often generated during the clearing stage of construction when trees are shredded using large tub grinders. Such materials are ideal for landscaping around established vegetation or large plants, but they should not be applied or incorporated into soil where permanent vegetation is planned. Typical groundcovers such as grass can have difficulty growing through woody material, and as this high-carbon material decomposes, it removes plant nutrients from the soil (through microbial processes), resulting in low soil fertility.

Erosion Control Blankets
Erosion control blankets (ECBs) vary widely in composition, price, and circumstances of appropriate use. In general, ECBs are more expensive to purchase and more time- and labor-intensive to install compared to straw mulch, but are comparable to hydromulching.

When selecting an appropriate ECB, important considerations are the slope ratio, duration of use, cost, and effectiveness compared to other viable options.

Despite the variety of ECB materials, the general method of installation is similar. The first step is proper slope preparation, including application of lime, fertilizer, and seed. The soil surface should be uniformly graded and without rocks, large roots, or other materials that could prevent good soil-blanket contact. After slope preparation, the top edge of a roll is “trenched in,” or staked into the ground six inches deep and buried, then rolled over itself down-slope.

When laying out the ECB after trenching, adjacent rolls should overlap by 3 inches on the sides and 18 inches downslope. Rolls are secured to the ground with lawn staples at a density defined by the Erosion Control Technology Council of 1.5 staples per square yard. Staples are generally put a foot apart across the top and every three feet along the sides on both overlapping and non-overlapping portions. Staples should also be installed down the middle of the roll in a diamond or square pattern.  

When installing ECBs, good contact with the soil surface is important to reduce soil bridging and washouts under the ECB. Correct installation—making sure the blanket is flush with the soil surface—is important for long-term erosion control and vegetative establishment.

Hydromulches
Like ECBs, there are many types of hydromulches available for erosion control. The selection of a hydromulch for a particular purpose depends on the slope and soil type where the mulch is being applied, the price of mulch application, and the effectiveness of a particular mulch type under a unique set of circumstances. The wide variety of hydromulches allows the applicator to select the optimum mulch for a job.

Unlike straw and ECBs, hydromulching requires specialized equipment, including a water tank with a mixer and a high-powered pump to apply the mixture to a slope. A water source, such as a pond, stream, or fire hydrant, near the site is also needed. The amount of water needed is based on the manufacturer’s recommended amount of material in the tank and the recommended application rate.

Hydromulch application does not require advance site preparation or direct access to the slope, making it suitable for large slopes. In addition, hydromulch does not contain netting that could entangle wildlife or degrade slowly.

Lime, seed, and fertilizer are sometimes mixed with the mulch, but this should be avoided if possible. Lime and fertilizer can harm the seed when left in the tank for too long and are actually more effective when incorporated into the soil prior to seeding and mulching. Seed applied with the mulch may land on the mulch surface, where it is unlikely to survive. Hydromulch is best applied after liming, fertilization, and seeding. 

Sprigging with hydraulic applicators, which involves putting plant sprigs or pieces in the tank and spraying them out with the mulch, has been met with some success as well, but this is generally limited to level areas with irrigation.

When applying hydromulch, follow the manufacturer’s directions for rate and method of application. The rate of hydromulch depends on the slope where it is being applied. Mulch can be applied at a known rate by calibrating the spray hose, but many applicators prefer to apply the mulch by “eyeballing” it until coverage is sufficient. Regardless of application method, the mulch should be applied from both the top and bottom of a slope to ensure uniform coverage. Several basic types of hydromulches are discussed in table 3, along with notes concerning application and appropriate circumstances of use.

Hydromulching cost varies with the type of mulch selected, the application equipment, water availability, and area size. The least expensive products are paper based, while the most expensive are bonded fiber matrix products. Costs can range from $0.41 to $1.15 per square yard, not including seed, fertilizer, or lime. As a general rule, the more expensive hydromulches, such as bonded fiber matrices, tend to offer better protection against erosion, but actual results are site specific.

Summary
Good ground covers are the ounce of prevention to avoid the pound of cure.  Regardless of the type of erosion control measure used, the long-term efficacy of the ground cover depends on proper slope preparation, correct selection and installation of erosion control materials, and the selection of the appropriate seed or plant mix for the location and time of year.   And in many situations where irrigation is not available, success is largely dependent on Mother Nature providing adequate rainfall.  SLDT

About the authors: Rich McLaughlin, Ph.D., is an Associate Professor/Extension Specialist and Deanna Babcock is an Extension Associate in the Soil Science Department at North Carolina State University. Rich can be reached at 919-515-7306 or This e-mail address is being protected from spam bots, you need JavaScript enabled to view it and Deanna at 919-515-8745 or  This e-mail address is being protected from spam bots, you need JavaScript enabled to view it