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This Is Why Mulch Turns White

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I was preparing my garden bed and raking back the mulch when I noticed the mulch areas were white. I was worried it might affect my plants, so I researched before planting.

Mulch typically turns white when it is infected with fungi. Many fungal species that might affect your mulch are not harmful, but some varieties may be pathogenic and harm your plants. Mulch can also be harmful when applied too thickly to plants.

There may be many reasons why fungi have grown in your mulch, and we will be talking about that in the next sections and the different fungi that can grow in mulch, the good and of course, the possible harmful ones.

What is mulch, and why are there fungi in it?

Finding white fungal growth in mulch, especially the wood chip mulch variant, is normal. These bacterial and fungal organisms help break down organic material, such as mulches.

What is mulch?

Mulch is mainly composed of organic or inorganic material spread around above the soil for landscaping and gardening purposes. You can mulch just about anything, from leaves, grass clippings, compost, and even stones and plastic sheets.

The plants we’ve chosen will collect and cycle Earth’s minerals, water, and air; shade the soil and renew it with leafy mulch; and yield fruits and greens for people and wildlife.

Toby Hemenway

As mulch can come from different materials, it is best to know the capabilities of each specific mulch, its benefits, and possible constraints. To ensure that you have a good base of knowledge about them, I have recently written an article on the best mulches for gardening. It covers organic and inorganic mulches, the mulches under them, and how to make specific mulches from existing materials in your homes and yards.

Fungi in mulch

While you should not eat these (would you want to eat the wood chips or compost anyway?), most species that affect your mulch are not health hazards or harmful.

The fungi typically occur because they feed on and decay the wooden organic material or feed on bacteria living amongst the mulch.

This process is generally very beneficial to your garden. As the mulch breaks down, it puts organic matter back into the soil, and fungal composting adds mycorrhizae into the soil, which benefits many plant root systems.

The process also makes nutrients more bioavailable within the soil.

Harmless Fungi

Some species of harmless fungi that might infect your mulch include:

Mold typeDescription
Slime moldsThese fungal organisms are generally yellow or orange fungi (as pictured above), although some can be white. The collection of organisms maybe just a few inches in diameter or extend several feet across your garden mulch.
Artillery fungus or shotgun fungus (Sphaerobolus)This fungus gets its name because it shoots spores into the air. These organisms produce small cup-like structures that dispense the black spores. The actual fungal structures themselves tend to be cream or brown in coloring.
Bird’s nest fungi and stinkhornWhile these can look rather ornamental, stinkhorn tends to be, as the name implies, stinky, and it can attract pests into your garden, such as flies.

A common fungal organism found on mulch is Fuligo septica, which has been “affectionately” dubbed dog vomit slime mold because it looks like a dog vomited up bilious material into your bed of mulch. This harmless mold is easy to identify in your garden.

The fungus Fuligo septica usually starts as a bright yellow foam across the mulch, starting slimy, and then gradually dries up.

At that stage, it looks brown before finally turning into a powdery white substance on your mulch.

While many fungal organisms are harmless to your plants, some produce toxic toadstools, which can harm animals. This can especially be a problem if you have dogs that might try to investigate your garden.

Pathogenic Fungi

While the fungi that may affect mulch are usually not harmful to people, some species can devastate your plants. Some of the varieties of harmful mulch that can infect your plants are:

  • Verticillium dahliae is a fungus that kills vulnerable plants when it has infected the mulch surrounding them, including plants such as ornamental shrubbery and trees.
  • Rhizoctonia solan is a fungal-based disease that can occur damping-off in seedlings when it occurs in fresh mulch.

These organisms may grow in damp and poorly aerated mulch, but they can also come when mulch is made from diseased trees, which can then spread to other plants.

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Treating Mulch Contaminated With Fungi

There are a few ways you can handle mulch contaminated with fungal organisms. Remember that there isn’t a product to treat the mulch to eliminate the fungi.

In most cases, it is a harmless process that can benefit the soil in the long run. The problem exists if you have a pathogenic fungus that might be affecting your living plants.

Get rid of the mulch.

The most expensive and time-intensive method of handling mulch is to get rid of it and not use the mulch any longer.

If you choose to get rid of the infected mulch, you need to contact city officials and see how you need to dispose of the material.

In many cities, you can bag the mulch in clear plastic bags and set them on the curb to be picked up and disposed of with other yard waste.

Compost the mulch

If you have a compost pile, consider composting your mulch. If your mulch pile reaches high enough temperatures, it will kill off the mulch.

Remember that the compost pile may also be a great place for mold to grow and spread if the temperatures are not hot enough.

Compost the affected mulch for a minimum of six weeks to kill off the fungal organisms before you take the mulch and apply it around your plants, especially if a pathogenic fungus is affecting it.

Covering the mulch with fungi with fresh mulch

A simple way to treat harmless fungi infecting your mulch is to cover it with fresh mulch.

Out of sight, out of mind, in this case, harmless fungi will have their air supply choked off, preventing it from continuing to grow and spread.

Preventing Mulch from Developing Fungi

While it can be difficult actually to prevent your mulch from developing fungal inhabitants, there are some different things you can try to cut down on the chances of it developing.

The main thing needed to prevent fungi growth in mulch is cutting back on how moisture levels of your garden, as a moist garden, particularly a shaded one, is a prime area for mold development.

Water only when necessary, and use methods such as drip irrigation to get water directly where your plants need it at the roots, instead of all over the mulch and plant leaves, where you run the risk of developing other conditions that might not be related to moldy mulch at all, such as powdery mildew.

Other Mulching Considerations

When you apply mulch, the ideal depth ranges between 1.5 inches and 4 inches, depending on what kind of plants it surrounds.

If your mulch is too deep, it chokes off the oxygen supply to your plant’s roots, which can stunt their growth or even kill them. It can even prevent water from penetrating the roots, causing some roots to grow up into the mulch, which may destabilize your tree or shrubbery.

Deep layers of mulch grow certain good fungi.

Applying a deep layer of mulch can also inhibit the growth of certain fungal organisms known as mycorrhizal fungi. These organisms are essential for plant health and often form a symbiotic or beneficial association with plant roots.

Fungi frying up in mulch can also cause irrigation problems to the plants

When fungi infect mulch, it may cause the plants to dry out. This process occurs as typically deep mulch layers cause the mulch to grow fungi.

These fungal organisms may form a slime layer and become water repellant, so rain and even drip irrigation may not penetrate the layer.

This is especially detrimental to young plants such as trees, which can suffer from this drought.

When you mulch around your plants, you want to keep the mulch at least several inches from the base. Mulching directly against your plants increases the chance that they may develop issues, such as getting too moist at the base, which can cause health issues and may even rot away your tree’s bark.


Conclusion on why mulch turns white

When you notice mold or fungal infections on your mulch, it can be tempting to discard it immediately, but that can be an unnecessary expense unless, of course, you have seen those growing in your mulch are the harmful ones described above.

Instead of throwing them out for the known harmless mulch, consider the benefits they can bring to your plant’s growth. If you feel the need not to see them, you can also cover the mulch with more mulch and choke off the fungi’s air supply so that it is fine to continue using.

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