Efficacy of invasive plant control depends on timing of herbicide application and invader soil seedbank density

Understanding the underlying temporal dynamics influencing invasive plant control outcomes is essential to achieve restoration and land management goals. Within grasslands, herbicides are commonly used as the main method for invasive plant control, but the efficacy of management may be dependent on seasonal dynamics, as well as the number of applications. Additionally, assessments to quantify invasive plant control are often limited to aboveground plant composition, overlooking the potential repository of propagules stored in the soil seedbank, and additional impacts on nontarget species. To ensure that an herbicide method is effectively controlling invader populations, while limiting impacts on the resident plant communities, both above- and belowground species responses must be assessed. We established an herbicide field experiment across different sites and years in Riverside, CA, U.S.A., to assess the control of a global annual invasive forb, Oncosiphon pilulifer. We investigated how seasonal herbicide management (early-season vs. late-season) and repeated annual herbicide applications (1 vs. 2 years) influenced cover and seedbank density of our focal invader and the resident plant community 1 year after treatment. We found that although late- and early-season-applied herbicides reduced invader cover, early-season applications were the only strategy to reduce invader seedbank density, indicating potential longer term management control. Lastly, native cover was limited in post-treated areas, suggesting that additional management strategies may be needed to overcome native establishment limitations in invaded grasslands.