Jstr, you can change your zone by clicking on 'My Account' near the top of the page.* Then click on 'Profile'.
I don't think 2 or 3 ducks will be enough to control the growth.* I'd rather see you grow veggies for the ducks.* It's safer and less chance of the duckweed escaping into the environment.* It could get on their feet and legs and spread from there.* I get nervous when people plan on using invasives.* Life has a way of changing and sometimes folks either lose interest or move on.* Whoever comes along next might not have the same enthusiam or knowledge to care for an invasive plant.* How about some native grasses or sedges that the ducks would eat?* You would need to put them in pots with stones on top of the soil so the ducks can't completely rip them up if your pond isn't an earthen one.
Do you already have the ducks?
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.