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Can Mulch Spontaneously Combust? (Mulch Fire Safety Facts You Should Know)

Can Mulch Spontaneously Combust? (Mulch Fire Safety Facts You Should Know)

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Most people wouldn’t even consider it a fire hazard. Yet, if you think about it, mulch is made from dried wood and stems that can burn!

The National Institute of Standards and Technology says mulch is a combustible landscaping. It can spread flames around your property in no time.

How does mulch catch fire, and what should you do to prevent it? Can mulch start burning by itself?

Find out in this article!

Can Mulch Spontaneously Combust?

Yes, mulch can start burning by itself!

In April 2021, a house in Virginia burst into flames after fresh mulch ignited. The mulch acted as a fuel, and the flames quickly spread to the home.

This isn’t an isolated case.

The National Fire Protection Association says more than 14,000 fires per year are caused by spontaneous combustion.

Why Does Mulch Spontaneously Combust?

Spontaneous combustion happens when a material reaches its ignition temperature by itself.

It occurs when a thick layer of mulch gets too compacted. Because of the absence of air in compact mulch, it starts to ferment and develop organic acids.

As heat builds up from the fermentation, it starts smoldering. The organic acids it produces, like alcohol, ammonia, and methane add to the flammability.

The combination of heat, acid, and dry atmosphere can cause the mulch to spontaneously combust!

Which Type of Mulch Is Prone to Burning?

According to the National Association of Landscape Professionals, fine mulch has a higher chance of compacting. There’s less space for oxygen because of the smaller particles.

Moreover, dark-colored mulch absorbs greater amounts of heat than light-colored mulch. This is useful for colder states, but it’s dangerous for places with hot climates.

In short, selecting the right mulch can reduce the risk of fire!

Which Types of Mulch Should You Avoid?

First, if you live in a dry area, you should avoid the following mulches since they can catch fire quickly.

  • Pine needles
  • Shredded rubber
  • Pine straw
  • Shredded red cedar
  • Hardwood bark
  • Pine bark nuggets
  • Cypress

Meanwhile, Tahoe chips and composted wood chips are better against combustion. You may want to look into buying mulch with fire retardant!

Factors That Affect Mulch Combustion

Here are some factors that increase the likelihood of your mulch catching fire.

1 – Wind

Strong dry wind can dehydrate your mulch. Moreover, once the mulch catches fire, wind can introduce oxygen to the mix.

It’ll strengthen the flame and may even spread the fire faster!

2 – A Hot Climate

We know that mulch can generate heat. However, a hot climate can speed up the process.

Mulch fires are common in the summer when temperatures are high. Houses in Virginia, Massachusetts, and Ohio are prone to these types of fires.

3 – Lack of Rain

Dry spells can suck moisture out of mulch.

Watch out if there’s below-average rainfall in your area. The lack of humidity means you’re more prone to mulch fires!

How to Prevent Mulch Combustion

Here’s how you can avoid a fire caused by spontaneous mulch combustion.

1 – Place Mulch in the Right Location

Wooden fences and decks can catch fire. Fire marshalls recommend keeping mulch at least 18 inches away from structures.

To add to this, mulch piles must be 30 feet away from each other.

You can create a defensible space that’s free from vegetation around your house. Only plant trees and shrubs with low flammability within 30 feet of your home.

Follow guidelines and get a permit from the fire department if you have more than 300 cubic yards of mulch.

Finally, never store your mulch near the house. You can keep it in the shed or cover it with a tarp away from sunlight.

2 – Discard Combustibles Properly

Be careful when you’re discarding cigarette butts, matches, or charcoal. These have the potential to ignite dry mulch.

Remember to use trash cans and approved receptacles, and don’t forget to clear the area of seasonal debris!

You should keep mulch piles away from places where people may carelessly throw combustibles.

Plus, educating the people around you about proper combustible disposal is a great way to prevent a fire!

3 – Fluff Your Mulch

Do regular checks on your mulch. If it seems too compact, you should use a fork to fluff it up.

It’s best to turn mulch piles over once every two weeks. Expose the center of the pile to air to prevent a build-up of flammable organic acids.

You may also do a sniff test. If the mulch smells like vinegar or ammonia, it’s a sign that it’s time to flip it.

4 – Keep Beds Moist

Regularly test the moisture of your mulch. If you’re experiencing a dry spell, water your mulch more often.

When you’re watering your plants, ensure the liquid seeps all the way through. This’ll prevent the mulch from becoming too dry.

Installing a sprinkler system may help you keep the mulch moist!

5 – Consider Gravel and Stone

Another great way to prevent a fire is to line a perimeter of gravel, sand, and stone around your garden.

If you have a choice, go for non-combustible decorative accents like metal fences and stone walls.

There’s less chance of the fire spreading if you surround your property with materials that aren’t flammable!

6 – Layer Mulch Thinly

The depth of your mulch depends on soil and plant type. In general, three to four inches of mulch is enough to do the job.

For people using weed barriers like cardboard, you may even lessen the thickness.

Avoid using more than six inches of mulch in your garden. A thick layer of mulch has a higher chance of compacting and fermenting.

Know that proper mulch maintenance can save you from an accident!

What to Do if You See Burning Mulch

When you happen to see a pile of mulch that’s smoking, be on high alert! A mulch fire can spread quickly, and the pile could burst into flames before anyone can react.

The first thing you should do is to put it out immediately with water.

After this, call for backup from the nearest building. There’s a chance that the mulch can re-ignite, especially if the pile isn’t soaked through.

Report the fire through 9-1-1 so an expert can come out and check it. While waiting, clear the area from debris or wooden structures to prevent spreading.

Final Thoughts

Can mulch spontaneously combust? Unfortunately, under the right conditions, it can.

Mulch may catch on fire if it develops organic acids and its temperature increases. Dry conditions can hasten the process.

Because of this, it’s important to maintain and track the condition of your mulch. Always keep it moist, and don’t store it near your house.

Lastly, you may want to share this article with your friends. Being aware that mulch is a fire hazard can save lives!


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