That's common bulrush, though it has other names. Magnificent plant, growing taller than a person. I remember being fascinated by them growing along the Basingstoke Canal when I was a bairn in the 60s. Shelter for frogs, and cover for emerging froglets. And cut them off for the flower vase. Eat the roots. This looks like the best deal for seeds. Prices for plug plants elsewhere are just silly. The link doesn't say how many seeds you get, but I think they're tiny, so maybe its countless thousands. Will update on this next year.