Is mulch combustible

Mulch, compost, and decomposition all generate intense heat and a risk of fire. The larger the pile, the more fuel and insulation is available to feed potential combustion. Larger piles also make it more difficult to extinguish a burning mountain of mulch.

Mulch that is piled up more than a few inches deep starts to heat up as it decomposes, which can lead to fires starting via spontaneous combustion.