Some pretty interesting things grow in mulch (besides plants!).  A few of them are really quite interesting and others are just plain disgusting.   Common organisms that grow on bark and wood mulches in the northeast include mushrooms, slime molds, artillery fungus and bird’s nest fungi. 

 mulched flower bed (90844 bytes)

Mulches are used in the landscape and gardens to conserve moisture, reduce weed growth and moderate soil temperatures.  They are also used to make garden beds more attractive.  Organic mulches such as wood and bark are decomposed by bacteria and fungi that use this material for food.  Other microorganisms feed on these bacteria and fungi. 


Many different fungi produce mushrooms.  The main ‘body’ of a fungus is composed of microscopic thread-like structures called hyphae which make up a network or mass of mycelium.  Once mycelium is well established and weather is favorable, some fungi form reproductive structures that are visible, such as mushrooms.  These vary in size, color and structure and last from a few hours to a whole season.  They often appear after a period of rain.  These decomposers are not harmful to plants or structures but some are toxic if eaten.  They can be left alone or removed.  It’s a good idea to wash your hands after handling them. 

 image 4Stinkhorns get their name from the foul odor they produce. (

Slime molds are initially slimy masses that may be brightly colored.  A common slime mold in our area is known by the common name ‘dog vomit fungus’ because that is exactly what it looks like.  When a slime mold dries out, the surface is a dry powder y mass of spores.  These organisms feed on bacteria in the organic matter and are not decomposers.   If you don’t want them in the garden, just remove them and discard them in the trash, a compost pile, or the yard or nearby woods or fields where they are not an eyesore.

Bird’s nest fungi are really fun to see!  The fruiting bodies or reproductive structures look like miniature bird’s nests or cups with eggs inside.  The eggs are spore masses and the spores are dispersed when hit by raindrops.   The nest or cup can be up to ¼” in diameter.  Sometimes the spores will adhere to surfaces like artillery fungus, but they can be easily removed and do not leave stains. 

Bird's nest fungi on mulch.

Artillery fungus is one mulch inhabitant that does cause problems.  On the mulch, the fungus is composed of white to cream colored cup shaped structures that contain a black sticky spore mass.  These fruiting bodies are oriented toward light surfaces such as light colored siding, fences, cars etc.  The spore mass is forcibly ejected and sticks to nearby surfaces.  It looks like a small speck of tar.  These spots are difficult to remove and leave a permanent stain. 

 image 7 (Cornell Univ. photo)

There are currently no fungicides labeled for control of artillery fungus.   Using bark mulch instead of wood chips or mulch may reduce its presence.  In one study at Penn State, blending mulch with 40% mushroom compost suppressed the artillery fungus.   High amounts of mushroom compost should not be used around acid loving plants like rhododendron, azalea and Pieris because of a high salt content. 

J. Allen