Hi JK--
I know you specifically asked about a chemical to kill the cattails. If, however, you might consider a non-chemical method AND you or a friend are willing to do some tubing or kayaking, the cattails can be killed mechanically.
My wife's step grandfather shared this method with me when cattails choked the entire bank of our pond into 5' water depth. He said simply cut the cattails about 6" below water surface. Then after a couple weeks, any new growth should be cut again. I would estimate I had a 90% kill rate after the first cutting of the cattails in our pond in the early '90's using this method. If memory serves, I think I only went into the water twice and the cattails were dead. It takes a couple years for microbes and nature to decompose the subsurface material. In the meantime the dying cattails can provide some cover for smaller fish. I remember carting wheel barrow loads of cut 'tails to a brush pile for burning as part of the process. Keep in mind if you apply chemical and don't remove the dead cattails, that organic material is left in your pond to decompose. The mechanical method of cutting and removing removes the material from the pond completely. Just thoughts on an alternative, non-chemical, method to kill the cattails.
