I guess I'm unclear why you don't just overwinter them directly in the water? Because of freezing? I have always just left mine in the pond and they do well, but I have temperate winters so that may well be the difference. It does freeze here, but not for the whole winter, perhaps 1-2 weeks now and again. I've had good luck leaving water lily, mint, pond weed, houttynia, and arrowhead directly in the pond.
I did try to overwinter some water lettuce and water hyacinth (which I think die with just a frost) in a tub in my basement laundry room, but my husband tossed it thinking it was something I forgot about. Not my best experiment, I DID forget about them and according to Gary it was just a bucket of mush...
