The following may be useful to some:

-In the winter of 2004-2005, I left a 4" deep shallow metal tray outdoors, in my -10 F. Zone 6b winter temperatures all winter long, which contained frozen water and the bare rootsystems of Yellow Flag Iris, Arrowhead, and Pickerel Weed. When they thawed, all of them died, except for the Iris tubers, which barely recovered, as all of the test plants were frozen solid in ice for months;

-In this past winter, 2005-2006, I filled a kiddie pool with 4" of soil and allowed snow and rain precipitation to be retained, so that the result would be frozen solid mud, all winter long. The test contained: cardinal flowers (allegedly freeze sensative), [no pickerel weeds], various unidentified marginals, Arrowheads, Watercress, Forgetmenots, some sort of plant which looks like elodia but grows above the water surface, Bog Arrum, Yellow Flag Iris, and Blue Flag Iris. All of these plants have thawed alive and are sprouting, with the exception of the Cardinal Plant - which was last seen vital and green right after it thawed, but I allowed the bog to dry out and I think they are, now, dormant, although when I flooded it last week, since all of the other plants have sprouted significantly.


Conclusion:

If you want to overwinter your marsh/marginal pond plants, don't sink them to the bottom of your pond where some of it will decay or drown, don't burry them bellow the frost line and cover with compost, don't dry them out in a garage; simply, dig a shallow trench, lay down some liner for moisture retention [I would not recomend water saturation, even though it worked fine for me, if it can be avoided], and leave them there under say 6" of mud until Febuary. This is what happens in nature. It has nothing to do with the fine roots of the plants going bellow the frost line - it is, likely, because the mud protects the root systems from much of the effects of water freeze.

I can't understand why all of those websites recommend digging the bog/marsh garden bellow the frost line - remember, do not attempt this with lilies or lotus, and this does not apply to the margin of the pond, where the roots are directly in water - as opposed to mud, which in winter is not consistently saturated with water, altough sometimes is.

Note: I left, as a separate experiment, Arrowhead "bulbs" in dry gravel outdoors and they sprouted alive in the middle of last summer (bucket outside in the heat of summer), after I forgot all about them - they were sprouting without water and fully sprouted days later, once placed in mud.

Thanks for sharing.

You have some real hardy plants!