I believe that is the plant I have, not the arrowhead. The problem I had looking at the pictures of Arrowhead is that my leaves are narrower and longer although the flowers looked the same. I was also confused by the 'cold hardy' label as that sometimes also means that a plant grows better where it gets a dormant, cold period. I don't have any blooms open on mine right now but I'll take a picture of the plant so you can see it. It certainly makes sense that I would have a Mexican native instead of one from the northern US. Whichever it is, I do like the long erect flower stems with white flowers all along them. It blooms off and on all year, and spreads rapidly. The one I have in my large pond long ago jumped its pot and is naturalizing among some rocks at one end of the pond. I pulled the ones I had in the roof pond out and stuck them in a bucket next to the pond and they have bloomed and thrived there. I'll post a picture tomorrow.
