Why is My Mulch Smoking? (3 Causes and How to Prevent)


If your mulch is smoking or steaming, you are probably wondering what is causing it and how to prevent it.  There are a few possible reasons that mulch will smoke, and the steps you take to prevent it will depend on the cause.

So, why is your mulch smoking?  Mulch will produce smoke or steam due to heat caused by bacteria during the decomposition process – just like in a compost pile.  Mulch will also appear to smoke or steam when watered due to certain types of fungus that grow in the mulch.  Finally, mulch will smoke if it is dry and a spark causes it to smolder.

Of course, there are ways to prevent mulch from smoking or steaming.  You may need to change your approach to landscaping, but it is definitely possible to avoid the problem.

In this article, we’ll look at each of the causes of smoking or steaming mulch and how to address the issue in each case.

Let’s get going.

Why is My Mulch Smoking?

Three common causes of smoking or steaming mulch are:

  • Decomposition – this occurs when bacteria begins to break down the mulch.  As bacteria multiply and break down the mulch, they generate lots of heat.  If there is enough mulch and it gets hot enough, it will begin to give off steam.  This steam is much easier to see in cool weather, or if you turn the mulch over with a shovel or pitchfork.
  • Fungus – sometimes fungus will grow in mulch.  This is much more common in damp mulch, which is more likely during humid conditions with lots of rain.  Fungi, along with bacteria, are a natural part of the decomposition process.  Some fungi will release spores that look like smoke when they get wet (from a hose, watering can, or rain).
  • Fire – if your mulch is dry due to drought, it can catch fire from a spark (such as from a cigarette or electricity).  Although it is rare, it is possible for a large pile of mulch to spontaneously combust (that is, catch on fire without a spark to start it).

Let’s take a closer look at each of these causes of smoking or steaming mulch, along with some ways you can prevent the problem in each case.

Smoking Mulch Caused By Decomposition (Bacteria)

This is a common cause of smoking mulch, and one that is commonly overlooked.  In reality, the mulch is not smoking – rather, it is steaming, due to the decomposition process caused by bacteria.

Why Does Mulch Steam?

If your mulch is made from wood chips, then it contains organic material (that is, carbon-containing compounds).

mulch
If your mulch is made of wood, then it is possible for it to decompose and start steaming, due to the action of bacteria.

Over time, this organic material will begin to naturally break down.  One way this happens is by bacterial growth.

When mulch begins to steam due to bacterial decomposition, it is similar to what happens in a compost pile.

First, the bacteria begin to eat the organic material in the compost or mulch.  Next, they multiply and spread through the pile.

The bacteria generate a lot of heat as they continue this process.  In fact, according to Cornell University, a compost pile can get up to 70 degrees Celsius (about 160 degrees Fahrenheit!)

Finally, when a compost pile gets hot enough, it will appear to be smoking.  In fact, this is steam caused by the heat in the pile.

This steam will be more obvious in cold weather.  You will also see steam when you turn the compost or mulch to aerate it.

If your mulch starts to decompose and then cold weather comes along, you will see lots of steam coming out of the mulch (more so if you turn it with a shovel or pitchfork).

Bacteria are more likely to start decomposing mulch under the right conditions.  This means slightly damp mulch will be more likely to decompose and start steaming.

Bacteria will not work in very dry mulch.  However, there is some danger of dry mulch catching on fire (more detail on this later).

However, this doesn’t mean you should be afraid to water plants growing in your yard just to avoid getting the mulch wet.  Decomposition by bacteria is a natural process, and the result will be compost that your plants can use to grow.

However, rapidly decomposing mulch might mean that you need to replace your mulch more often.

If you want to avoid steaming mulch caused by bacterial decomposition, there are a couple of things you can do:

  • Avoid over watering your plants – this will keep the mulch dry and slow down the decomposition process
  • Use artificial mulch or other landscaping materials – instead of wood chips, try using recycled rubber chunks or gravel as a landscaping feature.  Bacteria cannot break these down like they can wood chips or bark.
gravel small stones
Using gravel (small stones) for landscaping is an alternative to wood chip mulch which will avoid steaming mulch.

