PhilR :

There is a new product called habitat that Kelly Duffie has discussed on the fourm. You can search for references. I checked the label and it lists duckweed and many other pond and non-pond plants . It does not list watermeal but watermeal is a little like duckweed . I have used habitat with good results on pondweed and it is not on the label either. Prior to this use I contacted the manu. rep. and he checked with a prof at LSU who worked on its development and he said it would work and would be ok to use. See below.

http://www.pondboss.com/ubb/ultimatebb.php?ubb=get_topic;f=14;t=000155

Maybe Kelly will post on this later.

The cost was about $70 a quart which will treat about 1 acre with a mid strength application.I don't know if this will work for you but it would be worth checking on. As with all products read the label and use correctly. This product will kill many pond and non-pond plants including many trees if sprayed on ground over or near roots. ewest