Can Compost Get Too Hot and Catch Fire? Learn How To Prevent It

You may well have heard about compost heaps getting warm, but have you ever wondered can compost get too hot and catch fire? This might be a bit of a concerning thought, especially if your compost heap is close to your house, a shed, or any other potentially flammable materials.

Although it rarely happens, compost heaps can catch fire if the conditions are right. However, this is very unlikely to occur in your little backyard compost heap. It is more a hazard of commercial compost facilities – but we’ll still look at what could cause your compost heap to catch fire and what you can do to reduce the chances of this happening.

In this article, I’ll cover what causes compost heaps to catch fire and how you can prevent it.

What Causes Compost Heaps To Catch Fire?

Obviously, to catch fire, a compost heap needs to be very hot. If your compost heap is climbing above 160° F, you should be looking at ways to cool it down, because even if it isn’t at risk of catching fire, it is too hot for the worms and microbes that you need for effective composting to occur.

It’s a good idea to measure the temperature in your compost bin from time to time; this will let you know when your compost is getting too hot, but will also help you to see if it’s getting too cold to be effective. 

This is probably a bigger hazard in a backyard compost heap, and while cold compost doesn’t pose any safety threats, it won’t break waste down quickly. Temperature monitoring is important for both extreme heat and cold.

Check the temperature from time to time, especially during the summer months, if you are concerned about the possibility of a fire. Remember, the relevant part is the temperature at (or near) the center of the heap, not the temperature near the edge. Make sure you are measuring the inner temperature.

Usually, your compost can get up to around 200° F before you need to worry too much about the fire risk, but you should be taking action before these temperatures are reached because this much heat is not good for the organisms inside the compost, which are key to its operation.

What Creates Heat In A Compost Heap?

You might be wondering what makes a compost heap get hot in the first place. The heat is created by an exothermic reaction caused when microorganisms break down the food and garden waste inside the compost heap.

Because this heat tends to be trapped low down in the compost heap, the rest of the material insulates and keeps the heat in, causing the pile to gradually heat up. 

Turning the pile regularly can also increase the heat because you encourage the bacteria to work faster by giving them more oxygen. This produces more heat as they break the organic waste down.

A balanced compost bin needs greens (nitrogen-rich ingredients) and browns (carbon-rich ingredients) to operate properly. You are probably already familiar with the two kinds of waste, which need to be mixed to produce a healthy composting process.

It is worth noting that adding a lot of nitrogen is a sure way to heat your compost quite quickly. If, for example, you have a lot of grass clippings and you tip these into your compost bin, you might be surprised by how fast the compost bin gets hot. 

These are the main aspects that contribute to the temperature of a compost bin: the amount of microbe activity, how often the pile is turned, and the quantity of nitrogen-rich ingredients.

What Increases The Fire Risk?

What other conditions increase the risk of fire? A lot of oxygen in the pile certainly increases the danger, as fire needs oxygen to burn. Often, if your compost is composed of a lot of woody, tough materials that create air pockets, it is at greater risk of catching fire.

Having flammable materials is also an obvious way to increase the fire risk. Thin, twiggy, easy-to-burn waste will make your compost heap more likely to catch fire. Without tinder, a fire is unlikely to take.

Finally, uneven moisture distribution can be a problem. You might think that a wet compost heap is unlikely to catch fire, but this isn’t quite true. Water is a necessary part of composting, and a dry compost pile is often inactive – which means that it won’t be heating up, because the microbes are absent, or operating slowly.

Watering your compost could actually speed up the activity of the microbes and increase the fire risk, although it does depend on how thoroughly you do it. A damp pile is a fire risk, while a sodden one is not.

However, if you really saturate the pile, you’ll cause other issues, so it isn’t necessarily a good solution. Bear in mind that it’s pretty hard to soak the pile throughout, and a wet compost heap can still catch fire.

How Can I Prevent Compost Fires?

As mentioned, you probably don’t need to worry about compost fires if you are just composting in your own backyard, but if you are concerned, there are a few things you can do to decrease the risk of fire.

Keep Your Pile Small

The first involves keeping the size of your pile down. The bigger the pile, the more insulated the center will be, and the hotter it can get. A small pile will never generate massive amounts of heat, and therefore, it won’t reach high temperatures.

A compost pile should be more than three feet by three feet, but if your compost is getting larger than twelve feet (at the most), you need to break it down into smaller piles. This will also make it easier to handle and turn, so it is worth doing this even if you aren’t concerned about the potential for a fire to start.

Balance Your Browns And Greens

Secondly, avoid adding too much nitrogen. If you have a large amount of cut grass, assess your situation before you add it to your compost bin. The grass may provide valuable nutrients, but it will also sharply increase the microbial activity and boost the compost’s temperature fast.

If you have added a lot of nitrogen to the compost, try to balance this with a large quantity of carbon. Good sources of carbon include cardboard and sawdust. You should have significantly more brown material in your bin than green material; pay attention to this and increase the browns if necessary.

Turn The Pile

This might sound counter-intuitive because turning the compost heap does encourage it to heat up, but turning it also permits heat to escape and prevents too much from building up inside. Move hot material to the outside and cold material in, and keep an eye on the temperatures overall.

This sort of disruption will also help to break up patches of dry material and keep the air flowing.

You can also cool your compost by spreading it out and allowing the heat to dissipate. This is labor-intensive but does work if your compost is getting much too hot. Spread a tarpaulin on the ground to put the compost on, and then spread the material out on the tarpaulin and allow it to cool off.

When it is cool, put the pile back together. The excess heat should have dispersed. You can also water the compost layer by layer as you rebuild the pile; this will ensure you don’t have pockets of dry material mixed in with the damp material.

Monitor Your Compost

Keep a thermometer handy, especially during the summer months, and check what the temperature inside the heap is every few days. If you notice the temperature is climbing, you need to take action. Start by adding carbon to the bin and reducing the size of the pile if necessary.

Do Compost Heaps Steam?

If you see a smoke-like vapor rising from your compost heap, check it before you panic. Compost heaps often steam, especially on cold mornings, as their warmth causes moisture to evaporate into the air. This is not a sign that your compost heap is about to catch fire.

Steam is nothing to worry about; it just shows that the compost heap is nice and warm inside and the microbes are doing their job. If you are concerned, check the temperature. On the whole, however, steam is nothing to worry about.

If your compost steams a lot, you may wish to add some damp materials to it to replace lost moisture, but otherwise, content yourself with the knowledge that your compost is nice and warm and chugging along well, and you’ll soon have some wonderful soil improver for your garden.


A home compost heap is very unlikely to get too hot and catch fire. Usually, only very big commercial piles are capable of reaching sufficiently high temperatures, so you shouldn’t need to worry too much.

However, it has been known to happen, so pay attention to your balance of browns vs greens, the size of your compost heap, the moisture levels, and the air pockets. Remember, even if it doesn’t catch fire, hot compost will soon stop operating because the microbes won’t be able to cope with the temperature!