European bur-reed (Sparganium emersum) is a common plant in northern Minnesota. It is found in shallow water at the margins of lakes and swamps, in ponds and streams, and in backwaters of large rivers. From June to August it produces conspicuous, spherical heads of tiny white flowers.
The spongy leaves rising from the base of the plant may be stiff, erect, and up to 32″ long, or limp, floating, and up to 80″ long. The leaf-like bracts on the flowering stem are smaller and spread outwards or bend upwards.
There are 22 species of bur-reed worldwide, 9 in North America north of Mexico, 6 in Minnesota. European bur-reed is distinguished by its small size; leaves that are flat, not rounded, on the back; unbranched inflorescence with up to 6 pistillate heads; pistillate flowers with a single stigma; and spindle-shaped fruit that tapers to both ends.
When a flowering stem is not present the stiff leaves may be mistaken for narrow-leaved cattail. However, those leaves are rounded on the back, not flat.