Why Is My Mulch Moldy? (Plus How To Treat It)


If you are an experienced gardener, you might use mulch to keep weeds at bay in your garden.  If you are a homeowner, you might use mulch to make your house look nice and improve curb appeal.  Either way, if you have experienced mulch with mold on it, you are probably wondering why it happens and how to treat it.  I wanted to know the same thing, so I did some research to find out.

So, why is your mulch moldy?  Your mulch is moldy because it provides the ideal conditions for mold growth: organic material such as wood to feed on, humid air or soil, and warm temperatures (77 to 86 degrees Fahrenheit, or 25 to 30 degrees Fahrenheit).  You can treat mold growth on mulch by spraying it with vinegar to kill the mold, or by digging up and disposing of the mold.

Even though you cannot control all environmental conditions, there are still steps you can take to prevent the spread of mold to your yard or garden.  Let’s start off with a more detailed look at why your mulch is moldy, how it spreads, and how you can prevent it from happening in the first place.

Why Is My Mulch Moldy?

Many homeowners will use wood chips as a decorative feature in their front or back yards.  Gardeners may use wood chips as mulch, but they also have the option of using compost, grass clippings, fallen leaves, pine needles, paper, or cardboard as mulch.

For more information, check out my article on compost and mulch.

wood chip mulch
Wood chips are often used as mulch, although you can use many other materials like grass clippings or leaves.

Mulch does have many benefits, such as insulating soil from extreme temperatures and maintaining moisture levels.  However, mulch itself is often composed of organic material such as wood, plant fiber, or paper.  This organic material provides the perfect food for mold to grow – but conditions must be right.

Organic material alone is not enough to allow mold to grow.  Mold will only thrive when there is enough moisture in the mulch or in the air, and when temperatures are warm (77 to 86 degrees Fahrenheit, or 25 to 30 degrees Fahrenheit).

mushrooms
Mold and other fungi, like mushrooms, are more likely to appear when mulch is moist and warm, such as after a summer rain.

This is why you will often see mold begin to appear on mulch in the spring or summer, especially after a few days of rainfall.  As temperatures get warmer and mulch stays wet for longer periods of time, mold growth becomes more likely.

For more information, check out this article on mold from the EPA.

What Does Mold On Mulch Look Like?

Mold on mulch can have many different appearances, depending on the type.  You may see anything from white rings on the surface of the mulch to orange, yellow, or brown-colored patches.  Here are a few different types of mold and fungus, along with a description of each:

  • Shotgun or Artillery Fungus – Generally not harmful to plants, this fungus produces orange or cream “cups” that hold a black clump of spores.  This clump of spores looks like a tiny egg.  Given enough time, the cup will explode and send the spores up to 12 feet away to colonize more mulch or soil.  Be careful – the black spores that come out of the egg can stain cars or houses!  According to researchers at Penn State, a mixture of 40% used mushroom compost and landscape mulch can suppress artillery fungus.  For more information, check out this article on mold from the Penn State University Extension.
  • Slime Mold – Sometimes called “dog vomit” for its appearance, this mold may be yellow, orange, or another bright color.  A slime mold can grow up to a foot wide!  Slime mold may also grow on logs, so if you have firewood in your yard, watch out for this one!  For more information, check out this article from Wikipedia on slime mold.
  • Bird’s Nest Fungi – These fungi look like small bird’s nests with eggs in the middle.  When it rains, the “eggs” splash out of the “bird’s nest” to spread the fungi.  Bird’s Nest Fungi are generally not considered harmful to plants.  For more information, check out this article from Wikipedia on Bird’s Nest Fungi.
  • Stinkhorn Fungi – The top of a stinkhorn has an unpleasant odor.  This odor attracts flies, which land on the stinkhorn and spread its spores when they fly way. For more information, check out this article from Wikipedia on Stinkhorn fungi.
  • Mushrooms – a mushroom is simply the fruit of a fungus that grows underground.   This fruit contains the spores, allowing for reproduction and spreading of the fungus.  Mushrooms will not harm your plants, but some are poisonous to people or animals, so watch young children and pets near them!  For more information, check out this article from Wikipedia on mushrooms.

