Composting Temperatures – Can Compost Catch Fire?

If you keep a compost pile at home, you may have heard that compost can sometimes catch fire on hot days.

There have actually been quite a few reported cases of this phenomenon occurring, as compost and mulch offer all the necessary ingredients for a good blaze to get started and the larger the compost pile, the more dangerous the situation can get.

This doesn’t mean that all compost piles will suddenly burst into flames.

Following basic compost safety will guarantee that your home and compost pile remain fire-free. Below we explore the reason compost fires happen as well as what you can do to prevent them.

Why Do Some Compost Piles Get Hot?

Before we can explain how some compost piles can get hot enough to spontaneously combust, we’ll have to explain the difference between the two main types of composting: hot and cold composting.

Both techniques involve piling organic material such as kitchen scraps or lawn clippings to let them break down into fertile soil, but how each is done is what sets them apart.

Cold Compost

Cold composting is considered the more hands-off process, as plant matter is simply piled and left as is until it breaks down. Unlike hot composting, there’s no flipping of the compost required.

No oxygen is introduced into the compost and most of the bacteria at work are anaerobic ones, which don’t produce heat as a byproduct. Because of this, the compost pile stays cold and the composting process takes much longer, like a year or more.

You may also be interested in our guide on composting in cold weather for more tips.

Hot Compost

Hot composting, as the name implies, makes your compost pile hot and requires the constant flipping of your pile.

Flipping adds oxygen to the pile and feeds the aerobic bacteria present, which release heat as they work to break down your eggshells and garden scraps.

The benefit of hot composting over cold composting is that the composting happens much faster, in around a month or so if you get it right, and the heat kills weed seeds and pathogens that could spread to the rest of your garden.

How Hot Does A Compost Pile Get?

A hot compost pile can reach temperatures of 120 to 170 degrees Fahrenheit in just a few days and will continue to increase if left unattended, which is why it’s so important to constantly monitor your compost.

Temperatures of 200 degrees Fahrenheit are easy to reach, especially for larger piles. Although these temperatures may not necessarily lead to a fire, being this hot will definitely kill the beneficial bacteria and other helpful organisms in a compost pile in just a few hours.

It’s not hard to monitor the temperature of a compost pile.

Specialized compost thermometers that you can stick into your pile allow you to monitor the real-time compost temperature of your pile, allowing you to prevent it from getting too hot, killing your composting crew and potentially igniting a fire.

Can Compost Catch Fire?

Compost fires are most common in large, industrial-size composting operations, and it’s rare for home composting to lead to a fire.

However, it’s not unheard of and home compost fires can be especially dangerous if your compost pile is next to your house, shed or another flammable structure. A small fire can spread to these nearby sources of fuel and quickly become unmanageable.

When a compost pile isn’t regularly turned and watered, it begins to overheat, causing the outside of the pile to dry up. This leaves a hot inner core surrounded by a layer of dry and flammable material which serves to insulate the core and heat it up even more.

The internal pile temperatures continue to go up, and the outer layer becomes drier until suddenly it ignites and you’ve got a compost fire on your hands. The temperature danger zone for compost is around 300 degrees Fahrenheit.

How To Prevent A Compost Fire

Don’t throw away your compost pile just yet, though. As mentioned above, the risk of a home compost fire is pretty low due to the smaller size, and it’s actually very easy to prevent a blaze from happening in the first place.

Just follow these simple steps to ensure your compost pile stays within a safe temperature range.

  1. Keep your pile small. The larger the pile, the hotter it will get and the greater the risk of a fire. Your compost pile shouldn’t exceed more than 5ft x 5ft x 5ft. If you have more material to compost, it’s best to have multiple smaller piles than one large one.
  2. Make sure to regularly water and turn your compost pile. Make sure to water and turn your pile every few weeks to prevent overheating and the drying of the top layer. You don’t want your compost pile to be soggy, but a nicely moist pile won’t catch fire.
  3. Monitor the temperature of the pile regularly and especially keep an eye on your pile during hot weather.

Although compost fires do happen, most home gardeners never have to worry about this problem, especially if they follow the compost pile safety steps we’ve outlined here. Composting is a great way to recycle organic waste while returning the nutrients to your garden.

By understanding how composting works, you can ensure the safety of your home and family as well as the continued production of your garden.

About Justin Micheal

Justin has always loved gardening and caring for the outdoor spaces in his grandmother's backyard. He believes everyone can enjoy the space available to them, no matter how big or small. On Backyard Digs, he shares everything he's learned about growing a successful garden and maintaining and improving the landscape of a backyard.