Mulch, Mold, & Fungi

Mulch can be beneficial in many ways on plant beds, around foundation shrubs and
other gardening locations in your yard, but mold can threaten its benefits.

In landscape beds and gardens, mulch helps control weeds, prevent extreme soil
temperature fluctuation, decrease water evaporation and improve drainage. Mulch also
reduces mower and string trimmer damage on shrubs and trees by suppressing vegetation near their trunks. As it decomposes, mulch produces organic materials to improve soil and otherwise benefit plants.


You need to periodically re-apply mulch to continually get these benefits. Nuisance fungi occasionally grow on mulch. They include shotgun fungus, slime molds, stinkhorns, earthstars and toadstools. The shotgun fungus shoots masses of tiny black spore structures onto adjacent surfaces such as vehicles and home siding.

Slime molds are more unsightly than harmful. They don’t cause plant diseases and aren’t parasitic. Slime mold spores usually appear from late spring to fall. Abundant wet weather stimulates above-ground appearance of these fungi that initially appear slimy but quickly become dry and powdery when converting into spore masses. You’ll often see slime molds quickly appear and usually disappear in one to two weeks. They tend to reproduce in the same location every year.

Fungicide use isn’t recommended because slime molds aren’t harmful. When mulch hasn’t been composted, it might contain fungi that cause plant diseases. This situation is rare, however, and only occurs in non-composted mulch. Plant material fertility problems can arise when fungi in decomposing mulch remove nitrogen from the soil.

Insufficient moisture problems can develop when fungi permeate thick layers of dry mulch creating a surface that’s difficult for water to penetrate.

To gain the most benefit, you should use composted mulch with a high bark content and little wood material. Avoid finely ground, woody products that haven’t been composted.

If you buy fresh wood chips from a tree-maintenance firm, add water to the chips and allow them to partially compost for about six weeks. If this material doesn’t have fresh leaves, you can add some nitrogen to speed up the process. Avoid using fresh or partially composted wood chips near the house foundation because they can provide a food source for termites.

Immediately after you put mulch around plants or trees, soak it with water to enhance bacterial activity to initiate decomposition. Periodically wet mulch during the growing season. Avoid soured mulch because it tends to injure plants. You can spot sour mulch by its acrid odor.

Submitted by John Strang & Paul Vincelli, Extension Specialist, University of Kentucky