Off the top of my head I don't know of any willow specific herbicide that is approved for pond use, but Reward (when mixed according to the label recommendations) has whacked a LOT of different things for me, including horsetail rush.....

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3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).