I use round up on my cattails. I then have hit them with a weed wacker. Less come back up each time, but it is a battle. I try not to hit the water, but I have not seen any ill effects on the pond. I go around once a week and spray anything that does not seem to be wilting.
The problem is reaching the deepest cattails. I am not comfortable spraying them because most of the spray will go into the water. As soon as it gets warmer I will go out in my kayak and "glove of death" them.
