Flower:

A single spike at the top of the stem, egg-shaped to lance-oblong in outline, rounded to blunt at the tip, 5 to 13 mm (to ~½ inch) long, with 15 to 150 florets tightly packed and spirally arranged, each floret subtended by a single scale. Scales are 1.5 to 2.5 mm long, 1 to 1.5 mm wide, rounded at the tip, orange-brown to straw-colored with a green midrib that dries brown. Florets have a 2- or 3-parted style (often mixed in the same spike) and usually 3 stamens. The lowest scale in the spike is broader than the rest, surrounds about 2/3 of the stem, and lacks a flower.

Leaves and stems:

The 2 leaves are bladeless and reduced to sheaths on the lower stem. The upper sheath is firm, concave on the back, the front pointed at the tip with a tooth at the apex that is up to .3 mm long.

Sheath bases are green to straw-colored to brown to reddish. Stems are straight, ribbed, erect to ascending, usually of varying lengths, .2 to 2 mm diameter. Plants form dense clumps.

Fruit:

Each flower produces a single achene (seed), that drops off independently of the scale, the achene with a cap-like appendage (tubercle) at the tip that is clearly distinct from the rest of the achene. Achenes are .9 to 1.3 mm long, straw-colored to dark chestnut brown, smooth across the surface, shiny, lens-shaped in cross-section, urn-shaped in outline, rounded at the tip end and tapering at the base. Tubercles are greenish to brown, broadly triangular, at least 2/3 as wide as the achene and 1/3 to 2/3 as long as wide. There is no neck or constriction between the top of the achene and the base of the tubercle. Surrounding the achene are (usually) 6 or 7 barbed, brown bristles, slightly longer to much longer than the achene.

Notes:

Eleocharis obtusa is one of several clump-forming Spikerushes in Minnesota and can be difficult to distinguish from some of the others when achenes are absent. It is most similar to Eleocharis engelmannii and Eleocharis ovata, and all three were at one time considered a single species and are still identified as such in some references. All three may be found in a variety of wet habitats, from road ditches to wet meadows to receding shores to seasonal pools in rock outcrops. E. obtusa is distinguished by the shiny and smooth lens-shaped achene, bristles usually longer than the achene, and tubercle that is 2/3 or more as wide as the achene and 1/3 (or more) as tall as wide. The spike is normally egg-shaped to lance-elliptic, blunt to rounded at the tip, with orange-brown scales.

Like E. obtusa, the tubercle on E. engelmannii achenes is at least 2/3 as wide as the achene, but is proportionately shorter (less than 1/3 as tall as wide), and its bristles are typically much shorter than the achene. E. ovata has bristles about as long as E. obtusa, but its tubercle is less than 2/3 as wide as the achene and often about as tall as wide, its floral scales are usually purple-brown rather than orange-brown, and stems in a clump tend to be more widely varying in length.