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Why Does Mulch Steam – 3 Reasons and Solutions!

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Mulch is commonly used in landscaping homes and gardens. If you have ever grown plants in your home, you would know about the multiple benefits of adding mulch to the crops’ soil. While mulch has several uses, there are some confusing questions about laying this material down in your home. Several people will notice steam coming off from mulch, which makes them wonder, “why does mulch steam? “

There could be several reasons why you see steam erupting from the mulch in your garden. It could be due to a fungus, decomposition, the weather, or a few more environmental changes. Most of the time, it is nothing to worry about. So, if you have been trying to wrap your head around this confusing issue, keep reading to find out “why does mulch steam.”

Why Does Mulch Steam
Steam coming off the mulch. – via Reddit

Why Does Mulch Steam?

While mulch is not a necessity for your budding garden, in some cases, you may face a situation that makes it essential to use a mulch to avoid damaging your plants. This material is organic matter, making your plants more resistant to pests and insects. It helps maintain the temperature of the soil, which is very important in extreme weather.

Mulch retains water in the soil, so you won’t have to water it regularly. This keeps the issue of overwatering and underwatering at bay. It also keeps the weeds off, provides nutrients to the soil, and acts as a protective covering.

If it has so many benefits, then why does much steam? It is not always on fire. The steam has other causes behind it. Let’s find out what they are.   

Steam Due To Decomposition

One of the most common but overlooked reasons behind the steam erupting from your mulch could be decomposition by bacteria. Most mulch is organic matter, and it has carbon-containing compounds, especially if it is made from wood. All organic matter goes through a process of breakdown over time. This is called decomposition.

Bacteria present in the organic matter decompose the mulch, and it multiplies to increase in number as the organic matter breaks down. There is a lot of heat generated during this decomposition process, and it can go up to 70-degree Celsius. This heat keeps accumulating until it reaches a point where the mulch starts smoking and emitting steam.

The heat and steam from the mulch pile will be more intense if there is a thicker pile of mulch lying around. The steam is due to the excessive heat stored up in a pile. There is more steam in colder weather. If you shuffle the mulch, more steam will come out of it.

Bacteria need the right conditions to start decomposing. They work better on moist mulch. So, you won’t see any decomposition on dry mulch. If you see dry mulch emitting steam, there are other reasons behind it.

However, you don’t need to worry about keeping the mulch dry as decomposition is a natural process, and it is what prepares the mulch to be used as compost for your plants. You might need to replace the wet mulch more often as it breaks down.

What Can You Do?

If you are sick of mulch decomposing often and want to give it a rest, try using cedar mulch as it is resistant to breaking down more than other wood materials. If the mulch is for landscaping reasons, you can use gravel, sand, rocks, or some other material that bacteria cannot break down in your garden. Small rocks look pretty cool in the yard.

Also, remember not to overwater the mulch, as moisture is what provides a catalyst to the bacteria. Keep the mulch as dry as you can, and you won’t see as much steam.

Steam Due To Fire

Yes, mulch can catch fire, which could be the reason for the steam coming out from the pile in some cases. Mulch usually needs an accelerant to catch on fire, and there could be several.

If you or someone else threw a cigarette bud right after smoking it without putting it out properly, mulch could catch fire. You may have set up lights and lamps in the garden, which require wires outside. An electrical spark from these lighting accessories or buried wires could start a mulch fire. Another reason could be a lightning strike.

If you wonder whether mulch could start a fire on its own, then yes, it could. This process of starting a fire on its own is called spontaneous combustion. However, certain conditions will need to be met if the fire starts independently. The temperature needs to reach at least 300 to 400-degree Celsius before the fire can start.

Spontaneous combustion usually happens when the mulch is already wet and has undergone decomposition. The weather could become too hot, or there could be direct, bright sunlight over the decomposing mulch, leading to a fire.

What to Do?

Steam due to fire can be dangerous as it could threaten lives and spread to your house and the buildings nearby. To avoid fires from mulch, try to keep it dry and under a shade. Ensure there are at least 18-20 inches between the mulch and your house so that the fire won’t spread much.

Alternatively, you could go for rocks, gravel, and other materials for landscaping that don’t catch fire. Also, putting out containers for cigarettes and other such stuff is a great idea so you won’t have to throw them down. You should also ensure to spread out the mulch in thinner layers.

Why Does Mulch Steam 2
via Pikist

Steam Due To Fungus

Some fungi appear like smoke when they are touched. For example, if there is slime mold in the mulch and you rake it, or water hits it, a mixture of spores and dust will come off looking like steam. Most fungus feeds on bacteria, so it is a perfect environment for fungi to grow if the mulch is undergoing decomposition.

What to Do?

Fungus thrives in water and high humidity like during the rainy season. Your best bet is to keep the mulch dry so there would be less or no bacteria, and fungi won’t come to invade it. This fungus may not necessarily harm your plants. You can just shuffle and shift the mulch, and the fungus will disappear on its own in some time.    


I hope this clarifies why does mulch steam. Keep the mulch dry, spread in thin layers, and out of direct sunlight, and you will be fine. Make sure to work on the tips above to avoid any fires and other issues with steaming.

How do you deal with steaming mulch? Please share your views in the comment section below.