My white pickerel weed also died out. I don't think they are real hardy. I am interested in how well the hardy water cannas survive; mine did poorly and died. If you want a fairly aggressive marginal plant that does not grow deeper than 2ft consider the flowering rush. I have lots to get rid of. It is considered invasive and does spread fairly fast. Invasive is a relative term. I consider native cattails invasive. Cattails IMO are more of a problem and less of a benefit in ponds than flowering rush.
http://www.michigan.gov/invasives/0,5664,7-324-68002_71240_73848-368758--,00.html
http://www.luontoportti.com/suomi/images/10025.jpg
