Mulch is a layer of material intentionally spread over the surface of the soil. This practice is used to modify the soil environment and benefit plant health in several ways. The most common functions of a mulch layer are to reduce water evaporation from the soil, help regulate soil temperature by insulating the ground, and suppress the germination and growth of weeds. The variety of materials used means mulch can originate from industrial byproducts, agricultural waste, or highly processed recycled goods. Understanding the source of the material reveals how it will perform in a landscape setting.
Primary Wood and Forestry Byproducts
The vast majority of commercial wood-based mulch found in garden centers originates as a byproduct of the industrial logging and lumber processing industries. Log debarking is a necessary step before timber is milled into planks, and the stripped bark is a consistent and plentiful waste material that is then shredded and sold as bark mulch. This material is prized because the bark naturally contains high concentrations of lignin, tannins, and other phenolic compounds, which make the material highly resistant to microbial decay, allowing it to break down slowly over several years.
Sawmills also produce large volumes of wood chips and sawdust from the actual cutting of logs into lumber. These wood fibers, derived from the inner wood of the tree, typically decompose faster than bark because they contain less of the decay-resistant compounds. Heartwood generally contains more of these protective chemicals than the outer sapwood, which is more prone to rapid decomposition.
Beyond the lumber industry, a considerable amount of wood mulch comes from the arboriculture and land-clearing sectors. Tree removal companies and municipal green waste programs regularly chip whole trees, branches, and stumps that are not suitable for lumber. This whole-tree chipped wood is a blend of bark, sapwood, and leaves, which results in a product that breaks down faster than pure bark mulch.
Agricultural and Horticultural Waste Origins
Many organic mulches originate from seasonal farming operations and routine yard maintenance. Straw is the dried stalk leftover after the harvest of grain crops like wheat, oats, or rice. This material is inexpensive and low in nitrogen, making it a popular covering for vegetable gardens as it breaks down relatively slowly.
Hay is the entire grass or legume plant cut while green for animal feed. Its use as a mulch is less common because it can introduce undesirable weed seeds into the garden space. Shredded leaves, often referred to as leaf mold once partially decomposed, are a plentiful annual waste product from deciduous trees. This type of mulch naturally contains a good balance of nutrients and decomposes quickly, becoming a valuable soil amendment within a year.
Specialized agricultural waste, such as the hulls of cocoa beans from chocolate production, can also be processed into mulch. These husks are lightweight and fragrant, and are primarily sold as a decorative organic mulch in smaller quantities. Grass clippings from lawn mowing are another common horticultural source, though they are best dried or mixed before application to prevent them from compacting into a dense, water-repellent layer.
Recycled and Manufactured Sources
A different category of mulch is derived from materials that require significant manufacturing or recycling to become a usable product. Rubber mulch is one such example, sourced almost entirely from discarded vehicle tires, which are a considerable source of landfill waste. The manufacturing process begins with gathering old tires and mechanically removing the internal steel belts and fiber remnants using strong magnets.
The steel-free rubber is then shredded into small, uniform nuggets or chips suitable for landscaping or playground surfacing. This material is often coated with non-toxic pigments, such as iron oxide or carbon-based colorants, to create vibrant reds, browns, or blacks. Because rubber is an inert material, it does not decompose and offers a durable, long-lasting surface that does not need frequent replenishment.
Similarly, much of the brightly colored wood mulch on the market is manufactured from recycled wood waste, such as old hardwood pallets, construction debris, or scrap wood. These materials are ground down and then dyed with concentrated, water-based colorants. This coloring process is necessary because the raw, recycled wood fibers lack the rich, dark color of natural bark mulch.
Inorganic Mulches
Inorganic mulches are non-plant based and sourced from mineral resources. These materials are processed through crushing and screening to achieve a desired size and shape. Inorganic mulches offer a permanent, non-decomposing ground cover valued for drainage and aesthetic purposes. Common sources include:
- Stones
- Gravel
- Lava rock
- Crushed slate or brick