Mulching around plants helps keep out weeds, hold water in the soil and improve the appearance of planting beds. But the kind of mulch chosen can sometimes be very important. A damaging fungus, called artillery fungus or shotgun fungus, can develop in some wood mulches, especially chopped-up limbs, trunks and stumps of diseased hardwood trees such as maples and oaks. This type of mulch is sometimes obtained free or at low cost from tree-removal companies.
If used near houses, cars, garages or similar surfaces, the fungus can spray the surface with black, tar-like spots that are very difficult or impossible to remove. While the sprayed spores normally travel only a few feet, breezes can blow them farther and at times they even reach second stories of houses. Artillery fungus appears to be most prevalent in the Northeast, and has been extensively researched by scientists, especially at Pennsylvania State University.
A great deal of information about the fungus can be found on the Internet by searching with the words artillery fungus. A list of questions and answers by Penn State researchers is especially valuable – look for the psu.edu tag in the search lists. Pine-bark nuggets, made from softwood, are considered safest of wood mulches. Non-organic mulches, like black plastic and marble chips, are best. Wood chips from hardwood trees are best used around plants at some distance from objects that might be damaged.