Mulch is an organic or inorganic material laid on the surface of the soil to seal in moisture, cool the soil temperature, to reduce evaporation and soil erosion, and to act as a weed suppressor. The type of mulch selected varies with the application (paths, hillsides, open area, landscaping), the varieties of plant materials used (trees, shrubs, annuals, natives, succulents), landscape style (formal, cottage, prairie, xeric), product availability and personal preference.
Inorganic mulches include plastic sheeting, weed barrier fabric, rock, stone, gravel or sand. Sheeting and fabrics are covered over with one of the other inorganic mulches. Using both materials together creates a barrier between the soil and the stones so the stone, gravel or rock can't work their way down into the soil, making it difficult to cultivate the area for future plantings.
Plastic sheeting does a good job of suppressing weeds and works best used in unplanted areas and pathways. For landscaped areas, plastic is laid over the entire area, then circles are cut to allow access to plant. As plant roots extend out beyond the cut hole, the water cannot reach the growing roots. Once the hole is widened, the area no longer has the protection of the plastic, weed seeds move in and there is continued moisture loss. Plastic sheeting becomes brittle with exposure and has to be replaced every few years.
Weed Barrier Fabric
Woven fabrics allow for water penetration, while at the same time the material holds in moisture and serves as a barrier against weed seed germination. Fabrics stay in place well, so they work well on slopes. Reapplication is only required every 10 years, if at all.
Organic mulches (chipped bark, ground-up tree trimmings, cocoa shells) are available either in bag or bulk quantities. Applied in a 3- to 4-inch thickness, the mulch cools the soil, protects it from drying and erosion and suppresses weed seed germination. Organic mulches break down slowly, adding some nutrient value to the soil. They need to be replenished every three to four years.
Bagged chipped bark commonly comes from redwood tree bark; however, there are now a wide range of choices of bark from other species. Chipped bark is natural and uniform in color. The color fades over time. It can be quite costly to use over a large area.
Tree trimmings, a sustainable resource, include all parts of a tree or shrub that have been processed through a chipper. They are generally available in bulk/loose quantities from municipalities and arborists for a nominal fee. They are made up of larger pieces and differing colors and textures, so may not create the uniform look that chipped bark provides.
Cocoa shells are uniform in size, reddish-brown in color and offer a delicious chocolate scent. They work best in full-sun, dry applications, since shade and constant moisture can cause the hulls to mold. Cocoa shells are a bit more expensive, but they retain their color and break down more slowly, making maintenance and reapplication less of a chore.