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).

At the tip of the stem are 2 to 4 stalkless male flower heads, usually crowded and appearing as a single elongated head, 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.

Two to 5 female flower heads sit below the males, are slightly larger than the males, and are single in the axils of leaf-like bracts, the upper head(s) stalkless, the lowest head(s) stalked with the stalk at least partially fused to the stem so the head looks attached to the stem above the axil (known as supra-auxillary). Individual flowers have a single style at the tip of a green ovary and are surrounded by scale-like tepals (petals with similar sepals) that lack a darker spot near the tip.

Leaves and stems:

Leaves are alternate and basal, all floating on the surface, linear, to 4 feet long, to ¼ inch (2 to 6 mm) wide, hairless, toothless, flat on the upper surface, flat to convex on the lower.

Surfaces are green, the lower surface with green parallel veins and no keel. Leaves become more translucent below the water's surface. Stems are erect, green and smooth. Flowering stems rise slightly above the surface of the water.

Fruit:

Female flower heads form densely packed seed heads that expand up to about 1 inch (to 30 mm) diameter, the plump, spiky head maturing from light green to reddish-brown.

Fruit is elliptic to somewhat fiddle-shaped, the body 3 to 7 mm (to ~¼ inch) long, tapering to a short stalk-like base (stipe), the tip more abruptly tapering to a slender, straight beak shorter than the body.

Notes:

Narrow-leaf Bur-reed is an occasional to common aquatic in the northeastern quadrant of Minnesota, found in the quiet waters of clear lakes, ponds and slow-moving rivers, often in less than 3 feet of water, sometimes deeper. We have often seen it in diffuse colonies with the slender floating leaves all pointing in the same direction, almost like a parade—it's quite lovely.

There are 4 Sparganium species in Minnesota that have long, ribbon-like floating leaves; S. angustifolium is distinguished by widest leaves 3 to 6 mm wide, unbranched flower clusters, usually 2 to 4 male flower heads crowded at the tip of the stem, 2 to 5 female heads, single from leaf-like bracts, the upper female heads stalkless and the lower stalked but appearing to arise from a point above the bract axil (supra-auxillary), fruiting heads up to ~1 inch (3 cm) diameter, fruit with straight beaks shorter than the body. It can form colonies that may have few flowering stems.

Of the Sparganium species with floating leaves, Floating Bur-reed (Sparganium fluctuans) may be most similar but has widest leaves 6 to 10 mm, branched flower clusters, and fruit is distinctly red with strongly curved beaks. Small Bur-reed (Sparganium natans) is a smaller plant, under 2 feet tall, with a single male head at the stem tip and no supra-auxillary female heads. Unbranched Bur-reed (Sparganium emersum) sometimes has floating leaves but more often has stiffer, erect leaves that are keeled on the back and triangular in cross-section. American Eelgrass (Vallisneria americana) also has long, ribbon-like leaves, but with a very different vein pattern.