Herbicide-Induced Fragmentation: Regenerative Ability of Cabomba Fragments After Exposure to Flumioxazin

<i>Cabomba caroliniana</i> A. Gray (cabomba) is an invasive alien aquatic plant (IAAP) posing a significant threat to aquatic ecosystems in Australia. Its ongoing spread is primarily driven by its rapid growth rate and ability to readily regenerate from stem fragments. Flumioxazin, an effective herbicide for controlling cabomba, has been registered for use in Australia since 2021. However, exposing cabomba to flumioxazin can induce stem fragmentation, potentially facilitating further spread. This study aims to determine whether stem fragments of cabomba following treatment at different flumioxazin doses (i.e., 25, 50, 100, or 200 ppb a.i.) can regenerate new healthy shoots that could contribute to its future spread in a new environment, in either summer or winter. This study also aims to investigate how this regrowth potential changes over time after herbicide application. Results show that flumioxazin suppressed the regeneration of replanted stem fragments in a dose-dependent manner in both winter and summer. In winter, complete regeneration was suppressed at the highest concentration tested (200 ppb a.i.), while low concentrations (25 and 50 ppb a.i.) resulted in an average 45% lower regeneration rate and 93% lower regenerated biomass than the control. In summer, suppression of regeneration was lower; at 200 ppb a.i., partial regeneration (18%) occurred with a 97% biomass reduction. At lower concentrations (25 and 50 ppb a.i.), more stem fragments regenerated (66%) and biomass reduction was lower (69%) compared to winter. Furthermore, in summer, the plants gradually regained their ability to regenerate over time after herbicide exposure, regardless of flumioxazin concentration, while no such recovery occurred in winter at any concentration. The findings show that the highest tested dose (200 ppb a.i.) can effectively suppress cabomba regenerative ability, which will greatly reduce the risk of new infestations caused by dispersed fragments, particularly in winter, when cooler temperatures and lower light are suboptimal for cabomba growth. This suggests that winter may be a more effective season for flumioxazin application. However, since some regeneration still occurred in summer, even at the highest tested dose, the highest registered label rate (400 ppb a.i.) may be necessary to ensure effective suppression under warmer conditions. Further studies are needed to evaluate this higher dose and its long-term efficacy.