This Is Why Mulch Turns White – Garden Tips 2024

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why mulch turns white

Mulch is super-useful in the garden, but if you don’t know much about it, you may be alarmed if it suddenly starts turning white in places. Mulch mold is a type of fungus, and it not only looks peculiar, but it also smells strange. It might be cottony, thread-like, or crumbly – but what is it actually?

White mold mulch is perfectly normal. It’s more likely to form in damp conditions when bacteria feed on the mulch and form fungi. The white fungus is not usually harmful in any way. Unlike many mushrooms, it isn’t edible. 

What is mulch?

Mulch is an organic or inorganic material that we use to improve soil conditions in gardens and larger landscaped areas. 

It keeps moisture in the soil, keeps the soil cool in summer and warm in winter, chokes weeds, and stops new ones from germinating. It can also help to neaten up the appearance of garden beds. 

Organic mulch will decompose over time and you’ll need to replace it. But this usually takes years, and the bonus is that it helps to improve the quality of your soil. 

When choosing mulch for your garden, bear in mind that the woodier and dryer the mulch is, the fewer nutrients it will return to the soil. On the other hand, it will decompose much more slowly and won’t have to be replaced for a long time.

The question here is why mulch turns white. 

What causes mulch to turn white?

The mulch most people use in their yards is chopped or shredded bark or dead wood chips. Even though it lasts longer than leaf mold, wood mold starts to decompose as the bacteria and fungi in the soil start to feed on it. 

This white stuff feeds on all sorts of organic matter including compost, soil conditioner, and mulch. More properly known as saprophytic fungi, it breaks the organic matter down into humus that releases essential minerals that plant roots need. 

Carol Quish from the University of Connecticut’s College of Agriculture, Health and Natural Resources explains how the fungus starts as a single spore. It grows in a strand (hyphae), and groups of hyphae form mycelial mats. 

When you see a white mass or coating on your mulch, it may be a mycelial mat. If weather conditions are right, the fungus will mature and produce a fruiting structure above the ground.

The fruit of different types of white mulch mold has different forms and colors.

Unrelated to mulch, cucumber plants sometimes also get a white mold disease. This starts as powdery mildew and then develops white mold if the mildew is not controlled.  

Types of fungi in mulch

types of fungi mulch

Five fungi that grow in mulch are:

  1. Mushrooms
  2. Bird’s nest fungus
  3. Slime molds
  4. Artillery fungus
  5. Stinkhorn


Many different types of fungi produce a variety of mushrooms, some of which are labeled toadstools and are obviously poisonous. They appear in many guises – different colors, sizes, and shapes. 

Many are soft and fleshy and disappear as quickly as they emerge. Others will stay growing in mulch for days, weeks, or throughout the growing season. 

When mushrooms grow in mulch, the safest thing is to ignore or remove them. Researchers have found that most are poisonous. 

Bird’s nest fungus 

The bird’s nest fungus is another type of fungus mulch encourages. Of all the mulch fungus types, they are the most intriguing.

Bird’s nest fungi look just like miniature birds’ nests filled with tiny little eggs. Amy Gibbs and Brian Hudelson of the University of Wisconsin-Madison’s plant pathology department add more info. 

The little cup-shaped nests may be either brown, gray, or white, and the tiny eggs are brown or white. Of course, the “eggs” aren’t eggs at all but rather structures (Peridioles) that contain multiple spores. These fruits are about ¼ inch in diameter and they vary in shape. 

The way they form is fascinating. Because of its amazing cup (or nest) and egg form, bird’s nest fungi are able to disperse spores in a unique way. 

In stormy weather, the rain splashes the eggs out of their nests. Amazingly, they can travel more than 3 feet before they stick to something in the environment. 

When the “egg” dries, it will split open and release the fungal spores it contains.

Bird’s nest fungus is the least common type and researchers haven’t identified a favorite mulch.  

