Mulch is a protective covering placed around flowers, vegetables shrubs and trees to suppress the growth of weeds, retain moisture, reduce erosion and keep the soil warm in the winter. Organic mulches can rot, forming a mat that blocks air and water; decomposition can compete with plants for available nitrogen. Landscape fabric, also called fabric mulch, and plastic mulches are usually covered with organic mulches or rocks to make them look more attractive and less artificial.
Landscape fabrics are ordinarily treated chemically so they won’t rot. They are permeable, allowing both air and water to get to the soil. Although some grasses and sedges will grow through them, fabric mulches will block the growth of most weeds. Fabric mulch will last up to five years if it is covered with rock or organic mulch. It’s possible to use drip irrigation systems under fabric mulch.
The plastic used for mulch is always black; clear plastic would let in the sun and overheat the soil. Plastic will help control weeds and conserve water. It holds water in the soil. Plastic is cheaper. Plastic is useful for solarization, a process whereby the soil is covered by plastic and left to be heated in the sun, killing fungi and other plant pathogens.
Fabric is more expensive and complicated to apply. Fabric needs to be staked to the ground or perennial weeds will push it up. Most garden supply centers that sell fabric mulches also sell ground pegs or staples needed to keep them in place.
If plastic mulch is used on poorly drained soil, the roots may sit in water and develop root diseases. When a covering of organic mulch slides off the slick surface of plastic, the plastic becomes brittle and deteriorates quickly in the hot sun, yielding to weeds. Plastic does not allow air to easily get to the soil. The oxygen-starved soil can become sour, killing roots. Another consequence is that plants develop shallow roots, making them more susceptible to cold weather and drought.
When organic mulch is spread atop fabric or plastic to make them look more attractive, the organic topping can decompose, yielding a base for the growth of weeds. Pine bark or other organic topping with a coarse texture will last longer before weeds begin to appear. Covering plastic with a layer of pine needles or wood chips will also keep it cooler so it won’t break down as quickly.