Can a grass clippings pile catch fire?

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Also: How long does a compost pile stay hot?

Yes. Any grass can catch fire in one big pile of grass clippings but it can only happen when a combination of circumstances conspire together to make it happen. 

Here we’re looking at a phenomenon known as spontaneous combustion. This can happen when a pile of organic material has accumulated and the moisture level is at a point where heat is produced. A grass clippings pile that’s too wet will not catch fire. A high level of moisture will effectively seal a grass clippings pile from air getting into it.

Without air getting into a grass clippings pile it can’t catch fire. It will descend into a cold composting situation and it won’t get any more exciting than that.

For a pile of grass clippings to catch fire the conditions have to be right. The moisture level of the grass clippings has to be high enough but not too high. A pile of grass clippings that are too dry won’t succumb to spontaneous combustion and catch fire either.


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Looking into the subject and picking up on incidences where fire services have been called out to extinguish fires that have apparently started from overheating piles of grass clippings, there have been cases where dry grass clippings have been blamed.

One report suggested that a pile of dry grass clippings in a bin overheated in exceptionally hot weather. 


A dry grass clippings pile can catch fire


If a fire starts in a compost bin or pile of completely dry grass clippings, it will be nothing to do with any hot weather. Any heat from hot weather, alone, won’t generate a spontaneous combustion situation. It will, however, provide a very dry, potential fuel for a fire to occur.  

The ignition source that would easily lead to a fire in a completely dried-out grass clippings pile, would have to come from a spark generated from without. 

A pile of grass clippings can catch fire through spontaneous combustion but it would need to begin with a build-up of heat in the middle of the mass of grass clippings. Because of the insulating nature of grass clippings the heat doesn’t easily escape. It will build up to a point where it becomes so hot that there is an ignition in the middle of the pile.

Does this mean that grass can catch fire in your compost bin? In theory this is possible but there would have to be a significant volume of grass clippings in one big pile for it to have a risk of happening. Most compost bins are too small to hold the volumes needed. 

You may see some steam emerge from a compost bin the day after you’ve put in some grass clippings but this will only persist for a short while. Then the pile in the bin will cool down. If you’re concerned about the signs of heat coming from a compost bin, don’t be afraid to soak it with plenty of water. 

This will make it wet and cool it. The water will drain through the pile and the composting process will continue.

For a very large pile of grass clippings from a large lawn, the story may be very different to that of a small pile in a compost bin. A large mass of grass clippings from the cutting of a large lawn, inevitably means that there will be a large pile of material that could be a potential problem.

Where large piles suddenly build up, you need to take this seriously. You need to manage this from the beginning to reduce the risk. Don’t allow one large pile of grass clippings to build up. Make smaller piles. There will be heat generated, for a while, from smaller piles but it’s less likely to escalate into a problem.

 A fire ignition point is less likely in a smaller pile because there won’t be enough of a mass to make it happen. If you are looking at a pile of grass clippings that are steaming enough to cause concern, take a fork and dig away at the top of the pile. This will allow a significant amount of heat to escape rather than build up. It’s the heat build-up in a pile that can lead to combustion.



How long does a compost pile stay hot?

A compost pile should reach a temperature of about 130°F (54°C) and stay hot for 2 to 3 days. The heat in a compost pile will then reduce. At this point, turn the compost pile over. This will introduce more air, which will encourage the thermophilic bacteria to produce more heat and help the compost pile to stay hot.

Much will depend on the size of the compost pile. The heat will escape much more easily from a small compost pile because there won’t be enough mass of material to form enough insulation.

Bigger compost piles stay hot for longer

A large compost pile will stay hot for much longer due to more insulation from the larger mass. Where there is sufficient mass, a compost pile can stay hot for several days. It’s possible for compost to stay hot, or warm, for weeks.

You can expect the temperature of a large-enough compost pile to rise to 130°F (54°C) and possibly higher. It will reach the maximum heat that the specific compost pile conditions will allow. It will stay hot on a plateau trajectory for three or four days, then start to cool down.

The length of time a compost pile will stay hot not only depends on the size of the pile but the material that makes up the mass of the pile. It will also depend on the balance of materials involved. There needs to be an appropriate balance of ‘green’ and ‘brown’ material.

Get the balance right and any compost pile will become hot and stay hot for the maximum length of time that the mixture will allow.

When the compost has finished going through, what some may call, its juvenile stage of getting hot and then cool down, you can think about turning it over. 

Some people seem to think that a compost pile needs to be turned over when it’s still hot. The thinking is that a hot compost pile is using all its air and that, by turning it, we’re introducing more air. 

The fact that a compost pile is hot, demonstrates that there must be enough air in the mass of compost, for the actions to take place that generate heat. When you can see that the compost pile is hot and steaming, you don’t need to do anything to it.

Some people think that by turning a compost pile when it’s still hot will somehow produce more heat. This probably comes from the experience of seeing clouds of steam going up during the turning process. Seeing steam emerge from a compost pile when it’s being turned isn’t the compost getting hotter. It’s heat escaping that probably should have been left where it was.

You can take the temperature of the compost if you want to but this information will be nothing more than academic. Your compost pile will take its own time to heat up and cool down. You have no control over it.

Turning over of a compost pile can take place after it’s cooled down. This will introduce air into the mass to help with the next stage. This will be the cold-rot stage. When this happens there will be no more heat generated.

There are lots of ways to turn a compost pile. Any agitation is a good thing. A compost tumbler is just one way of maintaining regular movement. There are probing devices that can be pushed into a compost pile. These will open up airways into the mass.

The most positive way of opening up a compost pile is to dig at the entire pile and move it from one point to another. This will shake it all up. Such a comprehensive movement of compost may mean that it may not be necessary again, doing it once should be enough.

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