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); lateral flowering branches are absent. At the tip of the stem are 1 or 2 stalkless male flower heads, often 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.

Two to 6 female flower heads sit below the males, are somewhat larger than the males, and are single in the axils of leaf-like bracts, the upper head(s) typically stalkless, the lowest head(s) usually 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).

Leaves and stems:

Leaves are alternate and basal, mostly erect to ascending, linear, to 20 inches long, to about 1/3 inch (3 to 8 mm) wide, hairless, toothless, flat on the upper surface, weakly keeled on the back and triangular in cross section at least 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 seed heads that expand up to about ¾ inch (to 20 mm) diameter, the plump, spiky head maturing from light green to greenish-brown.

Mature fruit is shiny, the body elliptic to somewhat fiddle-shaped, 3 to 6 mm (to ¼ inch) long, tapering to a stalk-like base (stipe), the tip tapering to a straight beak 1.3 to 2 mm long, much shorter than the body.

Notes:

Clustered Bur-reed is considered a circumboreal species, native to parts of northern Asia, Europe and North America, its range only known to dip into the US in Minnesota and Wisconsin. In Minnesota, it was designated an Endangered species in 1984, downgraded to Special Concern in 1996 and de-listed in 2013 after biological surveys determined it was not as rare as previously thought and its typical habitat(s) not especially threatened. It is currently Threatened in Wisconsin. Habitats in herbarium records are often described as “moats” within floating mats, fens, and other wetlands. Most of the images here were taken in a wet ditch along a gravel road cut through a mixed conifer-hardwood swamp.

Of the Sparganium species with erect leaves, S. glomeratum is distinguished by the lack of lateral flowering branches, at least some supra-auxillary female heads, only 1 or 2 staminate heads, and mature fruit that is shiny and has a straight beak less than 2 mm long, distinctly shorter than the body. S. glomeratum resembles a small S. emersum, which also has supra-auxillary female heads, but typically has 3 or more staminate heads, sometimes has a lateral flowering branch, and beaks on the fruit are about as long as the body.