Skip to main content

What Is That Orange Fungus Growing in My Mulch?

Organic Mulch: What Is It and What Is It Hiding?

Organic mulch is an attractive, biodegradable way to dress up beds around flowers, shrubbery, and trees. It usually is composed of pine needles (pine straw), shredded wood, and chipped or shredded bark.

An organically mulched landscape saves homeowners time and money and is pleasing to the eye. Mulch will cut down on water loss due to evaporation, subsequently lowering your water bill. Mulch also reduces weed growth by shading the ground. This reduces the amount of money you spend on herbicides as well as the time you have to spend yanking weeds out of your flower beds.

Uninvited guests in organic mulch. But sometimes organic mulch comes with unwanted guests—spores. Spores that grow into a blob of slimy goo. Looking at this grotesque, slimy blob may make you wonder out loud, "What IS the orange fungus growing in my mulch?"

What Is That Orange or Yellow Fungus?

This sometimes orange, sometimes yellow substance is commonly called "dog vomit fungus." In the past, it was considered by scientists to be a member of the fungus family, but it has since been renamed "slime mold." But it's not really a mold either. In fact, since scientists aren't really sure WHAT it is, it is now classified as a protist.

Microbe World defines a protist as "... eukaryotic creatures <you-carry-ah-tick>, meaning their DNA is enclosed in a nucleus inside the cell.... They’re not plants, animals or fungi, but they act enough like them that scientists believe protists paved the way for the evolution of early plants, animals, and fungi."

According to Chris Reid of the University of Sydney, the term protist is used as a group for "everything we don't really understand."

Why don't scientists understand it? It is because slime mold thinks... and moves! And it does it without a brain.

Yellow slime mold

Yellow slime mold

Slime Mold Basics

The orange fungus growing in your mulch is a species of slime mold known scientifically as physarum polycephalum. These slime molds are single-cell organisms that feed on the bacteria produced by decomposing plant material, aiding the natural decomposition process.

Slime molds appear when the air is warm and wet. (Does anything reproduce when it's cold and dry?) They reproduce by generating millions of spores which are then dispersed either through the air or by contact dispersal. When the mold eventually dies out, it turns into a powdery white spot.

You may encounter slime molds growing in your mulch, or growing in slimy clumps on rotting tree stumps. Some people supposedly scrape the blobs up and scramble them, much like we scramble eggs.

Orange Fungus in Your Mulch Is Harmless

So the orange fungus in your mulch is really quite fascinating. Even though its kind is a billion years old and it feeds by enveloping its food in slime, it is just a harmless blob of protoplasm.

If you don't mind a little pulsating slime in your flower beds, just leave it be. It is part of the natural process of decomposition, and it will not harm you or your pets. It will eventually dry up and leave just a little powdery spot as proof it ever lived.

How to Remove It

If you really can't stand the sight of it, it can be washed away with a garden hose, or sprayed with a vinegar-water mix.

Slime Molds: Individuals or Collectives?

Scientists who study slime molds are amazed at the contradictions to "normal" scientifically understood behaviors. Slime molds are single-celled organisms without brains that at times come together to "move" and "think" as one collective unit.

The Individual

When food is plentiful, the slime mold cell will act as an individual, even when it is surrounded by fellow slimy cells. The cells move amoeba-like, oozing and shifting their shapes to suit their environment.

But when food is scarce, the individual cells will form a collective unit, thinking and acting as one unified organism.

The Collective

When the need to look for food arises, slime mold cells will come together to form either a collective blob less than 1/4 inch long, or it will form long tendrils up to several yards long.

In Greek mythology, the Hydra was a nine-headed monster that grew two heads whenever one was lopped off. This video is a microscopic time-lapsed view of the slime mold Physarum Polycephalum, the latter word Latin for “many-headed.”

In Greek mythology, the Hydra was a nine-headed monster that grew two heads whenever one was lopped off. This video is a microscopic time-lapsed view of the slime mold Physarum Polycephalum, the latter word Latin for “many-headed.”

Cellular Slime Molds

The minority of slime molds are cellular; when the single cells merge, they combine into one larger, many-celled blob. These are the smaller slime mold collectives that live in the soil until it is time to find a new food source.

When the time comes to move, the individual slime mold cells produce "signal" molecules that are detected by other slime cells. The single cells quickly move together and form a large mass of cells which then creeps to the surface of the soil. Then, if the blob is lucky, it will be stepped on and transported to a new area. If it is not transported, the blob will grow stalks and will start emitting spores.

Acellular Slime Molds

The majority of slime molds are acellular. When these particular cells come together, they actually merge into one giant single cell with millions of nuclei and a single cell membrane.

The giant, ameboid mass is filled with proteins that act as "muscles" and cause the slime mold protoplasm to flow back and forth, creating movement. The slime mold's movement has been clocked at a whopping 1/25 of an inch per hour.

Breaking Up Is Hard to Do

Physarum polycephalum has been studied by scientists for many years. Researchers once chopped the large blob into pieces and were astonished when the tiny pieces crept back together.


The SpongeBob slime mold (yes, really!) showed it was capable of "memory." Researchers put the slime mold at one end of a maze and food at the other.

The blob crept along, sending offshoot tendrils through every corridor of the maze. When the food was reached, the SpongeBob mold withdrew all the tentacles, leaving a slime behind in the dead-end corridors of the maze. When the experiment was repeated, the slime mold avoided the dead-ends and headed straight for the food.

