Before we resorted to a chemical, we decided to try another method. We saw this recipe for a weed killer using 2 gal white vinegar, 1 lb of table salt, and 2 tblsp liquid dishwashing soap. We mixed this in a 2 1/2 gal sprayer. MIX WELL. It takes a bit for the salt to be dissolved.....Spray the cattails 2 times a week. We saw results after the 2 sprayings. The cattails turned white and finally brown and died. We've used this on poison ivy also and it seems to do the trick. It also works on small patches of duckweed. It doesn't work as fast as the chemicals but it's cheap and safe......