Use of Fluridone for Hydrilla Management in the Withlacoochee River, Florida.

Fluridone at a theoretical sustained concentration of 10 to 12 µ g l -1 was applied over a period of 13 weeks to several lakes along a section of the Withlacoochee river in central Florida, to control hydrilla. Residue analyses indicated that although the theoretical concentration of 10 to 12 µ g l -1 fluridone was not sustained for 13 weeks, hydrilla was susceptible to lower concentrations over the long exposure periods that resulted from low water discharges that occurred in 1990. The combined fluridone concentrations and exposure times at a given site were expressed in Fluridone Exposure Days (FEDs), and these values were related to water flow data, particularly the influence of spring-fed tributaries. Hydrilla was controlled for 10 to 12 months in the upstream half of the target area, where fluridone exposure was in excess of 500 FEDs, but in downstream areas subject to 250 FEDs, hydrilla control lasted for only six to nine months. This treatment was repeated in 1991 under increased flow conditions, with only a 10-week application period. Effective weed control was achieved despite lower FEDs possibly because of the increased susceptibility of the actively growing plants and the stresses caused by high water levels.