One of the aquatic glyphosate formulations would work fine on cattails (Rodeo, Aquamaster, Glypro, Glyphomate 41). All but the last require the addition of a surfactant that is designed for aquatic use. Cattails are most susceptible to glyphosate after they have begun to form the catkin (seed head). Thorough coverage is needed for maximum control. Use the label-recommended mixing rate for the product you select.
Also, there's a new aquatic herbicide call HABITAT that does an incredible job on cattails. However, it is only available in 2.5 gal. jugs and is fairly pricey - mostly used by commercial applicators.
