Flower:

Round flower heads in a spike-like arrangement at the top of the stem and on branches arising from the axils of leaf-like bracts on the upper stem, with separate male and female flower heads on the same plant (monoecious).

Up to 12 male flower heads are at the tip of the stem or branch, crowded or not, each head stalkless and covered in dozens of petal-less flowers with yellow-tipped white stamens. The male flower heads open from the bottom of the spike up, turn brown, wither and drop off after pollen release, the naked part of the stem or branch usually persisting.

Female flower heads sit below the males and are larger than the males, with (0)1 or 2(6) stalkless flower heads per branch and those on the main stem more commonly stalked. Individual flowers usually have a 2-parted style and are surrounded by greenish to brown, scale-like tepals (petals with similar sepals) that have a darker spot near the tip.

Leaves and stems:

Leaves are alternate and basal, erect to ascending, linear, to 5 feet long, ¼ to ¾+ inch (to 20 mm) wide, hairless, toothless, keeled on the back and triangular in cross-section, though may be flat towards the tip. Stems are erect, stout, green and smooth. Flowering stems rise well above the surface of the water but do not typically rise above the leaves.

Fruit:

Female flower heads form densely packed seed heads that expand up to 2 inches diameter, the plump, spiky head maturing from green to brown.

Fruit is inverted pyramidal in shape, the body up to 10 mm (3/8 inch) long, half to nearly as wide, somewhat rounded at the tip and abruptly tapering to a straight beak 3 to 4 mm long, less than half as long as the body.

Notes:

Giant Bur-reed is the most common Sparganium species in Minnesota, a robust plant found in lakes, rivers and wetlands across the state. It is also one of the more easily identified as the only species with 2-parted styles on the female flowers. Most, if not all, flowers have this trait and the styles often persist while fruit develops. The fruit is also unique with its pyramidal shape and proportionately short beak. All other Sparganiums have single, undivided styles, more elliptic fruit and the larger species that otherwise may resemble it tend to have proportionately longer beaks on their fruit.