Landscaping defensible space: Plant flammability testing informs recommendations to reduce community fire hazard

• Identified links between plant traits and flammability through laboratory testing. • Provided science backed recommendations for landscaping defensible space. • Highlighted the role of community input in selecting species for fire-risk testing. • Developed a plant flammability index for Southern California WUI communities. • Developed a combustion chamber design to test plant flammability. The wildland urban interface (WUI) presents a unique planning challenge. Landscaped residential properties intersect with wild, fire-prone vegetation; however, WUI residents lack clear guidance on which plants pose higher or lower risk and how to manage vegetation to reduce fire hazard while also prioritizing landscaping design. This is largely due to the lack of information regarding how landscaping plants burn. This study provided a community-led approach to plant flammability testing, suggested an index to rank landscaping plant flammability, and offered landscaping guidance for residents of Southern California WUI communities. Community feedback revealed which ecosystem services (i.e., cultural: privacy) and plant species attributes (i.e., drought tolerance) were valued most by the community and helped identify common native species for flammability testing. Through laboratory flammability tests, interspecific differences in flammability were found as well as significant relationships between plant traits – hydration, branch, and leaf morphology – and flammability. Branching, mass, and drought stress were positively correlated with combustion intensity, whereas live fuel moisture (LFM) and stem surface area to volume ratio had a negative effect. These findings confirm the importance of minimizing plant mass near structures; carefully considering plant growth forms in planning; and maintaining healthy, hydrated plants in defensible space. With these considerations in mind, we discussed which defensible space zones could be best-suited for the tested plant species. Involving resident input and community horticulturalists in research direction allowed for targeted testing of species most relevant to the community of study and provided action-oriented collaborations with local stakeholders to improve fire-informed landscaping.