I have flowering rush in my pond and would like to kill it before it starts to grow for the year. Do anyone have any recommendations?

I had alot of flowering rush in my ditches.
Killed them successfully with gliphosate but there were no water at the time. I think that if you can reach them you can use a mop soacked of erbicide solution (just wet not dripping) to moisten the leaves carefully without the risk of polluting the pond.