I have a small pond entirely filled with cattails. Tall ones, too--8-10ft. I sprayed about 1.25 gallons of this at a 5% solution and nearly everyone turned brown within 2 weeks. I will probably have to spray a second time because those things grow so fast.
Mixed it with a surfactant, as suggested, for control of phragmites near my shop (a very difficult and invasive plant). Initial results are promising from first spraying; a second application may be needed.
The product may actually be working on the cat tails!! Surfactant is key and I blended my own as the waxy leaves are quite resistant to having a water based solution adhere to them.
It has been two weeks since my first application and I reworked the blend on one section an it appears to effective.