Why Does Fresh Mulch Smoke Sometimes?

Mulches are a great way to retain soil moisture, prevent weeds, and provide nutrients to your garden. However, you may have noticed your mulch piles sometimes look like smoke or steam is rising from them, even in regular weather. What exactly causes this?

Mulch can produce smoke or steam because of heat in the mulch pile, decomposition of organic material, or water vapor. For these reasons, mulch piles often make steam, but if proper fire safety measures aren’t in place, your mulch pile could start to smoke, indicating a fire.

In the rest of this article, I’ll explain how mulch piles build heat and how to prevent them from steaming or smoking. I’ll also discuss a few essential fire safety tips to prevent mulch fires, which materials you can use, and which ones to avoid.

Three Reasons Why Mulch Smokes or Steams

The main reason mulch piles smoke/steam is heat building up within the stack. Mulches tend to be the center of chemical reactions, so these release heat, or take it in, in the form of steam or smoke.

Let’s look at these three reasons why mulches smoke or steam.

The Decomposition of Organic Material Produces Heat

You can use two kinds of mulches—organic and inorganic/synthetic. Organic mulches help provide nutrients to the soil. In contrast, inorganic mulches are better at preventing weeds from growing and retaining soil moisture. 

If you’ve got an alternative method of clearing weeds, like using chickens or other weed control methods, you can use organic material to add nutrients to the soil.

Organic mulches can be materials like:

  • Wood chips or shavings
  • Bark
  • Leaves
  • Grass clippings
  • Newspaper
  • Manure
  • Straw or hay

Inorganic, or synthetic, mulches use materials like:

  • Plastic
  • Gravel or pebbles
  • Landscape fabric

Inorganic mulches last longer because they don’t decompose, but organic mulches can decompose quite fast.

The process of decomposing organic material involves exothermic reactions. In simple terms, this means that the process produces heat due to the breakup of the organic material. This heat can manifest as steam or smoke around the mulch pile.

Water Molecules in the Mulch Evaporate and Produce Steam

Water evaporation is the second reason for steam rising from your mulch pile. This process is particularly true if the weather in your area has been dry and warm. However, water evaporation occurs at any temperature, so warm weather isn’t necessary.

Organic material for your mulch, like wood chips or leaves, likely contains moisture. The water from the organic material might evaporate and produce steam when it hits the colder air surrounding the pile.

Heat Builds Up in the Mulch Pile, Causing Smoke

One of the biggest dangers of an improperly kept mulch pile is fire. Mulch can build up enough heat to combust spontaneously, so if you’re smelling smoke from your mulch pile, check if there’s a fire. If you suspect that there may be a fire within your mulch pile, call the local fire department and report it.

If you see ’smoke’—but can’t smell burning materials, the steam is likely due to the two things mentioned earlier.

How To Prevent Mulch From Smoking

Luckily, preventing mulch from smoking is relatively easy. There aren’t any specialized tools you need to use or fancy equipment, just good old-fashioned planning.

Here are four things you can do to prevent mulch from smoking and catching on fire:

Let’s take a deeper look into these measures and how they help prevent smoke or steam from mulch.

Use Inorganic or Synthetic Mulch To Prevent Smoke

The most straightforward prevention method is to use inorganic mulch. Smoke or steam is mainly caused by organic materials, like wood, leaves, or even newspapers. Replacing these materials with inorganic materials like gravel or fabric can reduce the fire risk.

Inorganic mulches also trap and contain heat, but their risk of catching fire is negligible. Most of these materials are not highly flammable, mainly gravel.

Space Out Your Mulch Piles To Prevent Fire

The most significant risk with fires is their ability to spread to other surfaces quickly. One way to keep a mulch fire contained is to spread out your mulch piles. It is best to keep your mulch at least 18 inches (46 centimeters) away from your home or any other structure.

Not only does this reduce the risk of heat building up, but it also ensures that if one pile catches fire, the others will be too far away for the fire to spread. Since there’s enough air circulation between the piles, the heat won’t build up as much.

Keep Your Mulch Piles Away From Your House or Shed

If there’s a risk of a fire starting, you will not want that anywhere near your house! If your home or shed is made of wood or contains other flammable materials, you should keep your mulch piles a distance away.

According to the Government of Massachusetts, new regulations prohibit anyone from keeping mulch piles less than 18 inches (46 centimeters) away from any building made of combustible materials. This distance is the minimum allowed in most states. It’s best to keep your pile at least 1-2 feet (12-24 inches) away from your house, shed, or other structure.

Replace Flammable Material With Non-Flammable Material

Although switching mulch material from organic to inorganic and non-flammable material is the easiest way to prevent a fire, sometimes that isn’t an option. Maybe your soil quality is poor and needs extra nutrients from organic mulch. Maybe some non-flammable mulch alternatives are unavailable or expensive in your area.

In this case, alternate patches of organic material with inorganic is the best thing to do. For example, if you’re using leaves as mulch, break it up with patches of gravel or rock. By separating them into patches, you can prevent a fire from spreading. 

If the leaves catch fire, they’ll spread until they reach the gravel, at which point they can’t go any further. This method is a great way to use both kinds of mulch while still maintaining fire safety standards.

Which Mulch Materials Are the Most Flammable?

If you live in a dry, warm climate, the risk of your mulch catching fire or producing steam is much higher. Some materials are more combustible than others, so it’s best to look for something that doesn’t present a fire risk.

According to the University of California, the three most flammable mulches are the following:

  • Cedar bark and pine needles: These materials are organic materials, making them more susceptible to fire or heat caused by decomposition.
  • Shredded rubber: Rubber is a non-organic material—but is still highly flammable. If these materials are in your mulch pile, they should be kept at least 30 feet away from combustible structures, like a house or shed.
  • Wood chips: Wood chips are flammable, though not to the same level as those mentioned above. Wood chips may catch fire and smolder, but the fire won’t become as big as it would with the other three materials. So wood chips can be used in patches, with a selection of non-flammable mulches in between.

Which Mulch Materials Are Non-Flammable?

If you’re worried about using flammable mulches, you can always switch to non-flammable materials. The tips I’ve mentioned above should also help prevent fires in your backyard.

Here are some non-flammable mulch alternatives:

  • Gravel or pebbles
  • Lava rocks (pumice)
  • Composted wood chips (smolders but with a slow spread rate)

These are almost entirely fire-resistant, but wood chips can still catch fire. However, composted wood chips will only smolder, and the fire spreads slowly, making it easier to put out.

Final Thoughts

Mulch can smoke or steam because of heat building up from different sources. Decomposition, water evaporation, and fire can all be possible causes of your mulch pile seemingly smoking.

You can prevent this heat build-up by ensuring that you follow fire safety standards or use non-flammable, inorganic mulches.

Alexander Picot

Alexander Picot is the principal creator of TheGrowingLeaf.com, a website dedicated to gardening tips. Inspired by his mother’s love of gardening, Alex has a passion for taking care of plants and turning backyards into feel-good places and loves to share his experience with the rest of the world.

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