Susceptibility to ignition of landscaping mulches exposed to firebrand piles or radiation

Abstract

Mulch products are widely used in landscaping at the wildland-urban interface (WUI), but burning mulch poses a significant ignition hazard to nearby structures. This study evaluates the ignition susceptibility of 10 mulches used in California, USA, exposed to smoldering firebrand piles or irradiation. The mulches included composted types, where products are piled and partially decomposed (bioretention, cal trans, marin vineyard, topper mulch, screened compost) and non-composted types (forest floor, redwood, black mulch, fir bark A and B). We found that all mulches except screened compost can be ignited to smolder by firebrand piles, with ignition more likely at higher firebrand mass and wind speeds. Following spotting ignition, non-composted mulches were more prone to transition from smoldering to flaming at critical wind speeds, which varied by mulch type. For piloted ignition, all mulches can sustain flames under irradiation, with non-composted mulches generally requiring lower critical heat fluxes due to the emission of more combustible gases. Particle size significantly influenced ignition thresholds, with smaller particles facilitating piloted flaming ignition and larger particles being more susceptible to spotting ignition and smoldering-to-flaming transition. This work deepens our understanding of ignition and burning behaviors of mulch, helping improve the prevention and suppression strategies against catastrophic WUI fires.