Landscape mulches are used to protect soil, conserve moisture, moderate soil temperature, and limit weed growth, as well as beautify and unify landscape plantings. Most mulches are mixtures of shredded wood and bark residues from lumber and paper mills, arboricultural and land-clearing operations, and wooden pallet disposal or recycling facilities.
Like other organic matter, wood and bark decompose over time. The primary organisms involved with their decomposition are bacteria and fungi. Bacteria are microscopic organisms that are not visible in the mulch. Fungi also may be microscopic, but many develop visible reproductive structures.
The fungi involved in the decomposition of landscape mulches are natural components of the environment. The most frequent types of fungi found in wood mulches include various mushrooms, slime molds, bird's nest fungus, and artillery fungus. These fungi are not harmful to landscape plants and no known health hazards are associated with them unless they are eaten. They can be found April through October, usually following rainy weather.
Bird's nest fungus has a fruiting structure that looks like a miniature bird's nest with tiny eggs. The eggs are actually the fungi's spores. This fungus is harmless and does not have to be removed.
Another harmless mulch fungus is the slime mold. These are the fungi that appear during damp conditions that look like the neighbor's dog brought up last night's dinner onto your mulch. While this fungus may look disgusting, it feeds off of bacteria in the mulch and is harmless.
The one fungus you will want to remove is artillery fungus. These resemble tiny cups with one black egg. The "egg" is the spores. Eventually, the fungus shoots this sticky spore "egg" into the air. If the spore mass lands on siding or a car, the sticky black mass is almost impossible to remove. If you happen to remove it, it leaves a brown mark. If you are having issues with artillery fungus, it is best to replace the wood mulch with an inorganic mulch, such a rubber or stone. Otherwise, you will have to keep an eye out for the fruiting, or sporing, structures and try to remove them prior to them releasing their spores.
Anne Lenox Barlow is a professional horticulturist who enjoys gardening with her family in Plattsburgh. She also chronicles her gardening experiences at her blog www.northcountrygarden.wordpress.com. She can be reached at firstname.lastname@example.org.