The appearance of a white film or patchy growth covering landscape mulch is a common sight. This phenomenon, which can range from a fuzzy coating to a slimy mass, signals the decomposition of the organic material. This is a normal and expected part of using wood-based products and is rarely harmful to plants.
The Primary Culprit: Fungal Decomposition
The white growth spreading across the mulch is primarily the vegetative body of fungi, known as mycelium. Mycelium is a network of fine, thread-like structures responsible for digesting the dense organic matter of the wood chips. These fungi are classified as saprotrophs, meaning they feed exclusively on dead or decaying material, making them beneficial decomposers.
The decomposition process recycles carbon and nutrients locked within the wood’s lignin and cellulose. This activity is encouraged by specific environmental conditions in the mulch bed. High moisture content, poor air circulation, shade, and warm temperatures create an ideal habitat for the mycelium to rapidly colonize the surface.
As the fungi break down the wood, they create humus, a dark, nutrient-rich material that improves soil structure beneath the mulch layer. The presence of this mycelial network indicates a healthy, biologically active environment.
Distinguishing Types of White Growth
The appearance of the white substance helps determine the specific organism involved and whether action is necessary. One common form is surface mold, often called slime mold or white mold. It may appear as a fluffy, cobweb-like coating or a thick, slimy mass that sometimes starts yellow or orange before drying to a white powder. These molds are mostly harmless, easily disturbed, and consume bacteria feeding on the decaying mulch.
A more problematic, though less common, white growth is the Artillery Fungus (Sphaerobolus stellatus). This fungus develops tiny, cream or orange-brown cup-shaped fruiting bodies containing a small, sticky black spore mass. The nuisance comes from its ability to forcefully eject these black spore masses toward bright surfaces, such as siding or cars. These spores adhere strongly, resembling small tar spots, and are difficult to remove without damaging the surface.
Strategies for Removal and Prevention
For common, fluffy white surface molds, treatment is generally unnecessary as they often disappear once the mulch dries out. If the appearance is a concern, simple mechanical agitation is effective. Gently raking or stirring the affected areas breaks up the mycelium, exposing it to air and sunlight, and disrupting its growth cycle.
If the highly adhesive black spots indicate Artillery Fungus, the entire affected mulch layer should be removed and discarded, as spores can remain viable for years. Alternatively, applying a new, deep layer of fresh mulch can suppress the existing spores and fruiting bodies. Using mulch types like bark chips or pine, rather than finely shredded wood, may also reduce the likelihood of this fungus establishing itself.
The best long-term strategy for minimizing any white growth is to manage moisture and aeration within the mulch bed. Apply mulch at a depth of two to three inches and avoid piling it against plant stems or trunks. Regularly turning the mulch with a rake promotes air circulation, which helps the material dry out and prevents the excessive dampness that fungi favor.