Reading Time: 5 minutes 🍃
Mulch has been known to combust spontaneously. If you aren’t careful, this blaze can spread to other parts of the garden, destroying years of hard work. But what makes mulch piles so flammable?
The build-up of heat within the mulch pile can cause spontaneous combustion. However, it’s common for an outside heat source, like a cigarette, to ignite the pile. Once the pile is hot enough, it will burn easily because of its amount of flammable material.
The good news is that you can do a few simple things to stop your mulch pile from catching on fire. Keep reading to learn some of the simple things you can do to keep your garden safe.
What Causes Mulch Piles to Catch Alight?
There are a few reasons why your mulch pile might catch on fire. Let’s look at each of them.
Flammable Materials
The most likely reason why a mulch pile will catch on fire is because of flammable materials. Most commonly, this will take the form of a cigarette placed on or near the fire pile. The mulch pile will be made of flammable organic material, like bark.
Because of this, it won’t take a lot of heat to catch alight. However, it’s rare for it to catch fire instantly. The pile will likely smolder and smoke for a while, while the heat builds up. When the time is right, it will burst into flame.
Spontaneous Combustion
Mulch piles can get very hot. This is caused by the bacteria multiplying rapidly to break down the organic materials and release the nutrients into the soil. When this happens, though, it will create a lot of heat. You might notice some steam or smoke released when turning your mulch pile.
But if the pile is thick enough and doesn’t get turned quickly enough, the heat won’t be able to escape. Because of this, there is a chance that it can spontaneously combust. While this is rare, it is possible, especially if your pile is more than six inches thick
.
This video goes into more detail about how this type of spontaneous combustion occurs:
How Likely is Mulch to Catch Fire?
There are a few elements that can make mulch more likely to start a fire; these include:
- Humidity. As the air dries out, the mulch pile catching on fire risks will increase.
- Temperature. The warmer the temperature, the more heat will build up within the mulch pile.
- Winds. A strong wind can help fan the flames, so the mulch heap will be more likely to catch alight.
- Human factors. As we’ll discuss, how damp the pile is, how often you turn it, and where it is positioned will all impact how likely a fire is.
It’s also important to consider the type of mulching material you use. According to a study from the Harrisonburg Fire Department, here are the most combustible mulch materials:
- Pine needles
- Shredded rubber
- Cedar branches
Though they weren’t included in the study, other combustible materials include dry garden waste, like cut grass and leaves. Manure can also increase the temperature of the mulch pile.
They found that the least combustible mulching materials were:
- Tahoe chips
- Wood chips
Preventing Mulch Piles from Catching Fire
We don’t have any impact on the weather conditions. But we do control the way that the mulch pile will be handled. When done right, we can drastically reduce the fire risk. Here are some tips you can use to help you do this.
Getting the Right Mulch Pile Size
One of the easiest ways to prevent a mulch fire is to limit the size of your pile. If you do this, you’ll be able to reduce the amount of heat that can build up within the pile. It’s recommended that you keep it to around six inches high. This can mean spreading the pile out across a larger area. Or it can mean using multiple mulching sites.
Keeping the Mulch Moist
One of the most important elements for you to focus on is stopping the mulch from drying out. If it does get too dry, there is a chance that it will catch on fire. This is especially important if you live in a dry, humid state. Plus, if you keep the mulch damp, you’ll be able to create an environment that allows worms and beneficial bacteria to thrive.
You should aim for moisture levels between 40 to 60 percent. This might mean watering your mulch pile a few times each week. It can help to have the same watering timetable you use for your garden. You should create a dip in the mulch pile’s top, allowing for superior water absorption.
Consider the Makeup of the Mulch Pile
The types of materials that you use in your mulch pile can determine how likely it will be to catch fire. If you feel the temperature in your pile is getting too high, there are some things you can add to reduce the risks. These include:
- Spreading some wood chips
- Using some sand. This adds bulk to the mulch and improves drainage when spread on the soil. But, since it has no nutritional benefits, don’t add too much. Make sure that you mix the sand thoroughly.
- Avoid mulching cigarettes. One of the most common mistakes is adding cigarettes to the soil. These can be the ignition source that sparks the fire in the mulch pile.
Turning the Pile Regularly
One of the simplest ways to prevent a fire is by turning the pile regularly. This breaks up the layers, stopping heat from building up. It also helps the organic material break down, introducing the oxygen bacteria need to thrive.
You’ll need to turn the mulch every three months in the garden. This will stop the soil from becoming too compacted and the heat from building up inside the mulch.
Moving the Mulch Pile to a Safer Location
Finally, these methods should significantly lower the odds of the mulch pile catching fire. But if you want to be on the safe side, you might want to move it away from your house and garden. This will limit the damage if a fire does break out.
It can also help to put a brick wall around the pile. This will keep any potential fires contained in the mulch pile. Make sure to keep it away from any chemicals that might accelerate the burn speed. This includes things like fertilizers or pesticides.
Consider using river rocks around the edge of your garden in a 15-inch barrier. This is akin to building a natural firewall, keeping any potential fire contained.
Conclusion
Mulch piles have been known to combust spontaneously. It is more common for them to require some heat source, like an unextinguished cigarette, to catch alight. The good news is that this is very rare.
You can do a few simple tasks to reduce fire risk. This includes turning the mulch regularly to release the built-up heat and keeping the pile moist. Also, spread the mulch to a maximum layer depth of six inches. This will make it difficult for enough heat to accumulate to start a fire. Not only will this stop the pile from catching alight, but it will also create the optimal conditions for bacteria to thrive, helping break down the mulch and release the nutrients into the soil.
Sources
- https://www.thespruce.com/why-landscaping-mulch-can-spontaneously-combust-and-how-to-prevent-it-5184551#citation-1
- https://soilyourself.com.au/what-you-need-to-know-about-mulch-and-fire-safety/.
- https://www.yourgreenpal.com/blog/could-mulch-catch-fire-and-burn-down-a-home