Landscape mulches are commonly used to conserve moisture, moderate soil temperature, and limit weed growth as well as for beautification of the landscape. Most mulches are mixtures of shredded wood and bar residues of hard wood or pine form lumber and paper mills, arboricultural and land-clearing operations, and wooden pallet disposal or recycling facilities. Wood and bark, as organic matter, decompose over time. Bacteria and fungi are involved in the decomposition. These organisms derive their energy for growth from the carbon-based compounds found in wood and bark. Both bacteria and fungi are microscopic, but fungi may be visible.
The fungi involved in the decomposition of landscape mulches are natural components of the mulch environment. 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 from April through October, usually following rainy weather.
Mushrooms or toadstools come in various colors, shapes, and sizes ranging from less than an inch to several inches tall. Some are soft and fleshy and disappear soon after they emerge; others may remain in mulch for a few days, weeks, or an entire growing season. Mushrooms may be poisonous if eaten.
Slime molds or “dog vomit” fungus are brightly colored (yellow, orange, etc) slimy masses that are several inches tog more than a foot across. These molds dry out and turn brown, eventually appearing as a white, powdery mass. These fungi feed on bacteria growing in the mulch. If their appearance is offensive, discard the slime mold in a compost pile, household garbage, or a spot in the yard away from existing mold.
Bird’s nest fungi resemble tiny grey to brown bird’s nests or splash cups with eggs. The nest is up to ¼ inch in diameter. These fungi may grow in large areas of mulch, but they are not a problem. The “eggs” or structures of the fungus splash out of the next when hit by a raindrop. These structures occasionally stick to surfaces, but they are easily removed and do not leave a stain. These naturally occurring fungi decompose organic matter and do not need to be removed.
The artillery fungus or cannon fungus or shotgun fungus resembles a tiny cream or orange brown cup with one black egg. The cup is approximately 1/10 of an inch in diameter. Areas of mulch with artillery fungi may appear matted and lighter in color than the surrounding mulch. The fruiting body of this fungus orients itself towards bright surfaces, such as light-colored houses or parked automobiles. The artillery fungus “shoots” its black, sticky spore mass which can be windblown as high as the second story of a house. The spore mass sticks to the side of a building or automobile, resembling a small speck of tar. They may be found on the undersides of leaves on plants growing in mulched areas. Once in place, the spore mass is very difficult to remove without damaging the surface to which it is attached. If removed, it leaves a stain. A few of these spots are barely noticeable, but as they accumulate, they may become very unsightly on houses or cars. To date there are no known ways to manage the artillery fungus.