“What’s Growing in My Mulch?!!”

By February 1, 2020 Blog

How many times have you heard that question from a panicked customer? Mold and fungi in mulch is a relatively common occurrence and there are some pretty interesting looking growths that can appear almost overnight. The good news is that, for the most part, they are relatively harmless. The bad news is that they can be unsightly and hard to avoid.

The fungi that helps decompose the organic mulch in garden beds are a natural part of the environment and play an essential role. That decomposition actually benefits the soil and ultimately the plants in the landscape. You want that to happen. But sometimes, several conditions can tip the balance and an overgrowth of mold ensues.

Why do fungi and mold appear in mulch?

They are there naturally. Things can get out of balance when the mulch is overly wet for a long period of time. Extended heavy rainfall, poor drainage, and too thick mulch can create an environment that is ideal for their growth. Make sure drainage is good, the mulch is not over 3 inches thick, and take time to rake and turn the mulch occasionally to help it dry out.

Let’s take a look at a few of the most common organisms that occur in mulch in the greater Philadelphia area.


Many different types of fungi produce mushrooms. The varieties are endless. Some can be poisonous if eaten, so be careful with small children and animals. One mushroom in particular is especially unsightly. The stink-horn. Not only is it ugly, but it smells and attracts flies. The flies land on the smelly tip, pick up the spores and carry them away to multiply elsewhere.

Mushrooms can easily be ignored. But if you find them too unsightly, you can chop them down and remove them from the landscape. Most mushrooms and stink-horns do not last long.

Bird’s Nest Fungus

These fungi produce small cups filled with round spore bodies in the bottom. They look like miniature bird’s nests! Raindrops fall into the small “nest” and the spores are splashed out to reproduce close by. They usually cover a larger area than other fungal growths and help decompose the mulch. This fungus is not harmful to plants or animals.

They can be left in the landscape but if a customer wants to remove them, just raking the area will help break them down.

Artillery Fungus

Here’s where things get a little trickier! Are your clients complaining of tiny black spots on their siding or on their cars? Chances are the culprit is Artillery fungus. These small fungi form white or brownish cups that are only approximately 1/10 of an inch across. They’re very difficult to find in the mulch. While they are in no way problematic for plants or animals, artillery fungi actively shoot their black spores out towards sunlight or bright, highly reflective surfaces. The dark tar-like spots are almost impossible to remove.

So, what do you do? Removal or raking infested mulch can interfere with their growth. Add fresh mulch yearly to suppress it, but make sure that it does not exceed 3 inches in depth. Organic mulch that is too deep just promotes the conditions that cause the fungi’s growth. One beneficial way to deal with Artillery fungus is to mix mushroom compost into the mulch at a ratio of 40%. This seems to reduce the occurrence of the fungi and has the added benefit of adding dark, rich organic matter to the landscape that will directly improve the health of the plants.

Slime Molds

Slime molds, otherwise known as “dog vomit” mold, look disgusting, but are harmless. They appear almost overnight and, at first, tend to be brightly colored. People often mistakenly think their dog or cat became sick and threw-up in the mulch. These soft, gelatinous surface molds can be yellow, orange, or white. They feed on bacteria in the mulch. They dry out to a dark color and then become white and powdery.

Use a shovel to scoop up the mold and remove it from the landscape. This is probably the most effective method of dealing with it. Raking the newly formed mound may get rid of it temporarily, but it leaves the organisms to reappear later.

Too much moisture can lead to the unsightly growth of fungi and mold in mulch. Deal with the root cause and have the knowledge to identify and confidently deal with these issues. Your expertise will let your business shine and your clients’ landscapes stand out for all the right reasons.