Corkscrew Rush, Juncus effuesus "Spiralis". 

It is one of my favorites because I love the way the stems grow in a corkscrew fashion.

The flowers, light green, are insignificant, as they are very small and sometimes missed when the plant blooms.

The plant grows in a "mound" and it a great specimen plant.

Rather than placing mine in the pond I grow it in an urn type planter that has no holes in it and sits on a pedestal as a specimen plant next to the pond.

Hardy zone 5 or higher.   Full sun