Act fast: improved control of Rhamnus cathartica with rapid foliar herbicide and herbaceous seeding

Many invasive shrubs re-establish quickly following initial removal by resprouting vigorously from surviving stems and germinating from remaining seeds. Foliar herbicide is commonly used to treat resprouts but may not suppress re-invasion in the long-term since even repeated applications are likely to miss some plants, and many invaders recruit readily from seed. We tested whether follow-up foliar herbicide and/or supplemental native seeding can suppress Rhamnus cathartica (common buckthorn) in deciduous forests by creating 28 experimental blocks across seven sites in Minnesota, USA. We imposed a factorial combination of seeding (sown in spring 2017 using a mix of 32 graminoid and forb species, or left to passive regeneration), and foliar herbicide application treatments (fosamine ammonium; “conventional/multiple” – applied in fall 2017 and 2019 – or “delayed/single” – applied only in 2019). We measured vegetation height, cover of functional groups, and buckthorn stem densities by size class in each of 2017, 2019, and 2021. Conventional herbicide application reduced buckthorn density by 71% one year after the first application and by 94% two years after the second application. Delayed herbicide reduced buckthorn by 88% by the end of the study. Seeding increased native plant cover and slightly reduced buckthorn abundance, but only when combined with conventional herbicide. Herbaceous plants increased with canopy openness, especially with seed addition. Using both conventional foliar herbicide and seed addition resulted in the lowest cover and density of buckthorn, and most robust native cover, reducing needs for future interventions.