Well, actually Paul, that's where I was thinking of using it. Here's a link to the MSDS for Rodeo
http://www.tcweed.org/pdf's/rodeomsds.pdf - looks like it's designed as an aquatic weed killer, but says to keep it out of waterways. Hard to kill aquatic weeds without going near the water....
You saw that other pic - there are millions of cattails. Is there something else that might work?