I don't know if Preen is labeled for use around ponds. That only stops seeds from germinating, if the plants come back from roots or grow from rizomes, Preen won't work.

www.hoosierpondpros.com

[Linked Image from lh6.ggpht.com]
http://www.pondboss.com/subscribe.asp?c=4
3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).