The cattails that lived last years attack are actually new shoots coming up. The best time to kill them is when they are very short. Then the biomass does not build up and take all year to decay. In Southeast Michigan new shoots will start coming up from now until just after the 4th of July or so I would guess.

I dont know all the politics or the science, but in Illinois it is against the law to spray any herbicide not approved for aquatic use in or around your pond. Always follow the label for any herbicide use.

Glyphosate still wouldnt be my first choice for submerged lake weeds anyhow.