Boy that is tough. Anybody in town going to volunteer a mechanical skimmer for a couple of days? Depth of pond not a real big influence on duckweed. Excessive nutrients are. Since it is floating most contact herbicides are easy to apply. Problem is you really might cause more problems killing it off all at once. I guess if dredging is in your future that does not matter as much. And it sure would have been better to nail it before it took over the entire pond.
I'd probably go with a Diquat product. Reward, Aquacide and others. Diquat dibromide is a fast acting dessicant. My only concern is will it sink or just die? If it floats, back to square one. Put out a flier, volunteers needed for one time get together skimming pond for 4th of July.
