Mulch that is piled too deeply, more than a few inches, can build up heat and spontaneously catch fire. Mulch fires start more readily when the weather is hot and it has been dry for an extended time. ... The distance between mulch piles keeps a fire from spreading from one pile to another, or to a building.

What causes mulch to smoke?

Mulch will produce smoke or steam due to heat caused by bacteria during the decomposition process – just like in a compost pile. Mulch will also appear to smoke or steam when watered due to certain types of fungus that grow in the mulch.

Can black mulch spontaneously combust?

So, can mulch, specifically black mulch, catch fire? Mulch fires are rare - that's something the experts we spoke with have confirmed - but they definitely can happen. 11Alive's Wes Peery said that spontaneous combustion of mulch is possible since the wood product creates heat as it decomposes.