What Is Mulch Steam? (Will Mulch Catch Fire?!)

What Is Mulch Steam

Ever noticed steam coming from your mulch?

This may be a surprising sight at first, but with a little investigation, we can explain why mulch produces steam.

And it has all to do with the decomposition process.

So, What Is Actually Happening?

Bacteria are responsible for decomposing mulch, specifically, it is the thermophilic bacteria (heat thriving and producing) that are generating the heat and subsequent steam.

In fact, temperatures can get up to 130°F – 170°F during the peak of the decomposition process.

As the microbes break down the organic matter, temperatures increase and heat is released.

In thicker mulch, like wood chips and bark, the center of the mulch pile can be especially hot due to a lack of air circulation and the build-up of trapped steam.

This is why steam often appears from within the thickest parts of the mulch. Steam also escapes the mulch when temperatures outside drop (ever noticed steam is more prevalent during early mornings?) or when the mulch pile is disturbed and trapped heat is released.

All of the steam is just a natural sign of the decomposition process as the bacteria breaks down the organic matter, releasing nitrogen, carbon dioxide, and minerals into the soil below.

Is It Possible For A Mulch Pile To Catch Fire By Itself?

Although very rare, it is possible for a mulch pile to ignite itself.

This is due to the high temperatures generated by decomposing bacteria. Heat and air circulation can cause spontaneous combustion if conditions are just right.

So what are the perfect conditions?

The mulch pile has to be pretty large to promote such a level of microbial activity – such as a pile that is over 1m in height. If you combine this with high outdoor temperatures, no aeration, and low relative humidity then you might end up combusting the mulch pile.

How Do I Prevent Mulch Steam (And Any Chance Of Combustion)?

Steam production from your mulch is a perfectly natural by-product of decomposition. Still, you may want to reduce steam production, and by doing so, reduce the chance of combustion as well.

To do this, you’ll want to keep the mulch pile aerated and spread out. Doing so will ensure that air is passed through the entire mulch pile, not just around its edges. This circulation of air will cool down the temperatures and reduce steam. Mulch piles that are thinner will also have less concentrated hot spots of microbial activity.

Bacteria love damp conditions, so if your mulch is super dry then the decomposition process may slow down significantly, and thus reduce steam output. Our advice is to water your mulch, but don’t make it too wet or things will get steamy again.

Certain types of mulch decompose at slower rates or not at all.

This includes some artificial mulches like stone and rubber. You can also opt for cedar mulch which is naturally resistant to decay.

Other Reasons Why Mulch Might Produce Steam

There are other less common reasons for steam coming out of a mulch pile. These include:

  1. Your mulch pile has caught fire from an external source. Cigarettes, nearby electrical sparks, or intense focused heat can all cause a mulch pile to catch light. This is especially likely during hot days when areas of the mulch may become very dry.
  2. In general, disturbing or raking mulch will cause it to steam as trapped heat is released.

Note: Fungus may grow in mulch piles and feed on the bacteria (as well as thrive in the humid and moist environment). Some fungi release dust-like particles and spores when disturbed or made wet. This can include slime mold and artillery fungus. These emissions can look like steam but are not actually steam.

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