For more information, check out this article from the University of Massachusetts Amherst on the types of mold that grow on mulch.

yellow slime mold
Slime mold is not harmful, but it can be unsightly, and it often stands out very well against dark mulch.

How Does Mold Spread?

There are several ways that mold can spread to your mulch from other places.

As mentioned above, some mold, such as bird’s nest fungi, will spread with the help of rain or water that move their spores around. To prevent this method of spreading mold, avoid spraying your mulch with water if you see mold growing on the surface.

Insects or animals may also spread the spores of mold.  For example, flies often spread the spores of stinkhorn fungi.

stinkhorn fungus
A stinkhorn fungus attracts flies, which then land on it and spread its spores around.

Some mold spores spread on the wind.  Just like dandelions spread their seeds on gusts of wind, mold spores can also find new soil or mulch to colonize by riding the air currents.

Finally, mold can spread if the mulch you buy already has mold spores in it.  In that case, the mold will likely grow if you provide enough moisture when you water your plants or when it rains.

If you buy mulch from someone who cuts down diseased or dead trees, then the wood chips are much more likely to be infected with mold spores.

Is Moldy Mulch Harmful To Plants Or Trees?

Most mold is not harmful to healthy plants or trees.  However, mold will grow on dead or fallen trees and plants to decompose them.  If the plants in your garden are weak due to nutrient deficiencies or over watering, then they are more likely to fall victim to mold.

fallen tree
Mold and fungus will grow on dead or fallen trees, so be careful about mulch from these sources.

Normally, there is no need to treat your mulch for mold.  If mold does grow, it is happening as a natural part of the process of decomposing wood and other organic matter.

If you are worried about mold harming your plants or trees, then avoid putting mulch too close to them.  Leave a few inches of bare soil around the base of your plants or trees.  This will prevent rain from splashing any mold spores from the mulch onto your plants.

How Do You Get Rid Of Mold On Mulch?

If you already have mold on your mulch, there are a couple of ways you can treat or prevent the problem.

One simple way to kill mold is to use vinegar.  Fill a spray bottle with vinegar, and then spray the mold with the vinegar.

spray bottle head
Vinegar in a spray bottle can help you to deal with mold or other fungi on your mulch.

The acidity of the vinegar should weaken and eventually kill the mold.  However, you may need to apply the vinegar a few times for it to take full effect.

Remember to avoid spraying vinegar on or near your plants, since the acidity will also kill them!  If you are worried about this, add some water to dilute the vinegar and make it less acidic.

In order to prevent mold from growing, you can also try to make your soil or mulch more acidic or basic.  Adding sulfur will lower the pH of soil, and adding lime (calcium carbonate) will raise the pH of soil.

A pH that is too high or too low will kill mold, but it may also harm any plants or trees that are nearby.  If you do opt for acidic soil or mulch to kill mold, try planting blueberries or azaleas.  They prefer more acidic soil, and may thrive where other plants would die.

One last simple way to kill mold is to dig it up and dispose of it.  However, there are several problems with this method.

First of all, if the mold is mature, you may spread the spores when you move it.  This can make the mold problem in your mulch worse in the long run.

There is also the danger of breathing in mold spores when doing this kind of work.  Exposure to some types of mold can cause symptoms similar to a cold.  This can be especially dangerous for people with asthma or allergies, so be sure to wear a mask before you try to dig up and remove mold.

orange slime mold
Some fungi have bright and interesting colors, but if you want to get rid of them, you might just have to dig them up and dispose of them.

Finally, even if you move the mold without sending the spores flying around, the mold may still take up residence wherever you dump it.  This means that you will need to find a way to dispose of it properly.  Mold does not need light to grow, so it can survive in places you might not expect.

Conclusion

By now, you have a better idea of why your mulch is moldy, how mold spreads, and how to prevent it in the first place.  You also have some ways to treat the problem if you need a quick solution.

I hope you found this article helpful – if so, please share it with someone else who can use the information.  If you have any questions or advice about moldy mulch, please leave a comment below.

jonathon.david.madore

Hi, I'm Jonathon. I’m the gardening guy (not guru!) who is encouraging everyone to spend more time in the garden. I try to help solve common gardening problems so that you can get the best harvest every year!

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