Flower:

Round flower heads in a spike-like arrangement at the top of the stem, with separate male and female flower heads on the same plant (monoecious), usually with 1 to 3 branches, sometimes none. At the tip of the stem are 3 to 7 stalkless male flower heads, not tightly crowded, each covered in dozens of petal-less flowers with yellow-tipped white stamens. Male flower heads turn brown, wither and drop off after pollen release, the naked part of the stem usually persisting for a time but eventually also withering away.

Below the male heads are female heads that are larger and not crowded together. On the main stem are 2 to 6 female heads, single in the axils of leaf-like bracts, stalkless or nearly so; branches have 1 to 3 female heads. Individual flowers have a single style at the tip of a green ovary and are surrounded by scale-like tepals (petals with similar sepals).

Leaves and stems:

Leaves are alternate and basal, mostly erect to ascending, linear, to 3 feet (to 1 meter) long, to ½ inch (6 to 12 mm) wide, hairless, toothless, flat on the upper surface, weakly keeled at best on the back, triangular in cross section near the base and more or less flattened above. Stems are erect, green and smooth. Flowering stems rise above the surface of the water and are much surpassed by the leaves.

Fruit:

Female flower heads form densely packed, spiky seed heads that expand up to about 1 inch (2 to 2.5 cm) diameter, maturing from green to tan or greenish-brown.

Mature fruit is dull, the somewhat fiddle-shaped body 3.5 to 5+ mm (to ~¼ inch) long, slightly constricted at or just below the middle, tapering to a stalk-like base (stipe) shorter than the body, the tip tapering to a slender, straight to slightly curved beak about as long as the body.

Notes:

American Bur-reed is a common aquatic in the arrowhead region of the state, with scattered populations in central Minnesota where it reaches the western edge of its range. It's found in the quiet waters of lakes, ponds and slow-moving rivers, usually in less than 2 feet of water, rarely deeper, and can become stranded on shores when water recedes. It can form large stands.

Of the Sparganium species with erect leaves, S. americanum is distinguished by the usually branched flower clusters with at least 1 female head per branch, female heads on the main stem all stalkless or nearly so, mature fruiting heads not more than 1 inch diameter (2 to 2.5 cm), and mature fruit that is dull and has a straight to curved beak about as long as the body.

The two most similar species are S. emersum (Unbranched Bur-reed) and S. androcladum (Branched Bur-reed). S. emersum has at least some female heads that are stalked, where the stalk is partly fused to the stem so it appears the head is attached above the axil (known as supra-auxillary), and only occasionally has any branches. S. androcladum usually has branches that are all male, with no female heads, and has larger fruiting heads (2.5 to 3.5 cm) with shiny fruit.