Mulch is organic or inorganic material spread over the soil surface, a universal practice in gardening and landscaping. Most commercially available mulch, composed of materials like wood chips, bark, and shredded plant matter, poses no significant risk to human health. However, because mulch is often a recycled product and supports biological activity, specific contaminants and natural processes can create localized hazards. Understanding these risks, which range from chemical residues to airborne biological agents, ensures safety during application.
Toxicity from Chemical Treatments and Contaminants
The primary chemical concerns related to mulch stem from the source material and any treatments it may have received. Modern colorants used in dyed mulches, such as iron oxide for red shades and carbon black for black, are generally considered non-toxic. The concern is not the dye itself but what the dye might be covering up in the source material.
Mulch produced from recycled construction and demolition debris presents a hazard if it contains wood treated with Chromated Copper Arsenate (CCA). CCA-treated wood, commonly used for decks and playground equipment before 2003, contains heavy metals, including arsenic. When this wood is ground into mulch, the arsenic can be released, potentially leaching into the soil or being inhaled as fine particles. Consumers should look for mulch certified by organizations like the Mulch and Soil Council (MSC) to ensure the product is free of CCA-treated wood.
Biological Hazards: Mold, Fungi, and Decomposition Byproducts
Mulch is an organic product that is constantly decomposing, a process involving various fungi, molds, and bacteria. Disturbing large, aged piles can release fungal spores into the air, which may cause respiratory irritation or allergic reactions in sensitive individuals. Spores from molds like Aspergillus can trigger severe reactions such as hypersensitivity pneumonitis, especially in those with compromised immune systems. Another hazard is the bacterium Legionella, which can be present in damp organic materials and may cause a form of pneumonia if inhaled.
A common issue with bulk mulch piles is the development of “sour mulch,” also known as wood alcohol syndrome. This condition occurs when mulch is stored in large, dense, waterlogged piles, leading to anaerobic decomposition (a process without sufficient oxygen). This decomposition produces volatile organic compounds (VOCs) that are highly toxic to plants, including methanol, acetic acid, and hydrogen sulfide. The resulting mulch has a distinct, pungent odor, often described as vinegary or rotten-egg. While the vapors are primarily phytotoxic, they can be irritating to human eyes and respiratory passages upon initial exposure.
Naturally Toxic Wood Types
Certain wood types contain natural compounds that can pose a risk if used in mulch. The most well-known example is Black Walnut (Juglans nigra), which produces a chemical called juglone. Juglone is a potent respiration inhibitor that is toxic to many plants, often leading to wilting and death. While the primary concern is plant damage, juglone is present throughout the wood, roots, and nut hulls, and is considered a respiratory toxin.
Direct contact with black walnut sawdust or mulch can cause skin irritation or welts in sensitive individuals. For pets, especially horses, using black walnut sawdust or shavings as bedding can cause acute laminitis (hoof inflammation). Dogs are susceptible to a mycotoxin called Penitrim A, produced by a mold growing on decaying black walnut hulls, making accidental ingestion a serious risk. It is generally recommended to avoid using uncomposted black walnut material in areas where sensitive plants, pets, or children may be exposed.
Safe Handling and Selection Practices
Selecting the right product and following proper application techniques can significantly mitigate potential risks. When buying bagged or bulk mulch, look for certifications. If purchasing recycled wood mulch, inquire about the source material to ensure it is virgin wood or free of CCA-treated wood. Before applying bulk mulch, smell it; a sharp, sour, or pungent odor indicates the presence of volatile compounds from anaerobic decomposition. If the mulch smells sour, spread it thinly in an open area for several days and water it to allow the toxic VOCs to dissipate.
Protective Measures
Protective gear should be worn, particularly when handling dusty or large volumes of mulch. Wearing gloves minimizes dermal contact with potential irritants. A simple dust mask or respirator prevents the inhalation of airborne mold, fungal spores, and fine wood dust particles. After working with mulch, wash your hands thoroughly before eating or touching your face.
Application Techniques
Avoid “volcano mulching,” which involves piling mulch high against tree trunks or plant stems, as this traps excessive moisture and encourages decay. Keep the mulch layer consistent and several inches away from tree trunks and plant crowns.