Mr. Roboto

Klaus-Peter Zauner at the University of Southampton, UK, has used slime mold to remotely control a six-legged robot. The slime was grown in a six-armed star pattern on top of a circuit that controlled the robot. When the slime was exposed to bright light over one area of the circuit, the mold slithered away, triggering the circuit and causing corresponding movement of one of the robot's legs.

Researchers believe slime molds may be of use in the future due to the miniaturization of electronics and technology.

Physarum polycephalum plasmodium, aka Spongeob

Physarum polycephalum plasmodium, aka Spongeob

Roll On Down the Highway

Researchers in Tokyo put food wafers onto a glass in the same pattern as Japan's major cities would appear. The scientists simulated natural obstructions such as mountains and rivers by shining light onto the glass.

The slime molds, averse to the lights, were able to recreate the major interstate patterns of Japan, something it had taken engineers decades to do.

He's Not Heavy, He's My Brother

Slime molds are capable of choosing the right balance of nutrition they require from two separate food sources. Two sources of food, one protein rich and the other carbohydrate rich, were placed in two separate areas on a piece of glass. The slime mold spread out and consumed the exact ratio of each nutrient that it required. That's great self-control!

Questions & Answers

Question: Can I pour bleach on orange mold slime?

Answer: Yes, you can, but it probably will not keep the slime mold from returning. Keep in mind the potential damage bleach can do to the surrounding plants.

If the slime mold is bothering you, you can rake it up and dispose of it. Note that the spores will still be in the soil and mulch, and after rain or high humidity, it will return. The best defense against slime mold is sunlight and air movement.


Jane Howard on July 25, 2020:

Found a large expanse of this in my compost bin. It looks just like a mass of scrambled egg. I find your article really fascinating. Never knew such an organism existed. Thank you. No I wonf be destroying it. Just monitoring its progress!

sarabjeet kaur on April 01, 2019:

nice blog

Jose on June 13, 2018:

I just built these flowerbeds and today I noticed orange looking mold growing on the wood. What’s this orange stuff and how do si get rid of it!

M.ortiz on June 09, 2018:

Thank you so much for explanning what this yellow/orange fungals was . I cried my heart out thinking the worst . In my 50s never saw something like it . Thank good I didn’t kill spongebob ..

Judy on June 09, 2018:

And I always thought it was racoon vomit with mold growing on it...its worst than I thought, mind of its own.

Pat Bothwell on July 14, 2017:

These fungus is growing in a large pot that I have a cucumber plant. It is growing in the soil but will it influence the cucumbers? I have 6 cucumbers on the plant right now and don't know if is safe to eat them. Thank you for your opinion.

Seanmu on June 14, 2017:

Forget Spongebob. I was thinking Steve McQueen and a fire extinguisher.

Gable Rhoads (author) from North Dakota on August 05, 2016:

I'm glad I could help and entertain. Who knew slime molds could be so entertaining. :)

Tricia on August 05, 2016:

This article is amazing! I am so thankful for your scientific explanation and terrific sense of humor. I was researching in order to learn how to dispose of the "fungus" without propagating, but now I just might leave it or try some experiments of my own! Thank you!!

Sciencebooksandcats on May 29, 2015:


Gable Rhoads (author) from North Dakota on July 28, 2014:

Thank you cat on a soapbox. :)

Catherine Tally from Los Angeles on July 27, 2014:

Fascinating information in a well-written hub! Thank you:)

Gable Rhoads (author) from North Dakota on July 06, 2013:

Thank you, MJennifer. This was an "exclusive title" that led me to the interesting life of slime molds. I love stuff like this. :)

Marcy J. Miller from Arizona on July 03, 2013:

I'm so glad I stumbled across this beautifully presented hub. I'm fascinated by the unusual, oft-overlooked creatures and gremlins ever-present in our midst. This introduced me to the "smarter" side of slime. We don't get to see too many slimy substances here thanks to the heat and dry air; our molds tend to be on the powdery side. (Heck, so does our dirt and our skin -- such is life in the desert.)

I hope to see this as HOTD -- it's really well done. -- MJ

Patricia Scott from North Central Florida on June 27, 2013:

Interesting. For the first time I spied this slime mold a few weeks back. It puzzled me but with all of my good intentions to look it up time slipped away and I never got around to it.

Now I know.

Thanks for sharing.

Angels are on the way to you . ps

Eugene Brennan from Ireland on June 22, 2013:

"slime mold thinks and moves" - sounds like good inspiration for a 50's B- movie!

I haven't come across this, however a green jelly type slime (like seaweed) appears on the surface of my yard when there is a lot of rain.

Gable Rhoads (author) from North Dakota on April 17, 2013:

Patty, you're welcome. It was an exclusive title, and since I did landscaping once. I thought I'd give it a go. It took me off on a whole other angle, though. :)

Patty Inglish MS from USA and Asgardia, the First Space Nation on April 17, 2013:

I have never seen orange fungus before! Thanks for writing about this invader.

Gable Rhoads (author) from North Dakota on April 17, 2013:

Thank you, Audrey. :)

Audrey Howitt from California on April 17, 2013:

Very interesting hub again--sharing these around!

Gable Rhoads (author) from North Dakota on April 14, 2013:

Thanks, Sheri Faye.

It was an exclusive title. I started out intended to do an article on fungi, but my research took me on a totally different tangent.

I'm glad you enjoyed it. :)

Sheri Dusseault from Chemainus. BC, Canada on April 13, 2013:

Wow, that was so interesting! Thanks