Indeed - since the plant grows from the tubers and roots, it would seem quite possible that a toxin from the source material could be found in other parts of the plant that grow out from it. I've shied away from trying any flower parts at all simply because there is so much confusion (and no regulation) about what cultivars are safe to use or in which ways. Hearing that similar species do show a tendency for toxins to sometimes exist in their leaves is very helpful cautionary information.
Water chestnuts, it sounds like, are easy to grow inside or outside in containers, so those might make great companion plants to a group of nice potted ornamental daylilies. Even with water chestnuts, however, it sounds like there are often-confused species that are invasive weeds, noxious, and should be very carefully avoided (see third link):
http://homeandgarden.sinitem.c...
http://www.ehow.com/how_866520...
http://www.ehow.com/how_757642...
