Why Is My Mulch Moldy? (Plus 3 Ways To Treat It)


Mulch is useful for keeping weeds at bay in the garden, and it can make your house look nicer when used for landscaping. However, mulch quickly loses its appeal when it has mold growing on it!

So, why is your mulch moldy?  Mulch is moldy when it provides the ideal conditions for mold growth: organic material to feed on, humidity, and warm temperatures (77 to 86 degrees Fahrenheit, or 25 to 30 degrees Celsius).  You can treat mold growth on mulch by drying it out, spraying it with vinegar, or by digging up and disposing of the mold.

Of course, you cannot control all of the environmental conditions in your yard. However, there are still steps you can take to prevent the spread of mold to your yard or garden.

In this article, we’ll start by looking at at why your mulch is moldy in the first place. We’ll also talk about how mold spreads and how you can prevent it in the first place.

Let’s get going.

Why Is My Mulch Moldy?

Mulch gets moldy when it provides the ideal conditions for mold growth:

  • Organic material for mold to feed on (wood chips is one of their favorite foods!)
  • Humidity (if you get lots of rain or if you over water)
  • Warmth (temperatures of 77 to 86 degrees Fahrenheit, or 25 to 30 degrees Celsius, seems to be perfect for mold growth)

Many homeowners will use wood chips as a decorative feature in the front or back yard.

fungus on wood mulch
You may see lots of different types of fungi or mold living on mulch made from wood chips.

Gardeners might also use wood chips as mulch in the garden, but they also have the option of using alternatives, such as:

  • Compost
  • Grass clippings
  • Fallen leaves
  • Pine needles
  • Paper
  • Cardboard

Unfortunately, all of these materials can also harbor mold growth under the right conditions.

wood chip mulch
Wood chips are often used as mulch, but you can also use grass clippings or leaves.

Mulch has several benefits in gardening and landscaping, including:

  • insulation (keeping soil from heating up or cooling down too fast)
  • water retention (preventing water from evaporating out of the soil)
  • weed prevention (smothering existing weeds or stopping the growth of new ones from seed)

However, mulch itself is usually composed of organic material such as wood, plant fiber, or paper.  This organic material provides the perfect food for mold to grow.

However, organic material alone is not enough to allow mold to grow.  Mold will only grow when there is enough moisture in the mulch or in the air (from rain or irrigation).

Mold also needs warmth to really thrive. Temperatures of 77 to 86 degrees Fahrenheit (25 to 30 degrees Fahrenheit) are ideal for mold to grow.

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. Some possible mold colors include:

  • white
  • black
  • brown
  • orange
  • yellow
  • green

You may see anything from white rings on the surface of the mulch to orange, yellow, or brown-colored patches.

yellow fungus
Fungi that appear on mulch can be yellow, green, orange, brown, black ,or white.

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

There are lots of different types of mold and fungi you could see on your mulch. Here are a few, starting with “shotgun” fungus.

Shotgun Fungus (Artillery Fungus)

Shotgun fungus is generally not harmful to plants. It produces orange or cream colored “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 exploding 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

Slime mold is sometimes called “dog vomit” for its appearance. It can be yellow, orange, or another bright color.

However, the name is misleading: slime mold is not really mold at all. According to the University of New Hampshire Extension, slime mold is descended from ancient amoebas.

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

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 Fungus

Bird’s Nest Fungus looks 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 fungus
Bird’s nest fungus spreads by spores when it rains.

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 Fungus

The top of a stinkhorn fungus has an unpleasant odor.  This odor attracts flies, which land on the stinkhorn and spread its spores when they fly way.

stinkhorn fungus
Stinkhorn fungus has an unpleasant odor, which attracts flies to help spread spores.

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.

fungus on ground
You might see any of a number of mushrooms growing on your mulch if it stays wet.

Mushrooms will not harm your plants. However, 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.

How Does Mold Spread?

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

  • water
  • insects
  • animals
  • wind
  • movement of mulch

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

sprinkler
Don’t leave the sprinkler on for too long, or you might end up with more mold than you bargained for!

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

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 & 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?

There are a few different ways to get rid of mold on mulch if it is bothering you, including:

  • Drying it out (not practical in all climates)
  • Treating it with vinegar (simple, but may harm nearby plants)
  • Digging it up and disposing of it (requires the most work)

By the way, you can avoid mold and decomposition of wood mulch by using cedar mulch, which resists rotting.

However, you might already have mold on your existing mulch. In that case, let’s look at mold removal methods in a little more detail.

Drying Out Mold

According to the Environmental Protection Agency, mold will not grow if there is no water present. However, it is not always easy to remove sources of moisture for mold.

Of course, you can water carefully with a watering can or install a drip irrigation system. This will help to avoid the over watering that is common with a sprinkler.

However, there isn’t much you can do about an extended period of rainy weather or high humidity. In those situations, you will need to try one of the other methods listed below.

Treating Mold With Vinegar

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.

According to Healthline, research has shown that vinegar can kill many different types of household molds. However, it does not kill every type of mold.

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

Keep in mind that the acidity of vinegar can act as a double-edged sword. 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 more acidic or basic.  Adding sulfur will lower the pH of soil, and adding lime (calcium carbonate) will raise the pH of soil.

Just remember that a soil 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 to kill mold, try planting blueberries or azaleas in that area. These plants prefer more acidic soil, and they may thrive where other plants would die.

Digging Up & Disposing Of Mold

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

shovel
Before digging up mold from mulch, wear a mask and use a bag to cover it up to prevent spores from spreading.

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.

To avoid this, cover the mold with a bag (ideally a biodegradable one) before you dig it up. This also gives you an easier way to move the mold after digging.

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 is 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

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.

You might also enjoy my article on preventing mulch from washing away or my article on how to prevent mulch from smoking.

I hope you found this article helpful – if so, please share it with someone else who can use the information.

If you want to read some of my most popular posts, check out the “Best of GreenUpSide” page here.  Enjoy!

~Jonathon

jonathon.david.madore

Hi, I'm Jon. Let's solve your gardening problems, spend more time growing, and get the best harvest every year!

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