Slime molds are fungus-like protists commonly found on rotting logs and compost. They move very slowly in search of decaying matter to eat. When food is scarce, individual cells swarm together to form a blob-like mass, like the “dog vomit” slime mold in the Figure below.

Slime molds have characteristics of both molds and protozoa. Under certain conditions, the slime mold exists as masses of cytoplasm, similar to amoebae. It moves over rotting logs or leaves and feeds by phagocytosis. The amoeba stage is called the plasmodium, which has many nuclei.

Slime molds were formerly classified as fungi but are no longer considered part of that kingdom. Although not forming a single monophyletic clade, they are grouped within the paraphyletic group referred to as kingdom Protista. More than 900 species of slime mold occur globally.

Only two sexes occur in most species, but Physarum polycephalum, a common yellow slime mold, has over 500 different sexes!

Not only do slime molds produce few resistant structures (except for spores, which are often overlooked or unidentifiable), but they live in moist terrestrial habitats, such as on decaying wood or fresh cow dung, where their potential for preservation is low.

However when food runs short, thousands of slime mold cells aggregate to form a mound. It secretes proteins that coat the stalk of the fruiting body and give it the rigidity it needs to send its spores out onto the ground in search of new food.

A plasmodium is a multinucleate mass of protoplasm which results from the fusion of amoeba-like cells. This is a “creeping” stage of the fungus so when sufficient water is available, slime molds creep or flow over many types of surfaces. They creep at a fairly fast pace and can actually move several feet in 24 hours.

While slime molds are definitely unattractive, they are not harmful. Slime molds often appear in the spring due to the fact that they get their moisture from the air and feed on the nutrients from whatever it is growing on, which is commonly your mulch beds.

All are basically harmless and are beneficial. Slime molds are not toxic, poisonous or venomous, and they help build up sandy soils by returning nutrients to the soil. Spores of slime molds move easily; they may have come in with the mulch, however, they also may have blown in from anywhere.

Slime mold can be prevented by reducing humidity. This can be done by watering early in the day during the summer—before the sun can heat any of the moisture left on the blades of grass.