What is Mulch/Controlling Fungi

Mulch, what is it?

Mulch is aged, shredded hardwood bark and wood. The majority of hardwood in mulch found in Pennsylvania is oak along with other hardwoods. Mulch is a by-product of the saw mill and until recently it was thrown away, buried or incinerated. Finally someone recognized the "waste" products of the lumber industry has a "real" value to horticulture and landscaping. Usually the mulch is allowed to "age" (or sit) to decrease the consumption of nitrogen so as not to deter the growth of plants. To balance this aging process requires a delicate mix of moisture, oxygen and aeration which may take months depending on the process used.

Mulch is decomposing wood. This decomposition increases the organic material in the soil. The minerals and nutrients from the mulch are utilized by the root systems of the plants. A thick 3" layer of mulch acts as a moisture sump by entrapping free water from rain and snow. It also works to slow moisture loss through thermal evaporation and condensation. The layer of mulch also impedes weed growth in the soil and the acidic nature slows the growth of more basic plant materials. Germination of seeds usually occurs rapidly in the upper layer (1/2) of mulch because of the rich moist substrate. The action of decomposition also releases heat, moisture, and may help insulate the root system of plants in the cold of winter and the dryness of summer.

Why Fungi?

Mulch is actually wood or cellulose in a state of decomposition. Cellulose is very difficult to decompose. A few insects and fungi and bacteria are capable of completing the task efficiently. Available moisture, as well as, freezing and thawing are mechanical devices that help breakdown mulch. Fungi and slime mold are almost always present in shredded wood. Spores, seeds of fungi, and other lower plants germinate in the moisture laden dark matrices of the mulch. The fungi's hyphae, with thread like fibers, are evident throughout the mulch. When two strains of some fungi meet they are able to reproduce. The fruiting body or mushroom is formed. The fruiting body is like the apples of the apple tree and the hyphae is like the tree - branches, roots, and leaves. The fruiting bodies can appear overnight and last a day or so. The purpose of the body is to produce enormous amounts of spores to increase the survival of the species. When the temperature and moisture situations are optimal, fruiting bodies are more common. That is why after new (foundation) plantings that are mechanically watered, there are often many new areas of fungi.

Where fruiting bodies erupt from the mulch, gardeners think there is a problem. Remember the fungi are necessary for the cellulose degradation and spores and the hyphae are omnipresent. The classic mushroom can be found in mulch. Sometimes there are groups of mushrooms, which are in a circular motif called a "fairy ring". The fairy ring is supposed to have been caused by gnomes dancing in a circle, however, it is just the relationship and growth of hyphae growing outwardly from a common center.

Bird's nest fungus and artillery fungus are members of the cup fungi. They are cup or saucer shaped, and some are even irregularly shaped depressions. Their normal colors are white, yellow, orange, and red. Many of the cup fungi are so small the fruiting bodies go unnoticed to the untrained eye. The spores from the cup fungi are mainly dispersed when water droplets from rain or mechanical watering hit them and cause them to explode upward and outward. These fungi grow best in old mulch, hence yearly applications of fresh mulch prevent eruptions of spores.

The artillery fungi's spores often stick to masonry walls, and sidings of homes. They are very difficult or seemingly impossible to remove. (There is a home remedy soap mix that does a good job if needed). Slime molds have the most hideous and obvious fruiting bodies. These bodies when moist, seemingly appear overnight, and look like dog vomitus. White, to gold, to pink, these bodies may dot a mulched area. They look ugly but are not dangerous.

Spores from fungi are extremely minute and can be carried long distances by wind, birds, or man. They are commonly found in greenhouses, woods, gardens, or any place where moisture and organic material allow germination.

Controlling Fungi

1. There are ways of dealing with the fruiting bodies. Once they appear, remove and dispose of them in the trash to reduce the amount of spores dispersed in that area. The other way to deal with the fruiting body is to interrupt the life cycle of the fungi. This is often difficult because the hyphae are usually not seen and the fruiting bodies usually come out overnight. One may lime the soil to sweeten it, possibly changing the pH necessary for sexual reproduction. A liquid lime mix sprinkled on the top of the mulch will suffice (Example: AGGRAND Lime Plus).

2. A dilute solution of sodium hypochlorite may be used to sanitize the mulch. From past experience we have found a 50/50 mix of water and common Clorox to be very effective. First remove visible fungi and loosen the top layer of mulch with a rake, then use a sprinkling can to wet the mulch being careful not to wet the plants themselves. Repeat in 3 days if necessary.

3. The mulch may be loosened and flipped on a regular basis during the rain season to help keep down the amount of available moisture for vegetative and reproduction growth. Adding chemicals to the mulch may hinder one's plants as well as the fungi - SO BEWARE! The available moisture lost to the fungi is also lost to the root system of the plant. The safest method of response is physical removal of the fruiting bodies to remove spore density.

4. If you experience spore spots from "artillery fungus" try washing them off with a mixture of 3 tablespoons of Cascade dishwasher detergent and 1 tablespoon Tide laundry detergent dissolved in 1 gallon of warm water. Scrub area with a bristle brush and rinse with clean water.

The fungi and their fruits are just part of nature's recycling system. Since decomposition of cellulose is achieved in part through fungal degradation, then it follows - where there is mulch, there is fungus. Do no despair, the gardener and fungi can live in harmony. Man is part of nature and must learn to understand and utilize it. The decomposition of wood improves plant species and the gardener need only experience a few unsightly masses on occasion.

For more information regarding mulch and fungi, please contact the Penn State Horticultural Department at (610) 489-4315.