I keep tropical lilies year round in the greenhouse, but they don't bloom much in the winter. They are much happier if I heat the water and keep them in a bright spot, though. The viviparous varieties, in particular, seem to be tougher and better able to handle the cold air. If it gets too cold, however, they form a tuber and tend to go dormant. I've never figured out how to wake them up again. Sometimes they come back, but often the tuber just remains and slowly rots away during the following summer.
There's a lot of different kinds of hardy waterlily roots, but mostly they creep along the soil until they run into something. The tropicals, though, tend to grow straight up into the water, and I've got a couple that are within inches of the water surface and really need to have been repotted long ago.
