Keven, I have sprayed cattails several times. Here is what I know. Do not spray them when they start turning green. That's way too early. You will not get a kill. Wait until they get more mature. Use 4oz. Rodeo per gallon of water with a good surfactant (2oz. per gallon). Be patient. They will start turning yellow in a week or two. Eventually they will die. Next year they will not come back.
I can assure you that this formula works.