A common question we receive in the Extension office comes from concerned homeowners and gardeners when a strange fungus-looking growth appears in their mulched beds — seemingly overnight. These growths, which resemble scrambled eggs or dog vomit, are slime molds. Their appearance generally occurs during or immediately following warm, wet weather. While the bright yellow foamy nature may resemble toxic sludge, the growths are not harmful to plants.
Despite their name, slime molds are not fungus; they are listed in the kingdom of Protoctista (Protista) and more closely resemble amoebas. There are over 700 types of known slime molds, but the one that we see most commonly is Fuligo septica, familiarly known as dog vomit slime mold. Unlike mushrooms and other fungi that secrete enzymes to break down and absorb their food, slime molds have fluid cell membranes and move around to find sustenance. Do an internet search of slime molds moving and you will see fascinating time lapse videos of plasmodial slime molds moving across surfaces, even traversing laboratory mazes in search of sustenance. Their ability to move is so extraordinary that slime molds are actually being studied in the development of technologies like driver-less vehicles.
Slime molds start out as a haploid spore that can remain viable for years waiting for optimal moisture and temperature conditions. Once conditions are right, they split open and release a flagellate swimming stage which then combines with another haploid to form a zygote. The zygote produces an amoeba-like body (plasmodium) that can move very slowly across mulch or other surfaces in search of food. Don’t worry about the scientific jargon — just think about how unique this is!
The organism feeds on the bacteria, fungi and yeast found on surfaces like soil, leaf litter and mulch. During feeding, the nucleus undergoes repeated divisions, but unlike most organisms these divisions happen without the creation of cell walls. Eventually multiple slime molds may also combine, fusing into one organism with multiple nuclei. The growth of the plasmodium may be 6 to 8 inches across up to a few feet wide. Probably something you don’t want to encounter without warning.
As conditions begin to dry, the plasmodium moves to higher locations for spore production. This search for higher ground can sometimes lead the organism up the stems and foliage of low growing plants. During this stage, the plasmodium changes from its yellowish color to a dull brownish cushion-like mass called an aethalia. The aethalia can be as much as an inch thick with the texture of a piece of bread. Instead of producing spores in specialized structures like the gills of a mushroom, the entire surface of the aethalia releases millions of haploid spores into the environment. This transition from toxic sludge (plasmodium) to dried bread (aethalia) happens within 24 hours. It might sound like something from a grade B horror movie, but how amazing!
Despite its appearance, which I hope you “appreciate” more now, and its repulsive name, Fuligo septica is completely harmless. If you have some dinner guests coming over during the day of an active plasmodia, you might even want to do a garden tour. After all, these things are … different. Once conditions dry, slime molds will disappear naturally, and because the organism is not considered a pest there are no products labeled for its control. Prevention starts with keeping mulch dry, and of course that would be challenging in damp weather.
To guarantee no slime, you would have to remove all organic mulches like wood chips and pine straw and replace them with rocks. Doing that would simply invite another set of problems. Instead, try your best to just appreciate slime mold’s occurrence in the landscape as part of the natural world. You have something special!