Shredded woody organic material, grass, or grain stalks applied to the soil surface to protect mineral soil from raindrop impact and overland flow.
Purpose: Mulch is used to cover soil, reducing rain impact and soil erosion. It is often used in conjunction with grass seeding to provide ground cover in critical areas. Mulch protects the soil and improves moisture retention underneath it, benefitting seeded grasses in hot areas but not always in cool ones.
Relative Effectiveness: Excellent-66% Good-17% Fair-17% Poor-0% (Replies = 12). Mulch was judged “excellent” in effectiveness by most interviewees, although many also noted that it is quite expensive and labor-intensive. It is most effective on gentle slopes and in areas where high winds are not likely to occur. Wind either blows the mulch offsite or piles it so deeply that seed germination is inhibited. On very steep slopes, rain can wash some of the mulch material downslope. Punching it into the soil, use of a tackifier, or felling small trees across the mulch may increase onsite retention. Mulch is frequently applied to improve germination of seeded grasses. In the past, seed germination from grain or hay mulch was regarded as a bonus, adding cover to the site. Use of straw from pasture introduces exotic grass seed. Forests are now likely to seek “weed-free” mulch such as rice straw. Mulch is judged most valuable for high value areas, such as above or below roads, above streams, or below ridge tops.
Implementation and Environmental Factors: Mulch can be applied most easily where road access is available because the mulch must be trucked in, although for critical remote areas it can be applied by helicopter or fixed wing aircrafts. Hand application is labor-intensive and can result in back or eye injuries to workers. Using a blower to apply the mulch requires considerable operator skill to get uniform distribution of the material. Effectiveness depends on even application and consistent thickness. Rice straw is not expected to contain seeds of weeds that could survive on a chaparral or forested site (too dry); however, weeds do germinate sometimes and could result in introducing new exotics to wildland areas. Other certified “weed-free” straws sometimes contain noxious weeds. There is concern that thick mulch inhibits native shrub or herb germination. Shrub seedlings have been observed to be more abundant at the edge of mulch piles, where the material was less than 1 in (25 mm) deep. Because of the weed and germination concerns, mulch should not be used in areas with sensitive or rare plants. Mulch can be applied in 100 to 200 ft (30 to 60 m) wide strips on long slopes, saving labor costs and also reducing the potential impact of the mulch on native plant diversity.