Mulch is used around plants to help suppress weeds and as a water-conserving measure; it keeps soil moisture from evaporating quickly. The mulch environment encourages the development of moldy organisms that are usually harmless to plants, though some are unattractive. Detrimental effects on plants from mold can be prevented or minimized with proper selection and installation of the mulch.
Mulch and Mold
Bacteria and fungi are part of the decomposition process of the organic material that's commonly used in landscaping, such as wood chips and compost. These fungi, or molds, aren't harmful to plants or a known health hazard, although you shouldn't eat them. They're most apparent between April and October, especially after rain. Some fungi can become a nuisance and cause disease in plants; often they're a result of mulch that's already infested or due to faulty application.
Shotgun or artillery fungus (Sphaerobolus) produces tiny, cream or orange-brown cup-like structures with a black mass of spores. the fungus shoots the spores in the air. The fungus isn't harmful to plants but they stick to the surface where they land and they're difficult to remove. Slime molds are bright yellow or orange slimy organisms measuring several inches to 1 foot across. Along with stink horn and bird's nest fungi, they have an ornamental quality, though stink horn has an unpleasant odor and attracts flies. Some fungi that are harmless to plants produce toxic toadstools.
Mulch made from diseased trees may carry harmful pathogens and infect plants. The fungus Verticillium dahliae causes the death of ornamental shrubs and trees and kills vulnerable plants when it's infested the mulch. Another disease, Rhizoctonia solan, is stimulated by the cellulose wood in fresh mulch and causes damping-off in seedling plants. Compost the materials at high temperatures to kill these diseases.for about six weeks before you apply the mulch.
Excessively deep layers of mulch are subject to high temperatures so that the mulch dries out. Fungi colonize the dry mulch, turning it into a water-repellant, moldy material, leading to the death of young trees from drought. Deep mulch also inhibits the development of mycorrhizal fungi, which has a beneficial association with plant root systems and is important for plant health. The ideal depth for mulch is 1 1/2 to 2 inches. Mulch made from decay-resistant trees, such as mature pine and cypress, should be used when possible.
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