Make a planter ring [instructions on this site] to contain them. You can anchor the ring to keep it from getting near the skimmer. Depending on the size and shape of your pond, you could also run a "line" of some sort across the pond -- it would need to be JUST submerged enough to act as a barricade to hold the plants back. I discovered this accidently, when I had to tie off my floating [Oase] skimmer to keep it in one place. I used a black, thick, slightly stretchy craft cording that's mainly used for making jewelry. Tied one end to the bottom of the skimmer [they provide places for that purpose] and ran the other end across the pond and secured it to a piece of PVC pipe that's a part of my "turtle" fencing. I noticed that the cording, while hidden, actually did keep stuff from floating past.