Slime mold 

There is a surprising variety of different slime molds, also commonly known as dog vomit fungus. They generally begin as brightly colored orange, yellow, or pink slimy, foam-like masses that spread.

As slime molds develop, they produce masses of tiny, dark spores that turn brown. As the spores dry out, they form a white, powdery mass. 

Slime mold isn’t a problem as such. It’s usually confined to a small area of mulch and it doesn’t last very long.

It’s safe to leave the fungi fruit produced by slime molds to decompose naturally – which they will do. Alternatively, you can remove the fruiting bodies and chuck them in the garbage or add them to your compost pile. 

Artillery fungus

Artillery fungus, also called shotgun fungus, isn’t pathogenic to living plants, but it can be horribly problematic. Its fruit is tiny, but it explodes and shoots spores in all directions. 

Carol Quish says it prefers growing on mulch made from “the interior of the tree” rather than mulch made from bark. It also prefers hardwoods. 

Staff from the University of Massachusetts (UMass) Amherst explain that when artillery fungus breaks down the mulch, it begins to produce tiny orange-brown or cream-colored cups. Like bird’s nest fungus cups (or nests), these contain tiny black eggs that are actually a mass of spores.

When the artillery fungus cup absorbs water it builds up pressure. Eventually, the cell will burst with enormous force, propelling the spores as far as 12 feet or more, and up to 6 feet high. 

Artillery fungus can be very damaging in a residential environment. It sticks to all kinds of surfaces from sidings to cars and is extremely difficult to remove. 


The stinkhorn is shaped like a horn and it stinks! It is also common and occurs anywhere that provides a food source from composted material. 

You can identify stinkhorns by the upright tube-like structures they develop. Amazingly, they can grow 6-7 inches in height overnight! 

While they are harmless, their smelly, slimy caps really do smell disgusting. But nature has her reasons. The smell attracts insects that pick up and carry the spores to new breeding grounds. 

How do you get rid of white fungus on mulch?

It can be difficult to control the growth of white fungus on mulch. One way to minimize growth is to rake the mulch often so the area dries out. 

Slime mold and stinkhorns are easy to rake. But you can also remove the affected mulch and simply replace it with new mulch. 

Otherwise, remove the affected mulch and pile it in a sunny area. Wet it thoroughly and leave it for a few weeks. 

Moisture will start the decomposition process. Then, as the pile starts to dry out, the heat should kill the mold. 

Turn the pile regularly so that the inside mulch doesn’t remain wet. 

If you don’t take action quickly, you might find you will need to get rid of all your old mulch. But that isn’t common. 

Researchers at the Pennsylvania State University (PennSTate) Extension have come up with a solution for artillery and bird’s nest fungus. Simplistically, a blend of landscape mulch and fresh mushroom compost is what does the trick. 

When all fungi mature, it is sometimes easier (if time-consuming) to remove the fruits manually, by hand.

Preventing fungi from developing in your mulch

Gardening pros generally agree that it’s a lot easier to prevent mulch fungus from forming than be faced with the challenge of getting rid of it. 

Generally, if you rake your mulch regularly you will help to prevent fungal growth. It stops the spores from getting a foothold and also keeps the mulch dry and aerated. 

Raking is more important after wet weather because mulch fungus thrives in moist conditions. 


The white stuff on garden mulch is guaranteed to freak out many amateur home gardeners. After all, they’ve brought in mulch to improve their garden environment.

Now what?

Happily, mulch mold generally breaks down organic waste into amazingly rich compost. Organic mulch is organic waste matter and all our gardens heed compost. 

Our garden tips are designed to help you identify the most prevalent forms of mulch fungus. Once you know what they are, you are likely to rest a lot easier. 

If you want to stop it from forming, there are simple steps you can take, like raking the mulch regularly. There are also ways to get rid of it. 

But whether you leave your white stuff to develop and rot on its own, or decide to remove it, consider mixing it in with mulch to create your own organic mixes.

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