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.
As does other organic matter, wood and bark decompose over time. The primary organisms involved with their decomposition are bacteria and fungi, which derive their energy for growth from the carbon-based compounds found in wood and bark. These compounds include cellulose, lignin, and simple sugars. 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 mulch environment. Some fungi, such as the artillery fungus, are "recyclers" and break down woody tissue directly. Other fungi, such as slime molds, consume bacteria and other organisms living in the mulch. 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.
This fact sheet describes four common types of fungi growing in landscape mulches in the eastern United States--mushrooms, slime molds, bird's nest fungus, and the artillery fungus.
Many different fungi produce mushrooms.
What do mushrooms look like?
They 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.
This fungus is called a stink-horn. It gets its name from the foul odor of the cap of the fruiting body.
Are they a problem?
They may be poisonous if eaten.
What should be done?
Appreciate their beauty, ignore them, or remove them.
slime molds, "dog vomit" fungus
species of Physarum, Fuligo, and Stemonitis
What do slime molds look like?
They start as brightly colored (yellow, orange, etc.), slimy masses that are several inches to more than a foot across. They produce many tiny, dark spores. These molds dry out and turn brown, eventually appearing as a white, dry, powdery mass.
A fresh, brightly colored yellow slime mold.
A fresh, brightly colored pink slime mold
Are they a problem?
No. These fungi are "feeding" on bacteria growing in the mulch. They are normally a temporary nuisance confined to small areas. What should be done? The fungi may be left in place to decompose. If their appearance is offensive, discard the fruiting bodies in a compost pile, household garbage, or a spot in the yard away from existing mulch.
Bird's nest fungus
bird's nest fungus
species of Crucibulum and Cyathus
What do bird's nest fungi look like?
They resemble tiny, gray to brown bird's nests or splash cups with eggs. The nest is up to ¼ inch in diameter.
Bird's nest fungus (Crucibulum spp).
Bird's nest fungus (Cyathus spp).
Are they a problem?
These fungi may grow in large areas of mulch, but they are not a problem. The "eggs" are masses of spores that splash out of the nest when hit by a raindrop. These spores occasionally stick to surfaces, as do the spores of the artillery fungus, but they are easily removed and do not leave a stain.
What should be done?
These naturally occurring fungi decompose organic matter and do not need to be removed. They are interesting to look at--show them to children!
species of Sphaerobolus
What do artillery fungi look like?
They resemble a tiny, cream or orange-brown cup with one black egg. The cup is approximately ¹/₁₀ inch in diameter. Areas of mulch with artillery fungi may appear matted and lighter in color than the surrounding mulch. Are they a problem? They may be a problem. The fruiting body of this fungus orients itself toward 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. You may also find them 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.
Fruiting bodies of the artillery fungus
Artilliery fungus spore masses on vinyl siding
What can be done?
Penn State researchers have discovered that blending 40 percent used mushroom compost with landscape mulch greatly suppresses the artillery fungus. Mushroom compost, or mushroom soil, is the pasteurized material on which mushrooms are grown. After the final crops of mushrooms are picked, the used compost is pastuerized a second time and removed from the mushroom house. This valuable by-product (sometimes called "black gold") is often made available to gardeners and homeowners. Used mushroom compost has physical and chemical characteristics that make it ideal for blending with landscape mulch to enhance growth of horticultural plants. In addition, mushroom compost contains beneficial microbes that compete with, or actually destroy, nuisance fungi such as the artillery fungus and bird's nest fungi. Homeowners are increasingly interested in controlling nuisance fungi without the use of chemicals. Blending used mushroom compost with landscape mulch offers a "green" and environmentally friendly solution to reducing the harmful effects of the artillery fungus.
Prepared by Elizabeth A. Brantley, Penn State Mont Alto; Donald D. Davis, Department of Plant Pathology; and Larry J. Kuhns, Department of Horticulture, retired.