Organic Versus Inorganic
Organic mulches are derived from natural materials that decompose over time. As organic mulches decompose, they add nutrients and organic matter to the soil and beneficial microorganisms like nitrifying bacteria and mycorrhizal fungi are enhanced while undesirable pathogens -- those that cause plant diseases are inhibited. Increased amounts of organic matter will improve soil tilth and drainage, increase soil moisture retention, reduce soil compaction, and attract earthworms. Because organic mulches decompose, they need to be replaced. Depending on the type of mulch used, replacement intervals vary from one to four years.
Inorganic mulches include stones, geotextile mats and landscape fabrics, and plastic mulches. Landscape fabrics and plastic mulches deteriorate with time and eventually require replacement.
Mineral mulches last indefinitely. These mulches include gravel, volcanic rocks and marble chips. Rubber mulch lasts for many years, but the color will fade eventually. Organic mulches, such as shredded or chipped hardwood, decompose over time. The rate of decomposition varies depending on the type of mulch, but most bark and wood-chip mulches last at least a year. Mulches made of leaves, lawn clippings or sawdust decompose rapidly and may require several applications each year to remain effective.
Stone, gravel, lava rock, pebbles= Very Long
Ground or shredded rubber= Very Long
Organic Mulches-Traditional Bark Products
Pine bark and nuggets= Long
Shredded hardwood=Medium Long
Organic Mulches-Recycled Products
Composted yard wastes (prepared from yard wastes such as brush, leaves, grass clippings, etc.)=Medium
Ground and dyed wood pellets=Medium
Compared with bark mulches, wood chips tend to lose more of their decorative appearance over time, weathering to a gray or silvery gray color. Because of this, people often renew wood chip mulches each year by adding an additional 3 to 4 inches of chips.
Sawdust mulch has an acidifying effect on the soil as it decomposes. In addition, sawdust layers are characterized by severe compaction and decomposition over a single growing season. Thus, it is necessary to fluff up and renew sawdust mulch each spring.
Hulls of cocoa, buckwheat, cottonseed
These are prone to blowing in strong winds and washing out after heavy rains.
Straw is easily blown by wind must be renewed annually.
Pine needles decompose slowly, are resistant to compaction, and are easy to work with.
They will finish decomposing in place, contributing humus, nitrogen, and other nutrients to the soil.
Crushed stone, gravel, volcanic rock
They are not blown about by wind, they do not harbor weed seeds or diseases.
Geotextiles (or Landscape Fabrics)
Used alone as mulches, geotextiles can be degraded by the ultraviolet rays of the sun.