Background Mitigation of wildfires at the wildland–urban interface (WUI) will be enhanced by understanding the flammability of plants growing in this zone. Aims We aimed to: (1) compare shoot flammability among wildland native, and both urban native and urban exotic ornamental plants; (2) quantify relationships between shoot traits and flammability; and (3) establish flammability scores to distinguish low- from high-flammability species. Methods Flammability and traits of field-collected shoots were measured and relationships quantified in 44 species from the Blue Mountains World Heritage Area, Australia. Key results In our study area, urban exotic plants were less flammable than wildland and urban native plants. Slow-igniting shoots had high fuel moisture and bulk density; short-burning shoots had low bulk density and volume; shoots recording low maximum temperatures had high fuel moisture, low bulk density and volume; and shoots with low biomass consumed in flames had high fuel moisture and low volume. Our novel flammability scores distinguished low-flammability (e.g. Lophostemon confertus) from high-flammability native species (e.g. Callistemon citrinus). Conclusions and implications Low-flammability plantings at the WUI should preferably use native species given potential ecological impacts of exotics. We suggest that future work should seek to identify broader suites of low-flammability native species.