The mature fruit is 6-8 mm. In flower during June and July. It can be distinguished from all other bur-reeds because it has 2 stigmas and the fruit is nearly square across the top. The other bur-reeds have 1 stigma, and the fruit tapers to the base and apex. Giant bur-reed is a persistent emergent found in shallow water and on wet substrates in marshes, bogs and margins of lakes and streams.
Muskrats use the entire plant and the seeds are commonly eaten by waterfowl and marsh birds.