Mulch is a deceptively harmless gardening necessity.
You need it to preserve ground moisture and protect your plants from the cold. However, did you know that, under certain conditions, mulch could steam and spontaneously combust?
Yes, mulch is a fire hazard that can generate heat without help from external flames!
In this article, I’ll discuss the science behind mulch combustion. Get ready as we uncover how to prevent mulch fires within your backyard.
An Overview of Mulch Fires
Mulch is a flammable material that can produce volatile compounds as it breaks down. Because of this, hundreds of mulch fires happen each year.
Last August 2023, a mulch fire in Houston burned for two days straight, causing widespread destruction.
In another incident, a mulch fire in Arizona produced so much smoke that it caused hazy conditions across the valley.
These fires can spread to nearby buildings and are potentially fatal.
Why Does Mulch Steam?
Mulch can catch fire when someone discards cigarette butts near it. The structure of mulch and its dryness means it can hold plenty of oxygen to fuel flames.
Yet, that’s not all. Mulch can steam and spontaneously combust because of chemical reactions.
When you leave a large pile of mulch untouched for a long time, the center of the pile starts decomposing and producing heat. Pair this with low humidity and a lack of airflow, and you’ve got a recipe for trouble.
According to FEMA, when the mulch reaches a high enough temperature, it can catch fire without outside help!
10 Ways to Prevent Mulch Fires
Preventing mulch fires means you have to follow guidelines to properly store your mulch. Here’s how you do it.
1 – Keep Mulch Piles Away
Researchers from the University of Nevada recommend keeping mulch 5 to 30 feet away from structures. Don’t place it beside your home as the fire from mulch can spread quickly.
Moreover, keep your area clean and be wary of objects near mulch piles. Plant debris, fences, and other combustible materials are potentially dangerous next to mulch.
2 – Surround It With Rocks
The same researchers suggest surrounding mulch with gravel, concrete, pavement, and rocks. In addition, you can take advantage of irrigation and create well-maintained flower beds.
Proper segregation is key for preventing mulch fires from spreading!
3 – Turn Mulch Over
Next, you must regularly turn mulch piles over.
The reason for this is that chemical reactions happen in the middle of mulch piles due to the lack of airflow. Turning a mulch pile over releases the flammable organic gasses.
This step is a great way to check whether the middle of your mulch is starting to produce heat!
4 – Avoid Thick Mulch Layers
Large mulch piles mean you have a higher chance of starting a fire. You have to get a permit for keeping more than 300 cubic yards of mulch!
You should only build small mulch piles and spread them in thin layers no higher than four feet. This way, you avoid creating dangerous gasses that spontaneously combust.
5 – Report Steaming Mulch
If you see a steaming pile of mulch, call your local fire department. Moreover, if you’re a business owner, it’s a good idea to train your staff to watch out for steaming mulch.
Steaming mulch is a sign that it’s at the ignition temperature. This means it could catch fire at any minute.
6 – Proper Fire Disposal
Be aware of where you dispose of cigarette butts and hot coals. You may want to wet ignition materials before you throw them away to prevent mulch fires.
On top of this, if you’re a business owner, you should have proper bins for smoking materials. Place receptacles near entrances or at designated smoking areas.
7 – Use Fire Retardant
Some hardware stores sell odorless and non-toxic fire retardants for mulch. All you have to do is either soak the mulch in the retardant or spread it with a bug sprayer.
Plus, consider using fire-resistant plants for your landscape. Some examples of these plants are lilacs, sedums, and sumacs.
8 – Buy Fire-Resistant Mulch
Most mulches are flammable, but some are more resistant than others.
The University of California says composted wood chips have the lowest burning characteristics. You should buy ones that are three inches long, and apply the mulch four inches deep at most.
Avoid shredded rubber, pine needles, and cedar bark. These mulches have the highest chance of catching fire!
If you have these types of mulch, a great tip is to store them 30 feet from your home. Doing so ensures fire won’t spread in case the mulch ignites.
In general, larger-grained mulch is better against fire, as it can retain moisture for longer.
9 – Wet the Mulch
In an interview with the CBC, a fire marshal suggests wetting the mulch to prevent it from smoldering.
However, he also warns that water may not fully seep into the mulch. In short, there’s still a chance for the mulch to catch on fire.
Because of this, you should keep a bucket of water or sand nearby. Pour it over a fire to stop ignition.
10 – Keep an Eye on the Weather
Finally, always be on the lookout, especially in summer.
The lack of humidity and the hot sun can increase the chances of a fire. What’s more, strong winds can fan flames from mulch piles.
Having CCTV cameras in your yard may give you extra time to discover and react to mulch fires!
FAQs
Why Is Mulch Hot?
Mulch becomes hot due to decomposition. As the wood breaks down, it releases heat and flammable gas.
As the internal temperature of mulch reaches 86°F and its moisture falls to 45%, it can spontaneously combust.
Can Mulch Burn Plants?
Yes. Decomposing mulch, also known as sour mulch, is harmful to plants. It can lead to plant injury and death because of the high temperature.
In addition, the alcohol, methane, and ammonia that decomposing mulch produces is also toxic to plants.
To prevent plant injury, you can wash the mulch with water. Spread it into a thin layer a week before use to air the toxins out.
Final Thoughts
Mulch is a useful addition to all plant beds. It helps retain moisture and prevents pests and erosion.
Yet, you should also look out for hot mulches.
If you don’t store your mulch correctly, it can decompose and accumulate heat through chemical reactions. Once it reaches a hot enough internal temperature, the mulch can spontaneously combust.
To avoid an accident, keep your mulch piles low, wet them, and turn them over regularly. Lastly, store mulch away from structures, and always report a steaming mulch pile when you see one!
Ben has a bachelor’s degree in construction engineering. When not constructing or remodeling X-Ray Rooms, Cardiovascular Labs, and Pharmacies, you can find him at home with wife and two daughters. Outside of family, He loves grilling and barbequing on his Big Green Egg and Blackstone Griddle, as well as working on projects around the house.
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