Landscape - Nuisance Fungi and Landscape Mulch

The following is information on different nuisance fungi that sometimes grow in landscape mulch.

Mulches are used in Delaware gardens and landscapes for many reasons. By suppressing vegetation near trees and shrubs, they keep mowers and string trimmers from damaging the bark. In landscape beds and in the garden, they control weeds, improve drainage, prevent soil water loss, lower soil temperatures, prevent soil erosion and, as they decompose they release minerals and leave behind humus which benefits the plants. Organic mulches generally suppress plant pathogenic fungi and enhance beneficial mycorrhizal fungi. For continuing benefits, mulches need to be reapplied periodically. However, mulches are sometimes misused, especially around trees where excessive mulch (volcano mulch) is placed against the trunk, a practice which is harmful to trees.

Earlier in June, rains and use of fresh wood chip or bark mulch in landscape beds resulted in a proliferation of nuisance fungi growing in or on the mulch. One that was prevalent is slime mold (sometimes referred to as “dog vomit fungus”) which produces yellow or whitish patches of mold which later turn gray as they dry out. This slime mold spreads over the surface of the landscape mulch, sometimes surrounding the stems of plants in the bed. Although nuisance fungi such as slime mold rarely harm plants, some homeowners and landscapers object to their appearance and thus seek ways to prevent or eliminate these fungi.

There are many examples of fungi that grow on or from landscape mulch. Examples include stinkhorns (Mutinus and other related species), bird’s nest fungus (Crucibularium), earth stars (Geastrum spp), assorted toadstools, slime molds (Physarum and other species), and the shotgun, or artillery fungus (Sphaerobolus). Of these, only the shotgun fungus is truly a nuisance because it shoots tiny black spore masses onto nearby surfaces such as home siding and cars. Fungi also permeate thick layers of dry mulch, creating a hydrophobic mulch which is not easily penetrated by water, thus causing irrigation problems. Fertility problems can result when the fungi decomposing mulch removes nitrogen from the soil which is needed by the plants.

With proper manipulation, mulches can be prevented from developing nuisance fungi while maintaining the benefits of mulch. Much work on microbes and mulch has been done at Ohio State University where they have found that hardwood mulches (commonly used in Kentucky), especially if finely ground, contain a large amount of cellulose which decomposes fairly rapidly and leads to nuisance fungi. Such mulches, if composted for a few weeks with added nitrogen, and maintained at moisture levels over 40%, will not develop nuisance fungi. Such moisture levels allow bacteria and other fungi to compete with the nuisance molds. Wet mulches are heavy and require more effort to transport; however, moisture contents of organic products up to 50% will not present excessive transport weight problems.

The following are suggestions for the landscape industry and for homeowners wishing to avoid nuisance fungi:
-Purchase composted mulch products.
-Use mulches low in wood and high in bark.
-Avoid finely ground woody products unless composted first.
I-f using fresh wood chips such as those from a tree maintenance firm, add water to the mulch and allow the pile to partially compost for six weeks. If the wood chips do not include fresh leaves, add some nitrogen to speed composting.
-Use coarse mulches, but do not apply them too deep.
-Soak all mulches with water immediately after application to enhance bacterial colonization.
-Do not apply mulch deeper than three inches.
-Do not use sour mulches (highly acidic mulches giving off an acrid odor) because they injure plants.

Reprinted with slight edits for DE from "Mulches, Mushrooms and Molds" By John Hartman in the current edition of the Kentucky Pest News http://www.uky.edu/Ag/kpn/kpn_09/pn_090707.html