do marginal plants (in 8-10" of water) need to be cut back above the soil and dropped to the bottom of the pond to survive. I plan on cutting the lililes back and leaving them in 30" deep water over winter.
I cut everything "in" the water (no matter how deep) back to about a hand width above water level, simply because it'll otherwise flop over and rot. Whatever's on dry land I leave until the spring; the growth catches the snow which insulates the roots.