About 3 years ago I planted a Thalia Dealbata in the small lake that is bordering the back of my garden in the Netherlands. As I suspected it not to be hardy, I took it inside during winter of the first year, where it barely survived. After the Water garden forum adviced me that the plant is reasoble hardy, I left it outside in the water next year and it survived perfectly. It now is a splendid large plant with nice purple ears, far too large to get inside again. As I understood the plant will survive as long as the roots which are about 30 cm below water level, will not freeze. Our Dutch winters are very mild the last few years, but a cold winter could come again. I would really regret to loose this plant, so I would like to know what to do if a winter with severe frost would come again. Does anyone has experience in protecting the roots of Thalia Dealbata (or other waterplants) from freezing?
Louis Harmsen
PS
I also posted this question to the Water garden forum
How to get a Thalia Dealbata through the Dutch winter
Somehow the picture the picture of the Thalia Dealbato did not come through, so here it is
I have this plant in a shallow pond here in SW France louis. It's fills a large pot and has flowered well this summer. Last winter was pretty cold - down to about -11C - and for quite a long time. The foliage went brown, but once spring came it greened up well and, as I said, went on to flower well.
Whether the long hot summers that we have here are necessary to get the plant in the right condition to withstand the cold I'm not sure, but the roots were not covered by that much water. The top of the pot was just under the water surface.
Hope this helps....