Landscape mulch serves multiple purposes: protecting soil, conserving moisture, regulating temperature, suppressing weed growth, and enhancing the beauty of your landscape. Most mulches are a combination of shredded wood and bark residues obtained from various sources like lumber and paper mills, tree removal operations, and recycling facilities.
Over time, the organic matter in wood and bark decomposes. Bacteria and fungi are the main organisms involved in this decomposition process, deriving energy from the carbon-based compounds found in wood and bark, such as cellulose, lignin, and sugars. Bacteria are microscopic and not visible in the mulch, while some fungi develop visible reproductive structures.
The fungi found in landscape mulches are natural components of the environment. Some fungi, like the artillery fungus, directly break down the woody tissue. Others, like slime molds, feed on bacteria and other organisms in the mulch. These fungi are not harmful to your plants, and there are no known health risks associated with them unless consumed. They are typically found from April to October, particularly after rainy periods.
In the eastern United States, you’ll commonly come across four types of fungi in landscape mulches: mushrooms, slime molds, bird’s nest fungi, and the artillery fungus.
Mushrooms
Common names
Mushrooms, toadstools.
Scientific names
Many different fungi produce mushrooms.
What do mushrooms look like?
Mushrooms come in various colors, shapes, and sizes. They can be less than an inch tall or several inches tall. Some are soft and fleshy, disappearing shortly after emerging, while others may remain in the mulch for a few days, weeks, or even an entire growing season.
This particular fungus, known as a stink-horn, earns its name from the foul odor emitted by the cap of its fruiting body.
Are they a problem?
Mushrooms may be poisonous if eaten.
What should be done?
Appreciate their beauty, ignore them, or remove them.
Slime Molds
Common names
Slime molds, “dog vomit” fungus.
Scientific names
Species of Physarum, Fuligo, and Stemonitis.
What do slime molds look like?
Slime molds start as brightly colored (yellow, orange, etc.), slimy masses that can be several inches to over a foot in size. They produce many tiny, dark spores. These molds eventually dry out, turn brown, and appear as a white, powdery mass.
A fresh, brightly colored yellow slime mold.
A fresh, brightly colored pink slime mold.
Are they a problem?
No, slime molds feed on bacteria in the mulch and are typically a temporary inconvenience in small areas.
What should be done?
You can either leave the fungi to decompose naturally or remove them if their appearance bothers you. Discard the fruiting bodies in a compost pile, household garbage, or an area of your yard away from existing mulch.
Bird’s Nest Fungus
Common name
Bird’s nest fungus.
Scientific names
Species of Crucibulum and Cyathus.
What do bird’s nest fungi look like?
These fungi resemble tiny gray to brown bird’s nests or splash cups with eggs. The nests can reach up to a quarter of an inch in diameter.
Bird’s nest fungus (Crucibulum spp).
Bird’s nest fungus (Cyathus spp).
Are they a problem?
Bird’s nest fungi can grow in large areas of mulch, but they are not problematic. The “eggs” consist of spores that splash out of the nest when hit by raindrops. Although these spores can occasionally stick to surfaces, including the spores of the artillery fungus, they are easily removed and do not leave stains.
What should be done?
These naturally occurring fungi decompose organic matter and do not need to be removed. They can be fascinating to observe, especially for children!
Artillery Fungus
Common name
Artillery fungus.
Scientific name
Species of Sphaerobolus.
What does the artillery fungus look like?
The artillery fungus resembles a tiny cream or orange-brown cup with a single black egg. The cup is about one-tenth of an inch in diameter. Areas of mulch with artillery fungus may appear matted and lighter in color than the surrounding mulch. These fungi orient themselves towards bright surfaces, like light-colored houses or parked cars. They “shoot” their black, sticky spore masses, which can be blown as high as the second story of a house. The spores stick to buildings, automobiles, and even the undersides of leaves on plants in mulched areas. Once attached, the spore masses are challenging to remove without damaging the surface and leave stains. Over time, multiple spots can accumulate, resulting in unsightly blemishes on houses or cars.
Fruiting bodies of the artillery fungus.
Artillery fungus spore masses on vinyl siding.
What can be done?
Blending 40 percent used mushroom compost with your landscape mulch can significantly reduce the presence of the artillery fungus. Mushroom compost, also known as mushroom soil, is the pasteurized material used for growing mushrooms. After the final mushroom harvest, the used compost is pasteurized again and removed from the mushroom house. This by-product, often referred to as “black gold,” is beneficial for blending with landscape mulch as it enhances the growth of horticultural plants. Additionally, mushroom compost contains beneficial microbes that compete with or destroy nuisance fungi like the artillery fungus and bird’s nest fungi. This environmentally friendly solution to combatting harmful fungi without using chemicals is becoming increasingly popular among homeowners.
Article prepared by Elizabeth A. Brantley, Penn State Mont Alto; Donald D. Davis, Department of Plant Pathology; and Larry J. Kuhns, Department of Horticulture, retired.
