AbstractFluridone was applied to a 98‐8‐ha lake in Orange County, Florida, USA, in five different treatment plots between October 1982 and February 1983 to control a severe infestation of Hydrilla verticillata. Hydrosoil residues and submersed aquatic plant biomass were monitored within the lake. Fluridone did not affect the submersed vegetation during the 4‐month fall‐winter treatment period. As water temperatures increased during spring, Hydrilla biomass declined at an average of 0.178 kg m−2per month. By summer (192 days after last treatment), the target species could not be found within the lake. Fluridone residues were detected in the hydrosoil immediately following treatments and generally peaked coinciding with the decline in aquatic plant biomass. The maximum fluridone detected in the hydrosoil was only 5 of the 2.25 kg ha−1applied, and this amount was obtained from outside of a treatment area. Residue concentrations were highly variable between sampling sites and sampling periods and unexpectedly increased 14 months after treatment. Winter‐killed marginal vegetation is a possible source of this increase. Detectable concentrations of fluridone, and vegetation control, persisted for a total of 86 weeks from the date of the last treatment and non‐detectable residues may have persisted after 86 weeks. This study indicates that a lower application rate might have provided adequate control of Hydrilla and possibly decreased residue concentrations in n
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