Smoking Mulch Caused By Fungus

Your mulch might also look like it is smoking if certain species of fungus take up residence.  For example, slime mold (also known as “dog vomit fungus”) will appear to smoke when it is watered.

orange slime mold
Slime mold is also known as “dog-vomit fungus”. This one is orange, but it can also be yellow, green, or blue.

In reality, a combination of dust and fungi spores is coming off of the fungus when water from rain or a hose hits it.

According to the University of Arkansas Extension, slime mold can appear on the surface of mulch as orange, yellow, green, or blue patches.  Slime molds will eventually dry out and turn brown, later turning white.

Slime molds feed on bacteria and yeast, which themselves feed on organic material (the wood chips in the mulch itself).  So, if your mulch has already started to decompose due to bacteria, it is providing extra food for slime mold.

Slime mold and other types of fungus are more common with high humidity and lots of rain.  Slime mold won’t harm plants, but it can have a strange and off-putting appearance.

If you want, you can use a rake to turn over the mulch anyplace where the slime mold appears.  In time, it will disappear on its own.

You can also scrape up the slime mold from the surface of the mulch and dispose of it.  However, the fungus might come back until it eats up whatever attracted it in the first place.

As with bacteria, the best way to prevent slime mold and other fungi in your mulch is to keep it dry or change your landscaping to feature artificial mulch (such as recycled rubber chunks or gravel).

Other types of fungus that might appear on mulch include:

  • Mushrooms (Toadstools)
  • Bird’s Nest Fungus
  • Artillery fungus
mushroom
Mushrooms (toadstools) might also appear in your mulch to help break down the wood chips.

You can learn more about these types of fungus, what they look like, and how to treat them in this article from the Penn State University Extension.

Smoking Mulch Caused By Fire

In certain cases, mulch can also catch on fire.  This is the one case where smoking mulch can be dangerous to the health and property of you and your neighbors.

How Does Mulch Catch Fire?

Mulch catches fire when the mulch is dry and something causes a spark to ignite the dry mulch.

For example:

  • If a smoker throws a cigarette into dry mulch, it could catch fire after he walks away.
  • A fire could also start due to an electrical spark from landscape lighting or buried wires.
  • It is even possible that a lightning strike could start a mulch fire.

No matter how it starts, a mulch fire can threaten buildings and people.  This is especially true with mulch made from recycled rubber chunks (since rubber is an oil product!)

In addition to threatening your health and property, a mulch fire could spread to other buildings nearby. 

fire
It only takes a small spark to start a mulch fire under dry conditions.

To avoid this risk, leave at least 18 inches between a building and any mulch used for landscaping.  As an alternative, you can use gravel for landscaping near a building.

Also, put out containers for smoking materials to help prevent mulch fires from starting.  You can learn more about mulch fires and how to avoid them in this article from Safety Insurance.

Can Mulch Catch On Fire By Itself?

Once in a while, mulch can catch on fire by itself, without any spark.  When this happens, it is called spontaneous combustion.

Although spontaneous combustion is rare, it is more likely when mulch is applied too thickly, or when it is left in a large, deep pile.

It really takes a perfect storm for this to happen.  For example, if the hot sun were beating down on a large, moist pile of mulch and bacteria had already started the decomposition process.

According to the Michigan State University Extension, a compost pile would have to reach temperatures of 300 to 400 degrees Fahrenheit before spontaneous combustion would occur.  (This is twice as hot as compost normally gets.)

If you are worried about spontaneous combustion, don’t leave a large pile of mulch in your yard or driveway for too long.  Use the mulch promptly and apply a layer that is not too thick for your landscaping.

Conclusion

Now you have a better idea of why your mulch looks like it is smoking.  You also know some ways to prevent the problem in the future, depending on what is causing it.

I hope you found this article helpful.  If so, please share it with someone 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 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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