Managing Strange-Looking Fungi in Landscape Mulch

Wood chips and shredded bark are commonly applied to landscape areas to control weeds and conserve soil moisture.  Wood chips and shredded bark are organic matter.  The decaying organic matter provides an ideal environment for some strange-looking fungi or fungal-like organisms when weather conditions are favorable.  Fungi that occasionally grow in landscape mulches in Iowa include stinkhorns, slime molds, and bird’s nest fungi. 

Stinkhorn  |  Slime Mold  |  Bird's Nest Fungus  |  Artillery Fungus  |  More Information


stinkhorn in mulch
Stinkhorn fungus in mulch.

Stinkhorns are aptly named as they are horn-shaped and foul-smelling.  They usually appear in cool, wet periods in late summer and early fall.

Several species of stinkhorn are found in Iowa.  Stinkhorns start off as an egg-like, golf ball-sized structure in the soil.  As the fungus develops, a stalk grows upward and is topped with a slimy cap coated with a mass of olive green to brown spores.  The putrid smell of the stinkhorn cap attracts flies and other insects.  The flies and other insects crawl on the stinkhorn, get covered with slime and spores, then fly away to other areas, disseminating the spores.  Stinkhorns range from four to eight inches in height. 

Stinkhorns are not harmful to plants.  Eventually, the stinkhorns wither away and disappear.  Individuals can scoop up and discard the fungi if their appearance or smell is bothersome. 

Learn more about stinkhorns in this article: Stinkhorn

Slime Mold

Slime mold is a brightly colored (usually yellow or orange) foam-like growth that occasionally appears in mulched areas in summer.  (It is sometimes referred to as dog vomit slime mold because of its bile-looking appearance).  Slime molds are primitive fungal-like organisms.  Slime molds feed on bacteria and other organisms in the mulch.  They do not harm plants. 

Slime molds are usually a temporary nuisance.  Within a few days, slime molds typically dry up and turn into white, powdery masses.  Most individuals simply let the slime molds dry up and fade away. However, the slime molds can be scooped up and discarded if they are objectionable. 

Dried slime mold on mulch
Dried slime mold on mulch

Learn more about slime molds in this article: Slime Molds

Bird’s Nest Fungus

Bird’s nest fungi resemble small cups or miniature bird’s nests.  The tiny fungi are only one-fourth inch in diameter.  The nests (fungi) are commonly brown, gray, or white and contain brown or white “eggs.”  The “eggs” are spore-containing structures called periodioles.  The eggs are splashed out of the nest by falling raindrops and adhere to nearby objects.  The periodioles eventually dry, split open, and release their spores. 

Bird’s nest fungi are not harmful to plants.  However, bird’s nest fungi can become a nuisance when the eggs stick to homes, cars, and other objects as they are difficult to remove.  Loosening the mulch with a garden fork or rake (permitting the mulch to dry out) creates a less favorable environment for bird’s nest fungi and should reduce their numbers.  Fungicides do not control bird’s nest fungi.

birds nest fungus
The bird's nest fungus grows on wood chip mulch.

Shotgun or Artillery Fungus

The shotgun fungus creates Small, shiny black spots that look like specks of tar.  These spores are very difficult to remove and must be soaked with soapy water.  Sometimes the spots are as high as 18 feet off the ground and attached to everything, though they show up best on light-colored house siding.  
One cause of the shiny black spots is a fungus called Sphaerobolus spp. with the common name of artillery fungus.  Artillery fungus is a wood-decay fungus common in moist wood-chip mulch.  The fungus forms fungal bodies very similar to cup fungi such as the birds nest fungi.

What gives this fungus its common name is its ability to “shoot” packs of spores as a dispersal mechanism. Black packs of spores that appear as discs (1 to 2 mm in diameter) are forcibly discharged into the air from the fungus on the wood chips. The spore packets are ejected because of the buildup of liquid that causes the cupped cells to burst and propel the spores up to 18 feet into the air. The spore packs are projected toward the light and a sticky substance ensures attachment where they land.
Black spots on the leaves of plants may look like a disease, insects or their products (frass) but inspection of the mulch surface will show the real source.  The spots can be a nuisance, but the good news is that these fungi won't cause any structural damage to the landing surface.  However, because of the sticky material the spore packs have, it is tough to scrub the spots from the surface of house siding or cars.  Management of artillery fungus is similar to the management of bird's nest fungi.

More Information

Last reviewed:
July 2023