Ask The Sietch: Can My Compost Pile Catch On Fire?

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Yesterday there was a local news story here about some nearby schools’ playgrounds being closed due to concerns about spontaneous combustion from “manufactured wood chips” that cover those areas. A security camera showed this stuff catching fire. (I assume this has to do with the fact that our temperatures this week are around the 104-degree mark.) Of course, the manufacturers say this is hogwash, but the firemen said they’ve been worried about this for some time. Have you heard anything about a problem with this product?

Anyway, during the story, they mentioned that compost piles can have the same problem if the compost isn’t turned. Is this true? I’ve inherited a house that came with a nice compost pile, but I haven’t turned it in the year since I’ve lived here.

I found this story, most likely the one Candance is talking about. So can a big pile of mulch and compost catch fire. You bet they can. And if they do, and the pile is big enough, it can be a big mess.

So how does of bunch of weeds, grass clippings, and table scraps suddenly catch fire? Before we get to that lets talk a bit about how composting works. Composting is the practice of speeding up the process of creating soil. In your average forest when leaves and branches and whatever fall to the forest floor a whole host of organisms big and small (worms, bacteria, insects, fungus, small mammals, birds etc) go to town, each one breaking down the plant matter until it becomes soil. Good soil is in essence good shit.

In a compost pile you are collecting large amounts of organic matter you want broken down into soil. One of the hardest workers in a compost pile is the lowly bacteria. They do most of the breaking down. To aid in this process you give the bacteria what they need most, oxygen and water. You give it water with a hose, and oxygen by turning it with a pitch fork every couple of days.

So how does this serene process result in a conflagration? Well as these little bacteria are going to town on the leaves and eggshells and whatever else you put in there they create heat (as most living things do when they move around). This heat can lead to the “smoking” you see over the compost pile early in the morning when the air is cool. This is not fire, it is just hot moist air condensing in the cool morning air.

steaming compost

If you have a large enough pile, and you fail to water it and turn it this is what happens. The outer layers of the pile begin to dry out, as more and more moisture escapes from these layers the bacteria stop working as hard, but deep in the core, where there is still plenty of water the bacteria are still going to town. The outer layer now acts as insulation, drying out but also keeping heat in the core. If you are unlucky, the outer layer can get so hot, and so dry that Blamo! compost fire. If you are wondering organic matter most often starts to be in danger of catching on fire when it reaches temperatures of around 300 degrees Fahrenheit. A simple candy thermometer or meat thermometer will let you know how high your compost is “cooking” at.

While the danger of fire is real, don’t get rid of that compost pile just yet. It is very easy to avoid a compost fire.

First, keep your pile small, if you have a lot to compost, make several small piles.
Second, turn and water your pile regularly, if your pile is moist (but not soggy) it will not catch on fire.
Third, during very hot parts of the year pay more attention to your compost pile.

These three simple steps will keep your compost pile producing rich soil, instead of ash.

7 thoughts on “Ask The Sietch: Can My Compost Pile Catch On Fire?”

  1. Thanks for answering so quickly, and thoroughly! My husband will no doubt be thrilled to hear that “we” need to turn (and water) the compost regularly. :)

    This is a great site – thanks again.

  2. I had never heard of compost piles combusting spontaneously – so to speak. If the temps have to reach 300 degrees F. One would think that the bacteria that does all the work would be killed before the temp could be reached.

  3. I’m a composter and volunteer firefighter so this discussion got me thinking. While it’s true that the organisms die off below typical ignition temperatures, long-term heat from composting can dry out the material and cause pyrolysis. This is the chemical decomposition of a substance into flammable vapors. The ignition temperature of these vapors can be significantly lower than the immediate temperature required to ignite the original material.

    Fire is rapid oxidation, and oxidation is something that’s supposed to be happening in your compost heap. To get an open fire going, the oxidation needs to accelerate so heat is produced faster than it can dissipate. This acceleration is usually limited by having little available oxygen because of the low air flow through an unturned pile. Turning the pile adds oxygen, but it also cools the pile, which is why a regularly maintained compost pile should not become a hazard. (Keeping the pile moist helps because water is really good at absorbing heat.)

    Oxygen starvation also means if there’s going to be a fire in a compost heap, it may be a smoldering one that puts out relatively little heat and light. Such a fire can flare up to a glowing or flaming fire if it’s given a burst of oxygen when you open it up to extinguish it. Higher than usual winds or removal of nearby barriers could also stoke a smoldering pile behind your back.

    If your compost heap is too hot, make sure you have plenty of tools, water, space, and helpers before you try to fix it. Remove and wet down pieces of it gradually, and don’t climb on it since it could contain dried cavities and collapse. Be aware that an undiscovered fire can persist a long time and travel underground in material such as the roots of nearby trees.

    Play it safe if there’s any question at all. My advice is to get your fire department involved early on. Better to have them come out for “no reason” than risk having them come later to spray a lot of water all over the place.

  4. I want to experiment with indoor gardening this winter and wanted to make compost in coffee cans that we can shake up once a week,.. will this work? and will i have to worry about combustion? I have kids and want to be safe,… Please email me the answer as i have a hard time navigating the web.
    Thank you very very much!

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