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Landscape mulch usually consists of hardwood shreds or bark chips, providing cover to hold moisture and add a finished look. Wood in mulch also provides a food source for fungi that are natural decomposers, breaking down plant material and utilizing organic matter. Without fungi, dead leaves, twigs and branches would clutter forests and landscapes. We see fungal fruiting bodies after growth of threadlike mycelium in soil and mulch. The most recognizable of these spore producing bodies are mushrooms, but sometimes they produce other structures, such as slime molds, stink horns, bird's nest fungi, and artillery fungus.

Most of these are harmless, providing decay of excess organic matter. Addition of fresh mulch yearly and raking to break up surface growth can prevent these fungi from sporulating. Artillery fungus can leave unsightly spots from spores deposited on siding, walls and cars.  Use of pine bark or nuggets rather than hardwood reduces growth of artillery fungus.

NFG 4/2019

FUNGI AND SLIME MOLDS grow in mulch composed of organic debris and wood chips. Fungi and fungus-like organisms such as slime molds are decomposers, and they do it very well to the benefit of our ecosystem. Without fungi, we would be deep in organic debris and fallen leaves. With adequate moisture, fungi will break down mulch into simple sugars they use to grow and survive. Most wood decomposers are harmless to people and animals. Fungal fruiting bodies and patches of slime mold are 95% water. They may be raked lightly to break them up, or they can be removed with a shallow shovel and the material discarded. On playgrounds, curious children can get very close, and breathe or eat this material. We consider these organisms harmless, however, people are very different and their immune systems are different.  There are allergies, asthma, and breathing related conditions that may affect individuals in our populations.  Children and their families should be made aware that there may be organisms on playgrounds and in yards that may be producing microscopic spores that may become air-borne. Air-borne spores may cause reactions in sensitive or immuno-compromised individuals.  Individuals (or their guardians) with health conditions should make judgment decisions about how much exposure can be tolerated. With our recent wet weather pattern, there will be an increased number of fungal spores in many environments. Raking will disrupt slime molds and they will dissipate naturally, or they can be scooped up and removed. Replacing mulch each year will bring in new organic material and organisms that colonize the material, so older mulch may not have as much visible fungal or slime mold growth. See the fact sheet for more info:

NFG 7/2013

Slim Mold on Mulch Next to Bricks
Slim Mold on Mulch Next to Bricks