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). A plant usually has 1 or 2 branches, sometimes none. At the tip of the stem are 3 to 6 male flower heads, 1 to 4 at branch tips, crowded or not, each head stalkless and 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 or branch usually persisting for a time but eventually also withering away.

Female flower heads sit below the males and are about as large as the males, with (0)1 or 2 flower heads per branch, those on branches stalkless, those on the main stem commonly on a stalk at least partly fused to the stem (known as supra-auxilliary). Individual flowers have a single style at the tip of a pale pinkish to greenish ovary and are surrounded by scale-like tepals (petals with similar sepals) that have 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 3/8 inch (4 to 10 mm) wide, hairless, toothless, flat, thin.

The upper surface is green, the lower surface paler, often whitish, with green parallel veins and numerous pale cross-veins. 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 diameter, the plump, spiky head maturing to red, drying to dull reddish-brown.

Fruit is elliptic to somewhat fiddle-shaped, the body 2 to 5 mm (to ~¼ inch) long, tapering to a stalk-like base (stipe), the tip abruptly tapering to a slender beak nearly as long as long as the body and becoming strongly curved at maturity.

Notes:

Floating Bur-reed is an occasional to common aquatic in the northeastern quadrant of Minnesota, where it reaches the southern edge of its range. It is found in the quiet waters of lakes, ponds and slow-moving rivers and streams, usually in less than 4 feet of water, sometimes deeper. It is aptly named, as all its leaves float on the water's surface.

There are 4 Sparganium species in Minnesota that have long, ribbon-like floating leaves; S. fluctuans is distinguished by widest leaves 6 to 10 mm wide, usually branched flower clusters, 3 to 6 male flower heads at the tip of the stem (crowded or not), strongly red-tinted fruit with beaks that are initially straight but become strongly curved with maturity. The flowers commonly have a pinkish cast even when first blooming. The submersed portion of leaves are frequently a translucent brown. It can form large colonies that may have few flowering stems.

Of the Sparganium species with floating leaves, all the others usually have unbranched flower clusters or occasionally 1 branch, fruit lacks the red tint and beaks are straight or only slightly curved at maturity; Narrow-leaf Bur-reed (Sparganium angustifolium) and Small Bur-reed (S. natans) also both have widest leaves usually under 6 mm, rarely wider; 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.