A safer (but less effective) alternative to
herbicides is to reduce the amount of
sunlight available for photosynthesis.
One approach is to add an approved
aquaculture dye to the pond. These
dyes are sold as weed-control agents
and tint the water blue to reduce light
penetration. The dye is usually effective
for several weeks. Note, however,
that using dyes may favor the growth of
mat-forming filamentous algae that float
high in the water column where there
will be adequate light even in dye-treated
water. Another way to reduce light
penetration is to keep the pond water
turbid (with suspended sediment) by
using aerators or other devices to stir up
mud from the pond bottom.