If it's the ditch lily they typically spread by bits of the plants getting broken off and transported somewhere else, such as by machinery, or floating along the creek. With the ditch lily you will probably find that any seed pods that do form do not have viable seeds. If you find some pods with seeds that are large and black then you can certainly try to germinate them but you're more likely to have success propagating them by digging some up and dividing them. They do spread though so you'll have more soon whether you plan to or not
The problem with the ditch lily is that it has an odd number of chromosomes so usually does not set viable seeds, although it can occasionally.
