When mulch burns: fire risks at the wildland–urban interface

Background Mulch is widely used for landscaping in wildland–urban interface (WUI) communities but can act as a fuelbed that facilitates fire spread from vegetation to structures. Aims Although prior laboratory studies have investigated mulch ignition thresholds, few have examined large-scale burning dynamics under outdoor conditions representative of real wildfire exposure. Methods Full-scale fire tests were conducted on 10 weathered mulch products, including 5 composted and 5 non-composted types. Key results Non-composted mulches can sustain flaming combustion, exhibiting faster fire spread, greater flame heights, and higher heat fluxes compared to composted mulches. Composted mulches generally produced weak or unstable flames, transitioning rapidly to smoldering combustion, which slowed fire spread. In some tests, ignited particles driven by wind roll across the fuel surface and ignite adjacent fuel. Particle size influences burning behavior, with mulch made of larger fir bark particles sustaining more vigorous flame spread vs smaller particles that primarily smoldered. Conclusions Comparison with prior small-scale studies confirmed that while laboratory experiments capture ignition thresholds, only large-scale testing revealed ember spotting, sustained flaming spread, and heat flux exposures. Implications These findings provide empirical evidence to guide defensible space design, mulch selection, and fire risk reduction strategies in wildfire-prone WUI communities.