diana
I wish I knew! I'm just a person with a pond like you. I tried keeping up with the reward treatment like Shan said.
But after a while it seemed that reward was not even working. The first few times the DW (duckweed) turned brown and was easy to see was dead. But the last few times I have sprayed I have seen no change.
Even when you do kill it it stays on the pond until the next heavy rain washes it away. I tried reward for close to four weeks and out of that time, I had a clean looking pond maybe 3 days. And that was after the first spraying.
I may try Sonar next year but I'm not sure how to proceed. I post a question (Tried Reward... now what) but have not got any replys.
Good luck to you Diana, if you learn something PLEASE let ME know!
