I grow a large, white calla lily directly in water in my flower garden. The mother plant is actually planted in the rocks of the waterfall. When I first planted the lily, I used rocks to anchor it in place. Within a week or so, it rooted firmly into the rocks and has stood upright ever since. I have divided the lily numerous times since it was planted a few years ago and I have smaller offspring planted in the stream. New leaves sprout up from the cut roots. I'm not gentle with the dividing. The calla lily is a tough plant and I use a sharp knife to cut off sections.Here in my zone 7 garden, the calla lily grows in a southeast location so that it receives morning through early afternoon sun. This lily needs sun to bloom. The leaves stay nice and green without scorching.
There have been times in winter when I've seen the calla lily literally encased in ice. The first winter, I cut the plant back in the fall and it was a bit unsightly. Since then, I've not cut it back and it looks fairly nice all winter. The calla lily blooms beautifully each spring. The large, pure white blossoms are so beautiful with the waterfall as a background.