Overwintering Plants

Question
I bought some plants for my pond this year that I have not had before. One is watercress and the other Yellow Floating Heart. Both seem to have been damaged by the frost we had last night. The watercress is in a pot, the other is submerged. Can these plants be saved? If so, what do I need to do to keep them over the winter?
I live in zone 7. The pond is small and shallow - more of a "water feature". It occasionally freezes over.

Answer
You can eat the rest of the watercress.  It's healthy and goes great on a sandwich.

I don't mean any offense to yellow floating heart but it is considered by the feds to be "noxious" and "invasive."  I would just get rid of it and pick a new plant next year.

Most plants native to zone 7 (a or b) can be preserved during a clod winter in one of two ways.

1)  Cut the plant down to about 1/5 of its original height.  This triggers the plant to stop using so many nutrients and it sort of hibernates.

2)  Prune the plant.  Place it in a trash bag.  Keep it wet.  Don't fertilize it.