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 8 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 4 female heads, single in the axils of leaf-like bracts, stalkless or nearly so; branches usually have none, rarely 1. 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, strongly keeled on the back to at least the middle of the blade, triangular in cross section and channeled towards the base. 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 1½ inches (2.5 to 3.5 cm) diameter, maturing from green to brown.

Mature fruit is shiny, the elliptic to somewhat fiddle-shaped body 5 to 7 mm (~¼ inch) long, slightly constricted near the middle, tapering to a stalk-like base (stipe), tapering at the tip to a straight or curved beak more or less as long as the body and may be hooked at the tip.

Notes:

Branched Bur-reed is an occasional aquatic in Minnesota, with scattered populations in our central and northeastern regions, where it reaches the western edge of its range. It's found in the quiet waters of lakes, ponds and slow-moving rivers.

Of the Sparganium species with erect leaves, S. androcladum is distinguished by the usually branched flower clusters, the branches usually with all male flower heads, 2 to 4 female heads on the main stem all stalkless or nearly so, mature fruiting heads at least 1 inch diameter (2.5 to 3.5 cm), and mature fruit that is shiny and has a straight to curved beak that may be hooked at the tip.

The two most similar species are S. emersum (Unbranched Bur-reed) and S. americanum (American 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. americanum usually has branches that have at least 1 female head, and has smaller fruiting heads (2 to 2.5 cm) with dull fruit.