What Is That Growing in My Mulch?

Artillery fungus growing on mulch

A common question we often hear from home owners is “what is that weird stuff growing in my mulch?” That funny looking stuff can be a number of mold or fungi fruiting bodies that are feeding off the decomposing mulch.

Most mulch used in landscape beds are a mixture of shredded wood and bark. This organic matter along with adequate moisture provides an ideal environment for fungi, mold and bacteria to thrive. As they are decomposers, fungi, molds and bacteria are natural components of the environment and are doing their job, breaking down rotting organic matter.

These organisms are not usually harmful to plants and pose no health hazards to humans or animals unless some are ingested. While not always visible, you can see fungi and mold when they produce their fruiting bodies, usually found between April and October when weather conditions provide adequate moisture.

Below we’ve highlighted the most common fungi and mold found in mulched garden beds:

Artillery fungus (Sphaerobolus)

Spore masses attached to siding

This fungus produces fruiting structures that look like tiny cream-orange-brown cups which hold a spore mass resembling a little black egg. These cups release spores, propelling them a distance of more than 12 feet and a height of 6 feet.  Spores stick to the surface of cars, siding, and leafs leaving small spots resembling tar. Once stuck these spores are difficult to remove without damaging the surface they adhere to and if removed will often leave a stain. Although several of these may go unnoticed, if allowed to accumulate these spore masses can become unsightly. At this time there are no known management techniques to quell the growth of this fungus other than minimizing use of mulch next to buildings and where cars frequent. Also regular disturbance of existing mulch through raking the surface may minimize moisture therefore suppressing fungal growth.


yellow slim mold

Slime molds (Physarum, Fuligo, and Stemonitius)

Often referred to as dog vomit mold, this brightly colored organism appears in small areas of mulched beds. Usually growing from several inches to more than a foot across, these molds materialize when humidity is high and temperatures are warm. There are over 700 varieties of this primitive single celled organism and are often found on any moist rotting vegetation from clumps of grass clippings to old rotten logs. The usually suspects appear bright yellow to orange, fade into brown and eventually turn into a white powdery mass. Removal of these masses is not necessary as they are not harmful although they can be deemed quite unsightly. Controlling these molds is best done by preventing excess moisture in mulch by altering irrigation systems and raking mulch to expose the organism to dry air.


Bird’s nest fungi  (Crucibulum and Cyathus)

Close up of the birds nest fungi

This fungus is aptly named as its fruiting bodies look like miniature brown gray cuplike nests that hold tiny spore filled eggs. These cups reach up to ¼ inches in diameter and when hit by rain drops project the eggs up to 3 feet into the air to latch onto anything it can. They can grow throughout large areas within mulched beds but are inconspicuous and unharmful. Unlike artillery fungus the spores released can be easily removed from surfaces without leaving a stain.






Stinkhorns can really offend the noise

Stink horn (Phallaceae and Clathraceae)

This diverse group of fungi are known to pop up suddenly in the garden and get there name from the fruiting body cap which is covered with a foul-smelling slime mass. The odor attracts flies and beetles which ingest the slime and in doing so collect spores on their legs, transfering them to other areas.





The toadstool

The most easily identifiable fungi, these fleshy, spore-bearing fruiting bodies of fungi typically produced above ground and reproduce by airborne spores. Mushrooms 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 and should be removed if small children have access to the mulched area. There is no reliable control for nuisance fungi other than to frequently disturb the mulch by raking the surface to dry out the area so it will be less inviting.




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