I did the same thing when my pond was filling. We were in the second year of a drought and the pond had filled about 1/3 of the way. I noticed 3 locations where willow trees had begun to grow in tight groups. I figured they would be great cover once the pond was filled so I let them grow with intentions to kill them and cut them before the pond was full so they were roughly 1' under the surface at full pool. I did not realize how hard they would be to kill. After 3 treatments of cutting the tops and brushing the top of the stems with straight Grazon P+D, they finally gave up.
