Mulch provides moisture retention, temperature regulation, and weed suppression. These organic materials serve as a food source for various fungi. Mulch fungus is a natural outcome of decomposition, as these organisms break down carbon-based compounds within the wood products. While this process is beneficial for soil health, the resulting fungal bodies can be unsightly or cause property damage.
Identifying Common Types of Mulch Fungi and Safety Concerns
The fungi found in mulch are mostly harmless decomposers, but their appearances vary widely. Slime molds, often called “dog vomit fungus,” present as bright yellow, orange, or white gooey masses that can spread quickly across the mulch surface. These are not true fungi but amoeba-like organisms that consume bacteria. They are not toxic and typically dry up and disappear once conditions become less moist.
Artillery fungus (Sphaerobolus) produces tiny, cream-colored cup-shaped structures that forcibly eject sticky, black spore masses. This fungus is attracted to light and can shoot spores up to several feet, leaving behind small, tar-like spots that are difficult to remove from house siding, cars, or other light-colored surfaces. Bird’s Nest Fungi (Crucibularium) resemble miniature gray or brown nests containing tiny spore “eggs” that are splashed out by raindrops. Stinkhorns (Mutinus) are also common, characterized by an upright tube structure with a slimy, foul-smelling cap that attracts insects to disperse its spores.
Most mulch fungi are not toxic to plants or humans, but any mushroom or fungal growth should be treated with caution. Some toadstool mushrooms that grow in mulch are poisonous if consumed. Wear gloves when handling any unknown fungal body, especially in areas accessible to children or pets. The primary danger comes from accidental ingestion, not from touching the growth.
Immediate Strategies for Removing Existing Fungal Growth
The first step is to physically remove the fruiting bodies and spores. For large, visible structures like slime molds or mushrooms, carefully scoop up the affected mulch using a shovel or rake, ensuring the surrounding material is also removed. The contaminated material should be disposed of in a sealed plastic bag and placed in the garbage, not added to a compost pile, to prevent spore spread.
For widespread or persistent growth, a mild topical treatment can help kill surface spores. A diluted solution of white vinegar and water (one part vinegar to four parts water) can be sprayed directly onto the affected areas. A baking soda solution also works by increasing the alkalinity of the mulch surface. Any solution application should be tested first and used sparingly, avoiding contact with surrounding plants.
Compacted mulch that has formed a dense, water-repellent layer, sometimes called a fungal mat, must be broken up. Using a pitchfork or hoe to turn and aerate the caked areas disrupts the fungal network, called mycelium. Breaking up these mats exposes the lower layers of mulch to air and sunlight, which helps to dry out the moist conditions that allow the fungi to thrive.
Long-Term Prevention Through Mulch Management and Aeration
Preventing the recurrence of mulch fungus requires altering the conditions that favor its growth: high moisture and low oxygen. The mulch layer should be maintained between two and four inches deep. Applying an overly thick layer, sometimes creating a “mulch volcano” around the base of trees, traps excessive moisture and promotes fungal growth.
Regular aeration of the mulch is essential for prevention. Using a rake to lightly turn or fluff the top two inches of mulch every few weeks introduces oxygen and allows the surface to dry out quickly after rain or irrigation. This prevents the formation of compacted layers and interrupts the fungal growth cycle by removing the damp, stagnant environment they require.
Moisture control is important, as waterlogged conditions accelerate decomposition and fungal activity. Reduce irrigation frequency in mulched beds and ensure water drains properly away from the area. When watering plants, aim the water directly at the root zone rather than soaking the entire mulch surface. Selecting mulch with larger particle sizes, such as coarse wood chips, can also help, as they break down more slowly and are less prone to fungal issues than finely shredded materials.