Submerged plants need to be below the ice to survive the winter months in the North. Any part of the plant that is allowed to freeze on the surface will turn to mush and add unwanted debris to the pond. In the Fall, you can net them out and re-bundle them. The tops are the part of the plant you want to keep. Bundle them in bunches of 6 to 8 stems about 8" long and replant in baskets of pea gravel or place weights on them to sink to the bottom of the pond.