The purpose of mulching soils is to provide immediate temporary protection from erosion. Mulch also enhances plant establishment by conserving moisture, holding fertilizer, seed, and topsoil in place, and moderating soil temperatures. Only the most common types are discussed in this section.
As a temporary cover measure, mulch should be used:
On disturbed areas that require cover measures for less than 30 days.
During the wet season on slopes steeper than 3H:1V with more than 10 feet of vertical relief.
Mulch may be applied at any time of the year and must be refreshed periodically.
Tackifiers shall be plant-based, such as guar or alpha plantago, or chemical-based such as poly-acrylamide or polymers.
Install mulch or tackifier products per manufacturer's recommendations.
1.8.3 Design and Installation Specifications
Mulch shall be compost, chipped site vegetation, hydro-mulch, wood-based mulch or wood straw, wood strand mulch, or straw. See Table 3 - 9: Mulch Standards and Guidelines for specifications, application rates, and additional information.
A minimum of 2” of mulch is required. Increase the mulch thickness until the ground is 95% covered (not visible under the mulch). Thickness may need to increase for disturbed areas in or near sensitive or other areas susceptible to erosion.
Mulch used within the ordinary high-water mark of surface waters should be selected to minimize potential flotation of organic matter. Compost has a higher specific gravities (densities) than straw, wood, or chipped material.
The thickness of the cover must be maintained.
Remulch and/or protect with a net or blanket any areas that experience erosion. If the erosion problem is stormwater and surface water related, then fix the problem and remulch the eroded area.
Table 3 - 9: Mulch Standards and Guidelines Meet the definition for “composted material” per WAC 173-350-100 and comply with standards in WAC 173-350-220, except the feedstock may contain biosolids or manure feedstocks. Be coarse compost meeting the following size gradations (by dry weight) when tested in accordance with the U.S. Composting Council “Test Methods for the Examination of Compost and Composting” (TMECC) Test Method 02.02-B. Have no visible water or dust during handling. Have soil organic matter content of 40% to 65%. Have a carbon to nitrogen ratio below 25:1. Carbon to nitrogen ratio may be as high as 35:1 for plantings composed entirely of plants native to the Puget Sound Lowlands region. Be applied a minimum of 2” thick (~100 tons/acre) though thicker application rates may provide more effective control. Do not use near wetlands or phosphorus impaired waterbodies. Compost can be later tilled into soils to help meet the requirements of BMP L613: Post-Construction Soil Quality and Depth as required per Minimum Requirement #5. Compost specifications are also contained in A900 - Compost. Chipped site vegetation shall: Have an average size of 2-4” with gradations from fine to 6” in length for texture, variation, and interlocking properties. Be applied a minimum of 2” thick. Do not apply on slopes greater than 10%. Do not use within 200 feet of surface waterbodies. Using chipped site vegetation is a cost-effective way to dispose of debris associated with clearing and grubbing material. The decomposition of the chipped vegetation may help impart nutrients for grass establishment. Be applied with seed and tackifier. May be applied without seed and tackifier if application rate is doubled. Have no growth inhibiting factors. Have fibers less than ¾” in length to ensure machinery does not clog. Be applied at 35-45 pounds per 1,000 sf or 1500-2000 pounds per acre with a hydromulcher. Wood-based mulch or straw mulch shall: Have no visible water or dust during handling. Be purchased from a supplier with a Solid Waste Handling Permit or a supplier that is exempt from solid waste regulations. Be applied 2” thick (~100 tons/acre) Wood-based mulch or wood straw is often called “hog” or “hogged fuel”. The preparation of wood-based mulch typically does not account for weed seed control so the inclusion of weed plants or seeds should be monitored and minimized or prevented during application. A blend of loose long, thin wood pieces derived from native conifers or deciduous trees with high length-width ratio. A minimum of 95% of the wood strand shall have lengths between 2” and 10” with a width and thickness between 1/16” and 3/8”. Free of resin, tannin, or other compounds that are detrimental to plant establishment and growth. Free from undesirable seed and coarse material. Applied 2”-3” thick (5 bales per 1000 ft2 or 2-3 tons per acre) Thickness may be reduced by half when used with seeding. Hand-application requires a greater thickness than blown straw to ensure required coverage. Held in place by crimping, using a tackifier, or covering with netting. Blown straw shall be held in place using a tackifier. Although straw can be cost-effect, straw can introduce and/or encourage weed species and has no long-term benefits so should only be used when other materials are unavailable. Do not used within the ordinary high-water elevation of surface waters (due to flotation).