The influence of fluctuating water depth on herbicide efficacy for Uruguay water primrose (Ludwigia hexapetala) control

Rapid fluctuations in water level are a common occurrence in many Florida waterbodies. While responses to fluctuating hydrology have been examined for many emergent invasive plant species, the influence these responses may have on herbicide efficacy is not well understood. Uruguay water primrose [Ludwigia hexapetala (Hook. & Am.) Zardini, Gu, & Raven] is an aggressive, emergent species that is difficult to control due to its immense creeping stem biomass. Given the difficulty in management, our objective was to determine how changes in hydrology can influence its control using herbicides. Mesocosm studies were conducted in 2017 to examine efficacy of a single herbicide treatment (imazamox plus carfentrazone) in relation to varying initial and final water depth. Mesocosms were either maintained at 20 or 40 cm inundation throughout the experiment, or were subjected to a change in water level following herbicide treatment (mesocosms were either flooded from 20 to 40 cm, or underwent a drawdown from 40 to 20 cm). We found that herbicide effectiveness increased for plants that were subjected to a low initial water depth followed by an increase in water levels after herbicide treatment. Shoot biomass above the waterline regrew to levels that were similar to the nontreated control when water depths were consistently low, consistently high, or fluctuated from high to low following herbicide treatment. These results indicate that changing water depth influences Uruguay water primrose control and that the pattern of change contributes to this. Aquatic managers may optimize control with late spring treatments when water is low just before the onset of the rainy season.