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Mulching

General

Mulching is the application of material to the soil surface to protect it from raindrop impact and overland flow. Mulch covers the soil and absorbs the erosive impact of rainfall and reduces the flow velocity of runoff, significantly reducing soil loss from a site.

Mulch may be applied after the site has been rough graded to control erosion. It provides a temporary cover that reduces soil loss and allows vehicular and foot traffic over the area. Mulch also provides benefits to the site beyond erosion control. Mulch forms a blanket over the soil, and moderates its temperature, conserving moisture and providing an environment conducive to seed germination. Mulch should be applied within 48 hours of the completion of seeding, or in hydroseeding applications, simultaneously (refer toPermanent Seeding and Temporary Seeding).

Mulching is a versatile practice that is applicable on sites where sheet flow is maintained and slopes do not exceed 3:1. Mulch has a limited life span, which varies with the material used and site conditions. It may not be used in channels or other areas where concentrated flow may occur. In these situations, erosion blankets or mats, which are more effective and may have a longer life span, should be used (refer to Erosion Matting). Mulching, while effective for smaller storm events, may not prevent erosion during larger storm events and is best used in conjunction with other management practices.

Advantages

  • Cost-effective
  • Easy to Apply
  • Protects the soil surface from raindrop impact, preventing erosion
  • Reduces evaporation from the soil and moderate soil temperature
  • Aids seed germination and establishment and hinders weed growth

Disadvantages

  • Ineffective on slopes steeper than 3:1
  • Ineffective with large storm events
  • May require frequent maintenance

Types

Mulch is available in a variety of types and should be selected based upon the individual site characteristics, such as slope, soil type, size, and time of year the mulch is applied. Regardless of the material selected, it must be free of weed and grass seeds that may compete with the establishing seed.

Straw

Straw is the most commonly used mulching material as it is cost effective and easy to apply. Straw from small grains, such as winter wheat, oats, and rye are generally used and can be spread by hand or with mulching equipment. Because straw is susceptible to the wind, it must be anchored to the soil by an approved method.

Wood Chips, Bark, and Wood Fibers

Wood chips are often used as landscape mulches and in specialized applications. They are generally more expensive, but do not require anchoring and may be obtained from a variety of sources. The wood used for mulch may be a hard or soft wood and shall be free of mold, sawdust, and other foreign materials, such as bonding agents and other chemicals.

Like all other organic mulches, wood chips are biodegradable. However, as wood chips degrade, they typically absorb a significant portion of the available soil nitrogen, making it unavailable for the establishing seed. Thus, depending upon the nitrogen content of the soils present on site, nitrogen fertilizer may need to be applied along with wood products to encourage the establishment of seed.

Tree bark, often obtained as a byproduct of the timber industry, is also used in landscape plantings and in areas that will not be closely mowed. Bark differs from wood chips in that it degrades faster and thus does not require added nitrogen.

Wood fibers consist of hard or soft wood that has been shredded in a hammermill, tub grinder, or other mechanical means. While wood fiber may not be used as a mulching material by itself, it is often used in conjunction with straw mulch in hydroseeding applications on steeper slopes and in critical areas.

Application

Rate

Mulch should be applied so that the soil surface is uniformly covered. This coverage rate corresponds with the application standards included in the following table. However, actual application rates may vary depending upon the individual site characteristics and the type of mulch used. The following table is intended for use as a planning tool only.

MaterialRate per AcreNotes
Straw1-2 tonsFrom small grains, should be tacked down or crimped
Wood Chips5-6 tonsTreat with 12 lbs. of Nitrogen per acre; not be used for fine turf
Wood Fiber0.1-1 tonMay be hydroseeded, not for use in hot weather
Bark35 cubic yardsShould be applied with a mulch blower or by hand. Not be used with asphalt tackifiers
Mulch may be applied by hand or by mechanical methods. Mechanical methods are generally much faster and more cost-effective, but may not distribute the mulch as evenly as hand application.

For hand application, the area to be mulched should be divided into sections with an area of 1000 square feet. Each section should then be evenly covered with 70-90 pounds of straw (roughly equivalent to 1½ - 2 bales). This method results in an application of 1½ to 2 tons per acre with a uniform thickness of 5-7 pieces.

Anchoring

Certain types of mulches, such as straw and wood fibers, are easily displaced by the wind and water. To keep them in place and effective, mechanical or chemical anchoring methods are applied. Mechanical means of anchoring include crimping and the use of erosion netting. Crimping is accomplished by a tractor drawn implement, similar to a farm disc, which draws the mulch into the soil profile in one piece. Crimping shall be performed on the contour of the land to prevent the formation of rills or gullies that may result from other application methods. Erosion nets, which are constructed of various materials such as plastic, wire, jute, cotton, or paper, are anchored on top of the mulch to hold it in place. Erosion nets are available in many types with a wide range of life spans. As a result, careful selection and adherence to all manufacturers’ specifications are crucial to their success.

Chemicals, called tackifiers, may also be used to hold mulch in place. Tackifiers hold the fibers together and reduce their susceptibility to wind and water erosion. Many types of tackifiers are available, including latex based products, asphalt emulsifiers, and natural products, such as guar gum. The type of product used will depend upon the characteristics of the site and the type of mulch used, however, regardless of the material selected, application should follow all of the manufacturers’ specifications.

Construction

  • All grading activities shall be completed and the area seeded before mulch is applied (except in hydroseeding applications)
  • Tackifiers shall not be applied in windy conditions
  • Mulch, when applied correctly, will have a uniform thickness of 5-7 pieces

Maintenance

Mulch shall be inspected weekly and after each storm event (including windy days) for signs of displacement and rill erosion. Necessary repairs and/or replacement shall be performed immediately to preserve effectiveness. Inspections shall continue until vegetation has been permanently established.

Method to Practice Practice Efficiency

Mulching efficiency is dependent upon many factors, including site characteristics, type of mulch used, rate of application, and atmospheric conditions. However, in general, when properly applied, mulching provides an efficiency of up to 88% (derived by using a USLE C factor of 0.12).

References