I tried to use a herbicide called Aquacide and had been told that it works well to eradicate cattails. The active ingredient in it is: Dichlorophenoxyacetic acid...17.5%. Unfortunately, when I called up Aquacide to order some for our Club I found out that they couldn't sell it to me in Alaska unless I had a Pesticide/Herbicide License which is sort of an equivalent of a FFL - except much harder and more expensive to get. In most States this product, and others like it, is available for a regular person without this license and if I lived in WA I would have gotten some and used it.
For those of you that can not use a herbicide because of land use restrictions (ie perhaps your permit for use on public lands prohibits you from applying a herbicide) then I was told if you have to cut by hand, cut them below the water line and here's why:
"I highlighted the hand/mechanical cutting followed by stem submergence on page 4. I think it’s worth a try. The stem submergence is critical to ensure the roots are depleted of oxygen, thereby killing the root. It is important to kill the root because cattails are able to reproduce through their creeping root system (rhizomes) and send up new vegetative shoots which can then disperse seeds. That’s why cattails are so invasive and can spread quickly. Both the living and dead cattail stems are capable of supplying the rhizomes with the necessary air exchange needed to survive, so both would need to be cut and submerged. This article had success with only a few inches of water covering the top of the cut stem whereas other research recommends 3-4 feet of stem submergence. Because there has been little work with cattail control in Alaska, it’s difficult to know what would be required. On a side note, this could be an interesting research project."