Notes from Ohio State University on herbicidal treatment of water lilies: http://ohioline.osu.edu/a-fact/0015.html
[On use of herbicides]...Effectiveness is greatly enhanced by treating a plant problem early, before it reaches serious nuisance levels. Emergent (cattails) and large-leaved floating vegetation (water lilies) are best controlled just after flowering when roots are weakest and the plant begins to send nutrients down to the root system. Submerged plants, duckweeds, watermeal, and algae should be initially treated in late spring, before high biomass levels are attained. In the case of algae, additional treatments may be necessary throughout the summer as re-blooms are quite common. ....
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