The efficacy of bensulfuron methyl (BSM) at 40 concentration and expos
ure time combinations was evaluated on Eurasian watermilfoil (Myriophy
llum spicatum L.) under controlled-environment conditions. BSM concent
rations ranged from 0 to 4600 mu g/l; exposure times ranged from 7 to
42 days. Efficacy was based on shoot and root biomass harvested at the
conclusion of each experiment. Herbicide injury was evident 1 week af
ter application on all treatments, and symptoms included leaf chlorosi
s, deformed leaves on shoot tips, downward bending of leaves at upper
nodes, stem necrosis, and formation of axillary buds. Following 7-, 14
-, 21-, 28-, 35-, and 42-day exposure periods at concentrations rangin
g from 10 to 4600 mu g/l, biomass was reduced 10 to 90% compared to un
treated plants. Increasing exposure time was more efficacious than inc
reasing concentration. BSM concentrations <10 mu g/l did not significa
ntly reduce growth. Effects on roots were variable depending on concen
tration and plant age. Only treatments of 230 mu g/l and higher inhibi
ted root growth on plants grown for 3 weeks prior to treatment. Regrow
th emerged from rootcrowns, axillary buds, and injured shoot apices, a
nd was evident 1 to 2 weeks following completion of the exposure perio
d and removal of the BSM-treated water. Plant death was not achieved a
t the concentrations and exposure times tested in these studies.