Mold in Mulch & Soil

Mold Types

• Artillery fungus, also known as white mold, and slime molds help break down the mulch, but they can also rapidly become a nuisance. Artillery fungus produces orange reproductive fruiting structures in the soil. Within the confines of the orange cup structure, a small black seed develops. Once the seed reaches maturity, the fungus ejects it high into the air. The seeds cling to the leaves and stems of landscape plants, and render them visually unappealing. The fungus can cause death and rotting of the plants. The black mold spores also stain the sides of houses and other structures. The soil and mulch may also develop slime mold. The mold grows across the soil's surface in large orange patches. Within the mold's midst, toadstools often develop.

Hardwood Trees

• Mulch from hardwood trees such as maples and oaks decompose and decay quickly because of the wood's large amount of cellulose. The rapid decomposition makes hardwood mulch the most prolific source of excessive mold growth in the landscape, according to the Ohio State University. Mulch that measures 3/4 of an inch in diameter breaks down quickly and causes accelerated microorganism growth to occur and mold to form. The rapidly developing microorganisms use the nitrogen in the soil to decompose the mulch, which causes the plants growing within the soil to undergo a nutrient deficiency. Always add nitrogen to fine hardwood mulch before applying it to the landscape.

Temperature Mold Growth

• The growth of microorganisms within soil produces excessive heat. The temperature within a heavy mulch pile often reaches 140 to 160 degrees Fahrenheit. Within the confines of the hot mulch, the microorganisms, known as thermophiles, flourish; but when the mulch is removed from the large pile for bagging or direct application to the landscape, the mulch begins to cool. The thermophiles require heat to live and reproduce, so they quickly die when the mulch cools. In the cooling mulch, the cold soil microorganisms such as mesophiles begin to develop and take over. When the mulch becomes wet from rain or irrigation, the fungi flourish and white mold appears throughout the mulch and soil.

Control and Prevention

• Avoid applying mulch at a depth of 4 to 6 inches or mold may develop. Apply mulch over the soil's surface at a modest depth of 1 1/2 to 2 inches to help prevent mold from occurring. Mulch often dries out when applied too deeply, which causes fungi to grow. The mulch forms a dense mat that ends up repelling water. The mat of water repellent mulch can cause a plant to die from drought even if it receives adequate water. Beneath the mat of mulch, the soil forms mold. Break up the mulch using a spade and irrigate to prevent mold from growing. All mulch has the capability of growing mold if the moisture content in the soil becomes high enough, even rubber mulch will form mold in excessively wet regions. When using wood mulch, make sure that each piece measures 2 inches in diameter or more to prevent rapid decomposition.