Spikes:

A single stalkless spikelet at the top of the stem, subtended by an erect bract ¼ to 2+ inches long that appears to be a continuation of the stem, so the spikelet seems to be growing out from the side of the stem. Spikelets are 5 to 15 mm (~¼ to ½+ inch) long, lance-elliptic, rounded to pointed at the tip, greenish to light brown when young turning darker brown with maturity, with florets spirally arranged. Florets are perfect (both male and female parts) each with a 3-parted style and subtended by a single scale.

Leaves and stems:

The 3 to 20 leaves are usually submerged or floating on the surface, very limp, thread-like to narrowly ribbon-like, elliptic in cross-section except C-shaped near the base, up to 1 mm wide, with 1 to 5 parallel veins and numerous cross-partitions. Leaves are crowded on the lower stem and sheath each other; sheaths may be finely fibrous. Flowering stems are very slender (to 1mm diameter), nearly round in cross-section towards the base though may be more clearly 3-sided below the spikelet, erect and rising above the water or weak and floating on the surface. Plants form mats from slender rhizomes and are typically submerged in water up to 3 feet deep but occasionally terrestrial, the terrestrial forms having somewhat broader and more erect leaves.

Fruit:

Fruit develops in summer, the mature achenes (seeds) dropping off individually. Scales are 4 to 6 mm long, about 1.5 mm wide, very thin and fragile, light brown to whitish, the green midrib turning brown, lance-elliptic, pointed at the tip sometimes with the midrib slightly extending. Achenes are 2.5 to 3.5 mm long, 1.5 to 1.7 mm wide, 3-sided, urn-shaped in outline with a beak up to .8 mm long, smooth and maturing to brown. Surrounding the base are 6 barbed, light brown bristles that are roughly equal in length and as long as or shorter than the achene.

Notes:

Schoenoplectus subterminalis, formerly Scirpus subterminalis, is an occasional bulrush found in wet places and often in standing water as much as 3 feet deep, usually in sandy or peaty soils. It resembles 2 other Schoenoplectus species in Minnesota, Schoenoplectus torreyi and Schoenoplectus pungens. All 3 share the common traits of a 3-sided stem and 1 to a few spikelets with an erect bract that appears to be a continuation of the stem, so it appears the spikelets grow out from the side of the stem. S. subterminalis is the most diminutive of the 3, distinguished by a very slender stem (to 1mm wide) that is nearly round in cross-section especially towards the basse, has a single spikelet, and up to 20 thread-like leaves that are usually submerged or floating.

By comparison, both S. torreyi and S. pungens have stouter, strongly 3-sided stems, usually more than one spikelet, and broader, stiffer leaves. Vegetative stands of S. subterminalis can be confused with other aquatic species, notably certain Eleocharis species, which have leaves that are round in cross-section for their entire length, lack the cross-partitions, and do not sheath the stem.

Compare these with other Bulrush species, which may differ by their round stems, leaf-like bracts, essentially leafless or distinctly leafier stems, globular or branching clusters of numerous small spikelets, or other traits not as above.